Lecture Notes, EES 1004, Sec 003, Earth and Environment Through Time (Historical Geology), 3:30-4:45 PM T&Th, Fall 2006

Ronald K. Stoessell, Office 1034, 504-280-6795, rstoesse@uno.edu or ronlondi@charter.net

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Links to Chapter Outlines & Reviews With Review Questions

Physical Geology (EES 1000, Dynamic Earth) is a prerequisite to Historical Geology (EES 1004). To pass historical geology with a C or better, without having a background in physical geology, is difficult. You will have to do extra work on your own time! Don't attempt this unless you are willing to buy a physical geology text and learn it on your own. The review of physical geology done in historical geology is brief.


These notes on the web contain the topics that are covered by the instructor in the class lectures. The lecture notes are not intended to be a book, and students are expected to use their class notes together with the class book, or an other historical geology test, or web resources to understand the topics. A list of review questions and specific topics for each chapter are given at the end of each chapter in the lecture notes. These are questions and concepts that the student should understand for the test. After a chapter is covered in class, this review list may be updated by the instructor. Students are responsible for checking the web site to get the update prior to a test.



Text: Levin, The Earth Through Time, 7th Edition



Tests: Three 100 point multiple-choice tests. The first test will cover Chapters 1-5, reviewing Physical Geology concepts, Fossils, and Evolution. The second test will cover Chapters 6-10, reviewing the origin of the solar system, the Hadean, Archean, Proterozoic Eons and the Paleozoic Era. The last test covers Chapters 11-15, reviewing the Mesozoic and Cenozoic Eras and the origin of humans.

The numerical grades of the 3 tests will be averaged and the final grade format is 87 or above, A; 74 up to 87, B; 61 up to 74, C; 48 up to 61, D; below 48, F. Students caught cheating will receive a zero on the test.

Students who miss one of the first two exams can take a makeup in the class period prior to the last (or third test). The makeup covers all of the material covered in the first two tests. A student who took both of the first two tests can also use the makeup to substitute for the lowest grade on one of those two tests. A student missing both of the first two tests will need excused absences for both tests in order to get two makeups, one of which will be given during final week. A student missing the third test with an excused absence can get a makeup during final week. Makeups during final week will be given at the time scheduled for finals for this class.

Class role will be taken.

You can contact me by telephone, email, and before and after class to set up an appointment. Otherwise, look for me in the Geochemistry Lab (1059) or in my office (1034) on class days in the afternoon. Note that the general class notes for this course (Earth and Environment Throught Time or Historical Geology) on my web page (accessed from www.ronstoessell.org) are often updated following the lectures.



Lecture and Test Schedule

The lecture schedule may vary but the three listed test dates are fixed. A review is scheduled prior to each test. The date of the review may be shifted up if the test material is covered faster than scheduled, freeing up a class period to be used for study for that test.

         Chapters                 Chapters                     Chapters
 
 8/21/06    1            10/03/06   6 K. Der.      11/16/06      15            
 
 8/23/06    1            10/10/06   6 & 7 K. Der.  11/21/06      review                
                                                             
 8/29/06    2            10/12/06   7              11/28/06      makeup on Chapters 1-10          
                                                                 (second to last class date)                           
 8/31/06    2            10/17/06   8              11/30/06      test 3 on Chapters 11-15                
                                                                 (last class date)                         
 9/05/06    4  K. Der.   10/19/06   9  				                 
                                                                
 9/07/06    4  K. Der.   10/24/06   10 				                        
                                                                
 9/12/06    3            10/26/06   review  			            
                                                                
 9/14/06    3            10/31/06   test (6-10)			                
                                                  
 9/19/06    5            11/02/06   11				           
  
 9/21/06    review       11/07/06   12 				                             
 
 9/26/06    study day    11/09/06   13 				                
 
 9/28/06    1st test     11/14/06   14 			                
            chapters 1-5  
                                                               
                                                   
 



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Links to Chapter Outlines & Reviews With Review Questions

Chapter 1 Chapter 2 Chapter 3
Chapter 4 Chapter 5 Chapter 6
Chapter 7 Chapter 8 Chapter 9
Chapter 10 Chapter 11 Chapter 12
Chapter 13 Chapter 14 Chapter 15



Chapter 1 "Introduction to Earth History" (p. 1-33)

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The chapter is mostly a review of general concepts from EES 1000, Physical Geology. Read the chapter and understand the following concepts and terms listed below. In addition, memorize the Geologic Time Table given in the outline. You will have to know the time table for all three tests.


Historical Development of Geology as a Science

Hutton proposed "Uniformitarianism" or "The present is the key to the past."

Principle of Uniformitarianism - You need to be able to explain how this principle is applied to explain how ancient rocks formed. The idea is to study the processes forming rocks today to be able to understand how they formed in the past. When we see diagnostic structures in rocks forming today than we can often assume that ancient rocks with these structures formed in the same way, i.e., the same processes occurred in the past as occur today because the same laws govern these processes.

Scientific Method - The scientific method is used in the study of the Earth and consists of four steps: (1) gathering data, (2) forming a hypothesis to explain the data, (3) making predictions based on the hypothesis, and (4) testing these predictions to see if they are true - if so, the hypothesis becomes a theory. A theory which makes many correct predictions is called a law; however, as new data becomes available, theories and laws always have to be modified, i.e., there is no absolute law or truth in science. An example is conservation of energy in reactions and normal chemical processes, the first law of thermodynamics. It doesn't hold in nuclear reactions in which mass is converted to energy.


Stratigraphic principles and biota (fauna and flora) succession used to establish relative time of formation


superposition, original horizontality, lateral continuity

Older sedimentary beds lie under younger sedimentary beds unless the beds have been overturned (superposition).
Sedimentary beds are initially deposited as horizontal layers (original horizontality).
Beds that are laterally continuous represent a single formation event which can cover a lot of time (lateral continuity).

intrusive relationships and cross-cutting relationships

Only a younger rock layer (intrusion, e.g., dike) can cut an existing rock unit.

principle of components

The components making up a sedimentary rock are older than the rock.

principle of biota (fauna and flora) succession

Evolution occurs along a pathway without repeating itself.


Radioactive dating is used to determine absolute time.

Absolute time measurements from radioactive decay involve measuring the amount left of an unstable isotope which decreases by one half during each half life. The unstable isotope decays to a daughter isotope which may also be unstable and undergoing decay.

For example, 14C radioactive carbon decays to 14N with a half life of 5700 years. After four half lives (22,800 years), the amount of radioactive carbon has decreased to 1/16th the amount initially present. After 10 half lives, the amount of any unstable isotope is so small that it is difficult to measure accurately. The necessity for a significant change in the unstable isotope and having enough of the isotope to measure limits the use of 14C to ages between 1,000 and 100,000 years. Other unstable isotopes (of generally different elements) with smaller and larger half lives are used to age-date rocks within smaller and larger time intervals. In the case of many isotopes, the amount of unstable isotope when the rock formed is not known; however, the unstable isotope is incorporated in different minerals within the rock but in different concentrations in each mineral. In this case, the dating is applied to the different minerals to compensate for the lack of knowledge of the initial concentration of the unstable isotope in the rock. For example, in 87Rb => 87Sr dating, each mineral starts out with the same ratio of 87Sr to the stable isotope 86Sr but with different concentrations of the unstable parent isotope 87Rb and of the stable daughter isotope 87Sr. The increase in the ratio of 87Sr/86 in each of the minerals can be used to solve for the age without knowing how much of the unstable parent isotope was initially present.


Unconformities represent missing geologic time

The three types are angular unconformity (erosion rruface separating two "non-parallel sedimentary beds), disconformity (erosion surface separating two parallel sedimentary beds) and "nonconformity" (erosion surface on top of a crystalline rock). Frequently, soils form when unconformities occur.


Isostasy and Isostatic adjustment

The crust is floating on the mantle (like a log on water) and moves up if material is removed, e.g., from erosion of mountains or melting of ice, and moves down if material is added, e.g., in a depositional basin such as the Mississippi delta. The continental crust is much thicker than oceanic crust. The thickest continental crust occurs with the tallest mountains - just as the log floating on a pond extends further below the water if it sticks up higher above the water.


Earth Structure

The temperature and structure of the earth passing from the center to the surface goes from hot to cold and from dense to less dense: inner core (solid iron with minor nickel and sulfur); outer core (liquid iron with minor nickel and sulfur - responsible for the earth's magnetic field); mantle (solid silicate except for plastic asthenosphere where convection cells occur); crust (solid silicate).


Plate Tectonics

Lithosphere plates are rigid and composed of oceanic and continental crust and the underlying uppermost mantle. These plates ride on top of heat-driven convection cells which are located in the asthenosphere and probably extend deeper in the mantle. The plates converge at subduction zones where one plate carrying oceanic crust is subducted. This produces shallow to deep (large) earthquakes and a trench with composite volcanoes (andesitic magma) overlying the subduction zone, forming from magma released by partial melting of oceanic crust (mafic rocks, basalt and gabbro) of the subducting plate. Plates diverge at spreading ridges (diverging boundaries), characterized by shallow earthquakes, diverging from a rift valley with shield volcanoes (basaltic magam) forming from partial melting of peridotite, an ultra-mafic rock in the underlying asthenosphere. Plates slide past each other along faults called transform boundaries, producing large, shallow earthquakes, e.g., the San Andreas fault. In addition, when two plates with continental crust on their leading edges converge, they fuse, forming high mountains of folded sediment from the sediment on the continental margins, and large earthquakes. Continental crust (felsic rock, granite) is not subducted because it is too light (low density). Two converging plates can only fuse after all the oceanic crust (mafic rock, basalt and gabbro) between the converging continental crusts is destroyed in subduction zones.

One additional interesting plate tectonic process takes place on the interior of plates. Hot spots or mantle plumes are mafic (basaltic and gabbroic) magma plumes rising up from the mantle and melting through the overlying plate, producing shield volcanoes on the earth's surface, e.g., the Hawaiian Islands. Because the hot spot is stationary in the mantle and the overlying plate is moving, a linear chain of shield volcanoes and/or fissures forms on the surface of the overlying plate.

Rock composition of the mantle, oceanic crust, and continental crust

uppermost mantle (ultramafic rock) same as other mantle rock but different minerals due to pressure changes, e.g., graphite to diamond - both are composed of carbon but daimone is the high pressure polymorph.

oceanic crust (mafic rock: basalt overlying gabbro) only about 7 km thick - less than 4 miles. Oceanic crust is created by partial melting of peridotite under diverging zones and destroyed in subduction zones.

continental crust (felsic rock: granite) - thicker than oceanic crust. Continental crust is created at subduction zones by partial melting of oceanic crust. Continental crust is never destroyed.


Geologic Time Scale

Please note that universal agreement does not exist on the Time Scale. For example, the Quarternary Period sometimes includes both the Holocene and the Pleistocene Epochs. And some authors use Precambrian as an Eon with Proterozoic and Archean as Sub Eons or even as Eras. The Hadean Eon is sometimes added as the earliest Eon to cover the time lacking a rock record on the Earth, taking time from the Archean Eon. Many authors use Tertiary as a Period and not Paleogene and Neogene Periods, and some authors use Carboniferous as a Period and not Mississippian and Pennsylvanian. The Time Table below is that consistent with your text and also with the concept that the Periods within an Era should roughly correspond to same length of time and that the Eons should correspond to the larger amounts of time. This splits the Carboniferous Period into the Mississippian and Pennsylvanian Periods and keeps the Archean and Proterozoic as Eons, not Eras. Use of Paleogene and Neogene Periods, rather than Tertiary Period, allows a split of a very long Period into three Epochs in each Period.


    Eon                Era           Period            Epoch

                            
                                    Quaternary         Holocene (Recent) Since the end of the last Ice Age
                                                        
                                                 ----------------- 10,000
                                     Neogene           Pleistocene  Ice Ages (sometimes included in Quaternary)
                                     (included in      Pliocene     Ice Ages begin, Isthmus of Panama forms   
                     Cenozoic         the Tertiary)    Miocene
                   (modern life)      
                                                 -----------------
                 age of mammals 
                    and birds        Paleogene         Oligocene   Himalayas Form
                                     (included in      Eocene      Alps form
                                      the Tertiary)    Paleocene   mammals and birds battle for supremacy on land
                                        
                 65 my -----------------------------

Phanerozoic                          Cretaceous (Latin for chalk) age of dinosaurs & first angiosperms, Rockies form

(well-displayed      Mesozoic        Jurassic   (Jura Mountains)  age of dinosaurs, first birds, Sierra Nevada form
                  Pangaea breaks up
   life)           (middle life)     Triassic   (3 fold division) age of thecodonts, first mammals, dinosaurs,
                                                                  and pterosaurs , hexacorals evolve   

               250 my ----------------------------- Greatest Extinction

                                     Permian   (Russian Province)
                                        age of reptiles, Urals form, major glaciation
                                     Pennsylvanian (US state)
                                        age of amphibians and reptiles, Appalachians form
                                     Mississippian (US river)
                                        age of amphibians and crinoids
                     Paleozoic       Devonian (Devonshire County)
                   Pangaea forms        age of fish, first gymnosperms and amphibians, 
                                        major extinction and glaciation at end of period 
                     (old life)      Silurian (Celtic tribe)
                                        abundant spore-bearing land plants, first fish with jaws 
                                     Ordovician (Celtic tribe)
                                        abundant stromatoporids and tabulate corals, first spore-bearing land plants
                                        major extinction and glaciation at end of period  
                                     Cambrian (Roman for Wales)
                                        age of trilobites and nautiloids, first fish
               540 my ------------------------------

Proterozoic (age of protoctists), part of Precambrian, Rhodina forms and breaks up, active plate tectonics, two
 major periods of glaciation, multicelled plants and animals evolve

2,500 my --------------------------------------------

Archean (age of bacteria), part of Precambrian, the earliest 600 m.y. lacks a rock record and is sometimes called 
the "Hadean Eon."  Oceanic crust forms during Archean and then first continental crust froms from oceanic crust.

4,600 my --------------------------------------------

Beginning of the earth, mantle on surface

Chapter 1 - Review Questions




Chapter 2 "Earth Materials: A Physical Geology Refresher" (p. 34-59)

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Rocks


Minerals

Rocks are composed of minerals. The bulk chemical composition of a rock is the average composition of the minerals. The most common minerals are the aluminum silicates that make up most of the igneous rocks, e.g., the micas (muscovite and biotite), the feldspars (orthoclase, albite, anorthite), pyroxene, ampohibole, and olivine. Other common minerals that occur in sedimentary rocks are composed of calcium carbonate (calcite and aragonite), iron oxides (hematite and magnetite), calcium sulfates (gypsum and anhydrite) and sodium chloride (halite). Minerals have different properties, i.e., melting temperatures, cleavage, hardness, etc. For this course, learning different minerals is not as important as understanding the different types of igneous, sedimentary, metamorphic, and hydrothermal rocks.


Igneous Rocks

(has a crystalline texture of interlocking crystals due to crystallization from magma)

extrusive (has small crystals) - examples, basalt (oceanic crust) & andesite

intrusive (has large crystals) - example, granite (continental crust), gabbro (oceanic crust), & peridotite (mantle)


Igneous rocks form from crystallizing magma and have a crystalline texture. They are composed of aluminum silicate minerals. These minerals range in composition from ultramafic to felsic in which iron, calcium, and magnesium-rich minerals are mafic minerals and silicon, potassium, and sodium-rich minerals are felsic minerals.

Intrusive rocks (plutonic) form under the earth's surface and have large crystals due to slower rate of crystallization. Extrusive rocks (often volcanic) form on the earth's surface on land or under water and have small crystals due to rapid crystallization from the faster loss of heat. If the magma cools very fast, a volcanic glass forms. Intrusive and extrusive igneous rocks of the same composition (same minerals) have different names. The chemical composition of the rocks varies from ultramafic to felsic as it changes from being rich in iron, magnesium, and calcium to being richer in silicon, potassium, and sodium. The ultramafic intrusive rock is peridotite; while its extrusive equivalent (komatite) is very rare. Within the earth's crust are three important estrusive and intrusive rock pairs: basalt and gabbro forming mafic rocks; andesite and diorite forming intermediate rocks; and rhyolite and granite forming felsic rocks. The viscosity of the magma increases going from mafic to felsic while its temperature of crystallization (rock melting temperature) decreases. Because more felsic minerals melt at lower temperatures than mafic minerals, more felsic rocks can be derived from more mafic rocks by partial melting the more mafic rock and then crystallizing the magma. mineerals that form in the crystallized rock will cover a more felsic range (than the parent rock) due to the more felsic composition. The process can be repeated several times to make an even more felsic rock. This is how ultramafic rocks making up the mantle peridotites are partially melted to generate magmas forming basalt and gabbro (oceanic crust) at diverging boundaries (spreading ridges) and hot spots, and how basalts and gabbros are partially melted to generate magmas forming andesite, diorite, and granite, making new continental crust overlying subduction zones.

Oceanic crust forms at diverging ridges and over hot spots from magma produced by partial melting of ultramafic rock in the mantle and is composed of gabbro underlying basalt. Continental crust is composed mostly of granite. Diorites and andesites form at subduction zones by partial melting of oceanic crust. The earth's mantle is composed of peridotite, an ultramafic intrusive igneous rocks.

An intrusive rock forming a tabular layer parallel to the surrounding rock structure is called a sill. If the tabular layer cuts the structure of the surrounding rock, it is a dike. A massive intrusion is a pluton. If the pluton has a large surface exposure (> 100 square kms), it is a batholith. A stock is a pluton with a smaller surface exposure.

Lava is magma on the earth's surface, and it can be emitted from a fissure or a vent on a volcano. Flat sheets of basaltic lava can occur, forming flood basalts which have columnar jointing due to crystallization on land and pillow structures due to crystallization under water. Volcanic eruptions of magma more felsic than basalt, often contain partly consolidated rock called tephra, e.g., ash and pumice. Tephra occurs because the more felsic magma is sticky, has a lower crystallization temperature, and moves slowly to the vent, making it likely to partially crystallize prior to the eruption. Ash and pumice form the extrusive igneous rocks called tuff and ignimbrite, respectively. Obsidian is a glassy felsic extrusive igneous rock formed from fast crystallization of lava.


Sedimentary Rocks

(generally shows stratification or bedding)

clastic or detrital - sediment lithified through diagenesis - exp., sandstone

chemical - exp., evaporites, some limestones, coal


Sedimentay rocks form from earth-surface processes and are usually bedded in layers. Sedimentary rocks are commonly composed of sediment grains of aluminum silicate minerals formed at higher temperatures and other minerals formed at lower, earth-surface temperatures, e.g., calcium carbonate minerals (calcite and aragonite), clays, iron oxides (hematite, limonite goethite), chert (microcrystalline quartz), evaporites (halite and gypsum), and recrystallization products such as dolomite (from calcite and aragonite) anhydrite (from gypsum), and quartz (from chert).

Detrital or clastic sedimentary rocks form from the weathered fragments (sediment) of preexisting rocks. The rock names are often based on sediment size: breccia and conglomerate [gravel size], sandstone [sand size], siltstones [silt size], and shale [clay size] and shales and siltstones are often called mudstones. Other rock names are based on composition, e.g., limestones are composed of broken fragments of calcium carbonate fossils and cherts are composed of microsiliceous (opaline) fossils; however, both limestones and cherts can be chemical rocks composed of calcium carbonate and opal precipitates, respectively. Breccias have angular grains while conglomerates have rounded grains. Note that the weathering process and stream transportation of sediment tends to round sediment, sort sediments by size (due to weight), and destroy mafic minerals relative to felsic minerals. So rounded felsic minerals are common in sedimentary rocks. Detrital rocks become lithified upon deposition, burial, and cementation of the grains together.

Chemical sedimentary rocks usually form as precipitates from water. Evaporites are generally composed of gypsum and halite and form as precipitates from evaporating sea water. Limestones are composed of calcium carbonate minerals (calcite and aragonite) which can be precipitated from sea water or fresh water when the water degasses carbon dioxide to the atmosphere. Limestones also form as cemented shell fragments of calcium carbonate minerals and hence can also be considered a detrital sedimentary rocks. Chert is composed of microcrystalline quartz and forms from lithication of hydrated silica (opal) either as a direct precipitate or as cemented opaline shell fragments. Petrified wood is chert precipitated from groundwater as a replacement for organic tissue. As with limestone, chert composed of shell fragments can be considered a detrital sedimentary rock.

Early in the earth's history, banded iron deposits formed as a direct precipitate of iron minerals, a process that is no longer possible because the high oxygen content prevents the build-up of iron in seawater. The calcium carbonate in limestones often recrystallizes through the addition of magnesium to form calcium magnesium carbonate rocks that are called dolomite. Organic rocks such as lignite and oil shale are also chemical sedimentary rocks, formed from the preservation and modification of organic plant tissue.

Sedimentary rocks typically have structures which can be used to identify the environment of formation. These include presence of diagnostic fossils of species living in particular environments, absence of bedding, and special types of bedding such as crossbedding and graded bedding, ripple marks that are symmetrical or nonsymmetrical, and the presence of mudcracks and varves (alternating layers of a thin, dark layer with thick, light-colored layer. The degree of sorting of the size of grains, the shape of the grains, the composition of the minerals, and the geometry of the sedimentary rock deposit can also indicate the environment of formation.


Metamorphic Rocks

(recrystallized rock)

foliated (has parallel crystal growth) - examples: slate, phyllite, schist, gneiss

nonfoliated (has nonparallel crystal growth)- examples: quartzite, marble, hornfels


Metamorphic rocks form by recrystallization of pre-existing rocks as the result of temperature and pressure. The rocks recrystallize because they are exposed to a different environment from that in which they formed. The recrystallization requires a fluid, and components in the fluid or associated gas phase may change the bulk composiiton of the rock, a process called metasomatism. A rock that has been heated so that portions have melted and then recrystallized is a migmatite.

Directed pressure on the rock will cause a preferential growth of crystals during recrystallization called foliation, giving the rock a layered appearance (like a sedimentary rock). Foliation typicaly develops during regional metamorphism associated with converging plate boundaries. The maximum temperature of metamorphism is the melting temperature of the rock. Metamorphism of shale under directed pressure will produce a slate (perfect foliation layers of microcrystalline micas), which then converts to a phyllite (not-so-perfect foliation of layers of megacrystalline mica), which then converts to a schist (less than perfect foliation with non-segregated megascopic minerals within the layers), which then converts to a gneiss (segregated layers of minerals of different colors), which then converts to a migmatite (contains portions of crystalline rock from partial melting and crystallization of the magma).

Typically, around a cooling pluton the rocks are baked (contact metamorphism) causing rocks without foliation, e.g., marbles from limestone, quartzites from quartz sandstones, skarns from shales and limestones, hornfels from shales or basalts. Cooling plutions are common in subduction zones and associated with hot spots in continental areas. Directed pressure metamorphism (with temperature) is also characteristic of converging plate boundaries such as subduction zones. These broad areas of convergence produce regional metamorphism, causing foliated metamorphic rocks to form (slates, phyllites, schists, and gneisses).

For a rock of a particular bulk composition, the minerals that form can often be correlated with the temperature and pressure of metamorphism. This is known as the metamorphic grade and is used to tell the temperature and pressure that the rock was metamorphosed at. The mineral assemblage of a particular metamorphic grade is called the metamorphic facies.

Other types of metamorphism include burial metamorphism of sedimentary rocks in which they are lithified and recrystallized as they are gradually buried. This is a low-grade metamorphism that occurs with deltaic sediments and they are usually nonfoliated. For example, this produces the transformation of lignite to coal, of smectites to illite to micas, of opal to chert to quartz.


Hydrothermal

(rock precipitated from hot water)

Common examples are veins in country rock containing quartz or calcite.



Chapter 2 - Some Review Questions




Chapter 3 "The Sedimentary Archives" (p. 60-103)

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Tectonic Settings

Tectonic settings include the craton which is the stable interior of a continent, e.g., Kansas; active continental margins which include volcanos and forearc and backarc basins associated with a subduction zone, e.g., Cascade region of western Oregon and Washington; passive continental margins which are usually the site of sediment deposition of river deltas on broad continental shelves, e.g., the Louisiana coast; and deep oceanic basins which include the abyssal plains and mid-oceanic ridges of the oceans and features along the ocean margins. Within each tectonic setting are distinctive environments of deposition which produce rocks with diagnostic features that can be used to identify those environments in ancient rocks.

Environments of Deposition

Some of the environments of deposition are discussed below with the characteristics or "sedimentary structures" used to identify them in ancient rocks. These sedimentary structures include fossils, e.g. marine shells of organisms; crossbeds which are nonparallel sedimentary layers deposited from a current of water or from wind; graded bedding which is a decrease in grain size upwards within a layer of sediment deposited from a turbulent mixture of sediment and water as it slows down; salt deposits precipitated from evaporating saline water; varves consisting of alternating thin and wide layers of lake-deposited sediment that are dark colored and light colored, respectively; and symmetrical and asymmetrical ripple marks found in stream deposits, wind deposits, and on beaches; and mud cracks found along the edge of low energy environments, e.g., lagoons.

Most sediments are deposited in shallow marine environments with only a minor amount deposited in the deep sea. Terrestrial environments include stream channels, natural levees of rivers, backswamps, lake deposits, wind-deposited sand dunes in deserts and on barrier islands, wind-deposited dust, and glacier-deposited sediment. On land, river transport of sediments tends to round the sediment and sort sediment by size; whereas, glacier transport does neither. Erosion features can be useful in identifying an environment. Rivers cut a V-shaped valley; whereas, glaciers cut a U-shaped valley and produce hanging valleys where glaciers once intersected. Glaciers also produce striations in the underlying bedrock. Wind transport causes sand grains to be pitted. Wind cannot carry anything heavier than sand and cannot carry sand grains more than a few feet above the ground, limiting erosion to the lower parts of standing rocks and producing arches and other interesting ventifacts. The lack of typical sedimentary structures is also useful, e.g., a lack of bedding, rounding, and sorting is typical of a glacier deposit. The shape of a deposit is a clue to the depositional environment, e.g., a beach deposit is wide and thin; whereas, an alluvial fan deposit is cone shaped at the mouth of a canyon.

Transitional and Marine - Shallow to Deep beginning with the continental shelf and ending with the abyssal seafloor.

Continental Shelf

River deltas composed of deposits from the distributaries with shoe-string shaped sand and silt channel deposits having small crossbeds and asymmetrical ripple marks; the channel deposits are bounded by natural levee silt deposits; which are in turn bounded by backswamp deposits of clay with organic debris (forming peat and coal) from the continents;

Barrier islands with a linear geometry with marine shells and with sand dunes making sandstones that have large crossbeds;

Beach deposits with a sheet geometry with symmetrical ripple marks and marine shells;

Lagoon deposits (separating barrier islands and the shoreline) with clay deposits and mud cracks;

Marshes and swamps deposits with clay deposits and associated plant fossils;

Coral reefs deposits with coral fossils of calcium carbonate;

Calcium carbonate sediment on carbonate platforms from shells and fecal pellets of marine organisms and from inorganic precipitation from sea water of oolites (spherical layered particles.

Note that the shape of a river delta along a coast can be a birdfoot (similar to the Mississippi River) if the wave action is weak, allowing the distributaries from the main channel to extend out into the oceans, or it can be shaped like a upside down V due to strong waves smearing the sediment along the coast as it is deposited. The latter is the normal shape of a delta. The longshore current in the Gulf of Mexico goes from east to west, carrying sediment westward from the Mississippi River delta; however, the sediment doesn't reach shore to rebuild coastal Louisiana because the river has been forced by man-made levees to build the delta far from the coast in deep water. Without these levees, the river would have switched its course (down the Atchafalaya) and built a new delta in shallow water near Morgan City.

Continental Slope

Sediment moves downslope in turbidiy currents (high density underwater debris flows) carving out submarine canyons with some turbidity current deposits.

Continental Rise

This is a wedge of sediment at the bottom of the continental rise containing turbidity current deposits with their characteristic graded bedding. A group of turbidity current deposits form an abyssal fan at the mouth of a submarine canyon. Lithified turbidity current deposits are called turbidites.

Abyssal Seafloor - plains and hills

Slow forming deposits form on the deep sea floor: deposits of calcium carbonate plankton shells (carbonate oozes on less deep portions which eventually become limestones; siliceous plankton shells (siliceous oozes which eventually become cherts); pelagic clays (from wind-blown clays from the continents) which become pelagic shales, and manganese nodules which are concretions (resembling cannon balls) forming at the seawater-sediment interface. The calcareous oozees do not form at the deepest depths because deep seawater is undersaturated with respect to calcium carbonate, causing the dissolution of these plankton shells as they descend.


Continental

Fluvial (river) Floodplain has the same features as the delta, i.e., rounded coarse-grained channel deposits of sand and silt (with a shoe-string geometry) with small crossbeds and asymmetrical ripple marks; associated width less-coarse grain natural levee deposits; and grading dark-colored fine-grained backswamp deposits containing coal and peat.

Lacustrine (lakes) deposits typically have varved deposits in which the sediment deposited in the summer forms a thick lighter-colored layer and the sediment deposited in the winter forms a thin dark-colored layer. These varves are best developed in areas where the lake freezes over in winter, e.g., near a glacier. Lakes also have beach deposits (symmetrical ripple marks) similar to marine deposits but with fewer fossils of organisms.

Glaciers deposits typically have moraine deposits (called till) that are unsorted, unlayered, and unrounded. Lithified till is called tillite. Glaciers carve striations in rock that they move across. Glaciers carry and deposit rocks (exotics) that are different from the native rocks in an area, and melting icebergs drop large boulders (drop stones) on the deep sea floor that are unrelated to the normal fine-grained deep sea deposits.

Desert deposits contain several different depositional environments. Playa deposits are dried-up lake beds with mud cracks and salt deposits in the center of valleys lacking external drainage, i.e., through-flowing rivers. Eolian sand dunes deposits become sandstones with large crossbeds, and ventifacts are faceted boulders, carved by sandstorms. Loess deposits are wind-blown dust deposits that are characterized by straight vertical faces in roadcuts. Alluvial fan deposits are flash flood deposits at the mouth of canyons that have a typical cone-shape geometry. Desert soils have a layer of caliche (calcite) near the surface from evaporating groundwater. Desert varnish is a dark layer of manganese oxide forming on the surface of desert boulders. Note that loess deposits are formed in non-desert areas in times of drought, and that eolian sand dunes also form on beaches near the coast.

soil zones are usually identified from their red color from oxidation of iron and the presence of preserved root structures and hard pan layers which could be a caliche (calcium carbonate) layer in deserts and a clay pan or iron pan layer in temperate soils such as in Louisiana.

Names of lithographic and biostratigraphic rock units

Rock units are named on the basis of distinguishable physical differences and the geographic location of the best outcrop exposure. A sedimentary rock unit can be delineated by lithology, producing a lithostratigraphic unit. A lithostratigraphic unit is not usually age constant, i.e., the age varies laterally. The lithostratigraphic units can be formations and their subdivisions are called members. The first word of the name is the "type" geographic location (where the best exposure is) and the second word is the lithology, e.g., sandstone, if the rock unit is composed of only one type of lithology. Otherwise, the second word will be formation or member. Formations are combined into a larger unit called a group, and groups can be combined into a system. A system is usually defined to correspond to the units deposited or otherwise forming in a geologic "Period." For example rocks in the Permian System formed during the Permian Period. This may appear to be a contradiction in the definition of a lithostratigraphic unit not representing constant time. However, Geologic Periods were defined based on worldwide unconformites at their boundaries so a lithostratigraphic unit would not be expected to cross the Period boundary.

A sedimentary rock unit can also be delineated by fossil assemblage, producing a biostratigraphic unit. A biostratigraphic unit corresponds to a time interval of the existence of the organisms represented by the fossil assemblage. The usual biostratigraphic unit is called a zone. The fossil assemblage consists of index fossils which are fossils of species that were widely dispersed, existed for a short time period before becoming extinct, and are easily recognized.

If a lithostratigraphic unit grades laterally into a different unit, e.g., a sandstone into a siltstone, this is called a facies change. This is a normal occurrence in the shallow marine environment in which coarse-grained sediments are deposited close to shore and fine-grained deposits are deposited in deeper water with less wave action. Facies changes move laterally as sea level rises and falls because the position of the shore moves landward as sea level rises (transgressive sea) and seaward as sea level falls (regressive sea).

As sediment is deposited, it compacts and the crust sinks so that the water depth decreases at a slower rate than expected. Eventually, sea level changes will change sediment deposition at any location in shallow marine water because it causes a shift in the position of the facies change. Of course other factors can cause a shift in the facies position, e.g., decreases in sediment delivered to the sea will shift the facies change landward and vice versa. If sea level is rising, we would normally expect the facies change to shift landward and coarse-grained sediments to be overlain by fine-grained sediments and the opposite to occur if sea level falls. If sea level falls to the point that the surface is exposed, no sediment deposition would probably occur and an erosion surface (disconformity) and/or soil surface would develop. Geologist use the vertical change in sedimentation and the location of erosion surfaces within vertical cores to delineate world-wide (eustatic) changes in sea level in the geologic past. The causes of sea level rise can be due to melting of the ice caps on Greenland and on Antarctica (which is happening today due to global warming) or it can be due to an increase in spreading at the mid-oceanic ridges. The latter is due to a decrease in volume in the ocean basins due to the seafloor rising and expanding as hot basaltic magma fills the oceanic ridge system. The normal changes in sea level are about 35 ft/million years; whereas, changes in sea level due to ice ages are about 330 ft/ several thousand years.


Geologic Maps

Geologic maps are used in this course to help understand an area's geology. The most common map shows the location of surface outcrops of formations together with information on the dip of the formations below the surface and the location of fault planes where they intersect the surface. Typically, such a map will also have a stratigraphic section for the area showing a vertical sequence of described units from top to bottom with information on age, depth, thickness, lithology, fossil content, composition, etc. Geologic maps of a single formation are commonly used to contour the depth below the surface to its upper or lower boundary or to contour its thickness. Geologic maps can be used to show the location of facies changes between formations that are laterally continuous across the area.

Chapter 3 - Some Review Questions

How would you recognize the following erosional or depositional environments in ancient rocks? This could be a sedimentary structure such as large or small crossbeds, a lack of bedding, presence of coal beds, the geometry of the deposit, type of fossil present, etc.


Know the answers to the questions below.


Chapter 4 "The Fossil Record" (p. 104-147)

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Fossils

Fossils are preserved remains or tangible signs of ancient life which can be found in sedimentary rocks, most commonly from marine environments, e.g., coral reefs and shales, but also from some terrestrrial environments, e.g., swamps. Hard parts such as bone, teeth, and shells are frequently preserved, and evidence of hard parts also includes permineralization (e.g., petrified wood), molds and casts. Soft parts can be preserved in amber and in anoxic (without oxygen) environmenst as petroleum, coal, and peat. Evidence of soft parts can also form from impressions and carbonization (e.g., outlines of fish in shales). Trace fossils represent indirect evidence of organisms such as tracks, trails, burrows. The index fossils used by paleontologists to define biostratographic units (zones) and for general correlation of widely separated rocks, and for dating rocks are the fossils of organisms that were widespread (generally marine), existed as a species only a short time, and are easily identified in rocks.


Definition of Life

Life has the capacity for self-replication (reproduction) and self-regulation (use raw materials to sustain chemical reactions). A virus does not fulfil this definition because it cannot self-replicate without entering a cell or use raw materials to sustain chemical reactions.


Taxonomy

Taxonomy is the classification of living organisms. Organisms are divided into kingdoms. Bacteria are single-celled organisms lacking a nucleus or chromosomes and other internal cell structures such as mitochrondia (for respiration) and chloroplasts (for photosynthesis). Their cells are called prokaryote cells and they are often lumped into one kingdom Monera or split into Archaeobacteria which include primitive bacteria and Eubacteria which include cyanobacteria (blue green algae). Nonbacteria cells are called eukaryotes and contain internal cell structures. Protoctista (also called Protista) contains algae which are simple single and multi-celled plant-like organisms, e.g., seaweed, and simple single-celled animal-like organisms called protozoans, e.g., amoebas. Phytoplankton (diatoms and nannobacteria) and zooplankton (foraminifera and radiolarian) are in this kingdom. The more complex kingdoms include the animal-like Fungi, the animals in Animalia, and the plants in Plantae.

Phylogeny is the evolutionary history of a species. Species in Fungi, Animalia, and Plantae represent an evolutionary process that begin with Monera, passed through Protista before evolving into more complex species.

The kingdoms are subdivided into phylum, class, order, family, genus, species

An example of taxonomy is the classification of humans: kingdom, Animalia; phylum, Chordata; class, Mammalia; order, Primates; family, Hominidae; genus, Homo; species, Homo sapiens


Clades

A cluster of species that share an evolutionary ancestry is called a CLADE. Within a group of related species, some traits are primitive and some are derived and hence are not shared by all species. The common traits imply a common evolutionary origin. A cladogram shows the progression of clades sharing fewer common traits as evolution proceeds.


Review of Important Organisms In Historical Geology

The groups are listed below and you should learn how they are related by evolution.


Monera

Bacteria cell fossils first appear in rocks of Archean Eon age.

Archaeobacteria - includes anaerobic bacteria which formerly lived on the earth's surface when there was little or no oxygen in the atmosphere.

Eubacteria - plant-like cyanobacteria (blue green algae) and animal-like bacteria responsible for decay of dead organisms. Cyanobacteria are photosynthetic and began the buildup of the earth's oxygen content in the atmosphere. They produce stromatolites which are boulder-shaped rocks composed of layers of algae mats covered with sediment.


Protists or Protoctista

Protists first evolved in the Proterozoic Eon. The earliest non-bacterial fossils are of acritarchs, thought to be single-celled phytoplankton, possibly a type of dinoflagellate. However, paleontologists think that plant-like eukaryotes (such as phytoplankton) evolved from animal-like eukaryotes because animal-like eukaryotes only have mitochondria and plant-like eukaryotes have both mitochondria and chloroplasts.

Protozoans - single-celled animal-like. Protozoans are thought to have evolved from an animal-like bacteria engulfing another (smaller) animal-like bacteria which survived to become a mitochondria in the cell. This explaijns the presence of DNA in mitochondria that is unrelated to the DNA in the chromosomes of a cell. Need to know important zooplankton: radiolaria ( have silicious (opaline) shells which produce siliceous oozes) and foraminifera (have calcium carbonate shells which produce calcareous oozes on the seafloor).

Algae - Thought to have evolved from a protozoan engulfing a cyanoacteria which survived to become a chloroplast within the cell. They are single-celled and simple multi-cellular plant-like organisms that have fertilization external to the plants in water. Fertilization in higher plants occurs within the parent plant. Learn important phytoplankton: diatoms (have silica shells) and nannoplankton (have calcium carbonate shells) and dinoflagellates (have chitin shells).


Plantae


Multicelled plants containing organs of tissue. They evolved from algae in the Late Proterozoic and are listed below in order of evolution:

Spore-bearing plants> require moisture to reproduce. These include moses (primitive nonvascular plants) and spore-bearing ferns (vascular plants). They evolved in the Ordovician Period and were the first land plants, living in moist environments.

Vascular plants evolved in the Devonian Period. Vascular means that fluids can move up and down the plant stem.

Gymnosperms - naked seed-bearing plants, e.g., conifers such as pines and cycads, which evolved from spore-bearing plants in the Late Devonian Period and colonized dry land.

Angiosperms - flowering plants, e.g., oaks, which evolved from gymnosperms in the Cretaceous Period. These are the dominant plants today with a shorter reproductive cycle than gymnosperms.


Animalia

Multicelled animals are composed of organs of tissue and evolved from Protozoans in the Late Proterozoic. Skeletons first appeared at the beginning of the Cambrian Period (beginning of the Phanaerozoic Eon and Paleozoic Era which accounts for the abundant fossil record since this time.

Invertebrates

Unless otherwise noted, the major invertebrate groups first appeared in primitive forms in the Cambrian Period.


Phylum Coelenterata (Cnidarians)

They are suspension feeders with radial symmetry. Examples include floating jellyfish and sedentary corals (tabulates, horn corals, hexacorals) which form calcium carbonate reefs in warm, shallow waters.


Phylum Porifera

Sponges are suspension feeders which often contribute calcium carbonate skeletons to coral reefs, e.g., stromatoporoids.


Phylum Annelida (segmented worms)

These evolved in the Proterozoic Eon and are burrowing animals (worms) with a well-developed digestive tract


Phylum Arthropoda

Arthropods evolved in Proterozoic Eon and are segmented, joint-legged animals with a hard exoskelton. (Although the exoskelton was lacking in the Proterozoic Eon.) Arthropods include the trilobites, insects, crustaceans, spiders and scorpions.


Phylum Brachiopoda

These are bivalves that lack mirror symmetry and include articulate and inarticulate brachiopods.


Phylum Bryozoans (moss animals)

Evolved in the Ordovician Period: moss-looking corals which form colonies which help form reefs of calcium carbonate, e.g.,lacy bryozoan.


Phylum Mollusca

This group includes bivalve molluscs (clams), gastropods (snails), cephalopods (nautiloids, ammonoids, belemnoids, squids, octupus)


Phylum Echinodermata

They have radial symmetry. Examples include starfish, crinoids, (sea lilies), sand dollars, and sea urchins.


Vertebrates

Phylum Chordata


Have a vertebrate column with a spinal cord. Cambrian conodonts were the earliest vertebrates. The following classes appeared in order of evolution - jawless fish, jawed fish, sharks and bony fish, amphibians (from lobe-finned fish), reptiles (including therapsids and thecodonts), dinosaurs (from thecodonts), pterosaurs (from thecodonts), mammals (from therapsids) and birds (from lizard-hipped dinosaurs). Jawless fish evolved in the Cambrian Period, and sharks and bony fish evolved from jawed fish in the Devonian Period. Amphibians evolved from bony fish in the late Devonian Period, and reptiles evolved from amphibians in the Pennsylvanian Period. Therapsids and thecodonts were reptile groups that evolved in the Permian and Triassic Periods, respectively. Dinosaurs and pterosaurs evolved from thecodonts in the Triassic Period, and birds evolved from dinosaurs in the Jurassic Period. Mammals evolved from the therapsids in the Triassic Period.

Evolution

Evolution means to change or "Descent with Modification" for a species and was first proposed by Charles Darwin

Organic evolution refers to changes in populations of the same species. A basic restraint on evolution is the body structure of what is already present in a population, i.e., because evolution remodels rather than starts with a new design.

Natural Selection and Speciation

During evolution, a species as a whole can evolve in the course of time or can rapidly give rise to additional species. Natural selection is used to describe the gradual transformation of a species through transferral of selected traits through breeding from survival of the fittest. Speciation is used to describe the rapid development of new species from another species. Often, the new species is geographically isolated from the remainder of the parent species, thereby lacking the full genetic variability in the general population. Also, often no competitors exist in the environment.

Adaptations refer to specialized features of plants and animals that allow them to perform functions, e.g., the development of seeds in gymnosperms and the development of eggs in reptiles enabled both groups to colonize the dry interiors of continents.

Changes in Gene Pairs Control Traits

The process of evolution involves changes in the gene pairs which are responsible for hereditary factors. These changes can involve genetic mutations of sex cells when dividing or or exposed to chemicals and/or radiation which are subsequently preserved through breeding or by new gene combinations through breeding. Each gene is a chemical segment in a DNA (deoxyribonucleic acid) strand within a chromosome in the nucleus of a cell. The chromosomes exist in pairs, half supplied by each parent during breeding.

DNA has the structure of a twisted rope ladder (double helix) in which the sides of the ladder are composed of alternating sugars (S) and phosphates (P). The steps are composed of pairs of nucleotide bases: either adenine (A) and thymine (T) or guanine (G) and cytosine (C). Genetic point mutations of DNA can occur during cell division or by an external source such as radiation or chemical exposure. It these mutations occur in sex cells thay can be passed on to offspring through breeding in which each individual of a pair contributes half the chromosomes by way of a gamete (e.g., egg and sperm). If the mutation is favorable, the individual is more likely to survive and continue to pass on his or her genes. In this way species evolve or change with time. Genetic point mutations probably result in speciation, a quicker process than natural selection.

Species may also evolve, in the absence of genetic mutation, through a new gene combination in breeding. This is a long-term process which is called natural selection. If the combination is favorable, the individual is more likely to survive and continue to pass on his or her genes, making it more likely for the gene combination to reoccur in related offspring. However, to produce a completely new species, genetic mutation may be required.

Terms

homology - The development of different functions for organs in different species which shared this organ in a common ancestor, e.g., bat wings, human hands, dog paws, and whale flippers all have the same basic five-finger structure. This was part of the basic evidence for evolution cited by Darwin.

vestigial organs - organs developed in ancestral species and retained through evolution but not serving a purpose, e.g., our ear muscles, the pelvic bones of whales and some snakes. Again, this is part of Darwin's evidence for evolution.

natural selection - survival of the fittest.

artificial selection - domestic breeding

mass (rapid) extinction - large numbers of species become extinct every 26 m.y. to 30 m.y. and this appeasr to be associated with asteroid impacts on the earth. The asteroid events deposits trace metals (e.g., iridium) in sediments which have been identified in the geologic record. These trace metals were within the asteroid which was vaporized upon impact. The vaporization process ejected trace metals into the atmosphere to spread arourd the earth and then slowly settle out onto the earth's surface. An explanation for the cycle is that the sun has a companion star, a red dwarf, not yet identified which is circling the sum every 26 to 30 m.y. When this "death" star or "nemisis" star passes near the region where comets are located that pass through our solar system, the comet orbits are disrupted, increasing the probability of collision between the Earth and a bolide or large meteorite associated with the comets. The variation from 26 m.y. to 30 m.y. could be due to the passage of other stars that disrupts the orbit of the companion star.

pseudoextinction - evolution into a new species, suggesting that the ancestors have become extinct.

adaptive radiation - rapid expansion of a group into many new species as the result of a friendly environment, e.g., other competing groups have become extinct or introduction into a new environment lacking competitors, e.g., the adaptive radiation of mammals in the Cenozoic following the extinction of the dinosaurs at the end of the Mesozoic. This is part of speciation.

adaptive breakthroughs - development of key features that along with ecologic opportunities allow adaptive radiation to take place. In corals, the development of a porous skeleton allowed more rapid growth and the development of the symbiotic relationship with dinoflagellates allowed for removal of their produced carbon dioxide as well as a convenient food supply.

extinction rates vary between different groups. Mammals have a survival rate of 1 to 2 m.y. The average extinction rate for all groups is about 3 m.y. Human species have been here for at least 200,000 years.

evolutionary convergence - adaptive radiation evolution of two different taxonomic groups, e.g., marsupials and placentals to produce similar-looking species to occupy similar environments, e.g., Tasmanian wolf and the wolf.

iterative convergence - evolution of a taxonomic group producing similar-looking species at different time periods; however, they are not identical because evolution does not repeat itself.

phylogeny and ontogeny

The history of evolution of species is called phylogeny. The sequence of development of an individual from its origin by fertilization of an egg to its death is called ontogeny. Frequently, phylogeny is preserved in ontogeny in the different embryonic stages, leading to the saying "Ontogeny recapitulates phylogeny." The preservation shows up in common embryonic stages between species having a common ancestor group. This evidence for evolution was cited by Darwin.

Evolutionary Trends

Cope's rule - Organisms increase in size through evolution (now somewhat discredited)

Organisms become more complex.

One common evolutionary trend is to eliminate the adult form or the latest stages of development or by retaining some of the juvenile or embryonic features. This involves the transfer of sexual maturity to an earlier juvenile stage and the arrestment of development at that stage. Amphibians that have an aquatic juvenile stage may remain in that stage, never progressing to four-legged terrestrial adults. The axolotl evolved from the juvenile stage of a salamander, through the loss of thyroxine from the thyroid gland, which is needed for transformation to an adult salamander. The domestic dog has lost the adult form found in wolves. Dogs like wolf pups bark, but adult wolves do not. Humans have lost the adult ape form through evolution. We lack the body hair and pointed faces of adult apes, retaining the juvenile, lesser hair, and flat face of young apes.

Significant evolutionary processes

Most evolution probably involves speciation, evolving rapidly from existing species, rather than by natural selection, the slow transformation of an existing species through breeding.

The evolutionary trend may be controlled by species selection in which the significance of a species in evolution is related to its length of duration and its rate of producing descendant species. If a species has a long length of duration and produces a lot of descendant species than more of its traits are likely to be passed on.

Dollo's Law - Evolution doesn't reverse itself to perfectly duplicate an earlier extinct species.


Mass extinctions of species have occurred throughout the earth's history. these extinctions have been postulated as due to temperature changes on the earth's surface, possibly resulting from meteorite hits that have periodically occurred. Mass extinctions have occurred throughout the earth's history but there does appear to be a 26 million year cycle that is marked by mass extinctions. This has given rise to the Death Star hypothesis, a dark sun rotating our sun every 26 m.y. and bringing comets and meteorites with it to bombard the solar system.


Chapter 5, Earth Structure and Plate Tectonics (p. 148-195)

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Plate tectonics is a theory that is consistent with the observations about the interior of the earth and its crust. Seismic wave modelling indicates the earth has a molten outer core, so heat is flowing to the surface. Seismic waves also indicate the asthenosphere, underlying the uppermost mantle is plastic and hence can move in convection cells set up by this flow of heat towards the surface. The rigid plates of crust and uppermost mantle in plate tectonic theory move on top of these convection cells in which the upward arms occur at diverging boundaries and the downward arms occur at subduction zones. The processes at the diverging boundaries and hot spots account for the formation of oceanic crust with shield volcanoes and the processes at the subduction zones in converging boundaries account for the destruction of oceanic crust and the formation of continental crust with composite volcanoes and the inclusion of remnants of oceanic crust within continental crust. The complex folding of layered sedimentary rock into mountains occurs at converging boundaries where continental crust is sutured to continental crust following the destruction of intervening oceanic crust on the leading edge of plates in subduction zones. This process has produced the super continents such as Pangaea and the highest mountain ranges on the earth's surface. Breakup of the continents by rifting explains the formation of oceans and the occurrence of salt deposits and red beds along passive continental margins. The young age of the ocean basins together with the trend of increasing age from the diverging boundary at the center of an ocean to subduction zones at their margins and the old to young trend from the center of the continents to their margins are consistent with the processes associated at diverging and converging boundaries, respectively. The large faults that occur between plates and along fracture offsets of spreading ridges with strike-slip motion are consistent with the horizontal movement as one plate slides past another. The theory has been able to link all aspects ot the structure and composition of the interior and surface of the earth.

History of Plate Tectonics

The Frenchman Antonio-Snider-Pellegrini published a map in 1858 suggesting that a supercontinent had broken apart to form North America, South America, Europe, and Africa, using the puzzle-like fit of the continents. Even earlier, the Englishmen Bacon had proposed this possibility.

Fossil evidence pointed for a connection between the continents. The name Gondwanaland, proposed by the Austrian geologist Edward Suess in the late 19th century, originally referred to a hypothetical southern hemisphere super continent containing land bridges connecting India, Antarctica, South Africa, South America, and Australia. All of these areas contained flora of fossil plants called the Glossopteris flora which are best exposed in Gondwana, India. The most conspicuous Glossopteris genus is the Glossopteriss seed fern. Early geologists speculated that land bridges of felsic rock had connected the continents and then subsequently sank. However, lower-density felsic rock cannot sink in the higher-density mafic rock.

Alfred Wegener, a German meteorologist, published "On the Origin of Continents and Oceans" in 1915, a book proposing continental drift in which a supercontinent called Pangea split apart in the late Mesozoic. Wegener used different lines of evidence besides the puzzle-like fit of the continents. He pointed out the African rift valleys were early stages of a continental rifting process. He used geologic structures and fossil evidence to support the occurrence of Pangea as well as paleoclimate data which different from present-day climates.

The South African geologist Alexander du Toit became the main supporter of Wegener and showed that striations from Permian glaciers indicated the impossible movement from oceans onto present landmasses. Du Toit supported continental drift with the present-day common occurrences of earthworm genera, the common presence of the Early Permian fresh-water fossils of Mesosaurus genera, and the Early Triassic mammal-like herbivore reptiles Lystrosaurus genera, and the common Gondwana sequence which includes Permian tillites, Triassic dune deposits, and Jurassic lava flows. Du Toit also pointed out that a reconstruction of Gondwanaland allowed the mountain ranges along their margins to form a continuous chain between the continents. Du Toit proposed that Pangea did not form until the late Paleozoic. The northern supercontinent Laurasia and the souther supercontinent Gondwanaland existed prior to the formation of Pangea. Unfortunately du Toit and Wegener could not come up with an adequate force to allow the continents to push through the oceanic crust.

Wegener confused the issue by pushing for an early Cenozoic split of Pangea, rather than the early Mesozoic split. This dating error confused palaeontologists who realized that not all of the evolution occurring on separate continents could have occurred within the Cenozoic time frame

In the 1950s', apparent magnetic polar wandering was used to support the movement of plates. Remnant magnetism can give the apparent location of the magnetic poles at the time the rock formed. The remnant magnetism from dated (by radioactive means) igneous rocks of the same geologic age on different continents usually points to different locations of the poles as a function of geologic time, indicating separate movements of the plates rather than true polar wandering.

In 1962, the American Harry Hess published the "History of Oceanic Basins," proposing the creation of sea floor at the midoceanic ridges and its destruction at trenches or subduction zones. He was able to explain the young age of the sea floor. He also could explain the movement of the continents as being carried along with the sea floor, rather than being pushed through the oceanic crust. He supported his evidence with the small numbers of volcanic seamounts, the presence of guyots (submarine volcanoes with wave eroded flat tops that occurred as the plate carrying the inactive volcano moved it below sea level. He proposed mantle convection as the force moving the sea floor and continents. In 1963, Vine and Matthew explained the magnetic sea-floor anomalies symmetrical to the midoceanic ridges using Hess's hypothesis.

Evidence for Plate Tectonics from Magnetic Stratigraphy

The earth's magnetic field shows two changes with geologic time. First, the declination (difference between the magnetic poles and the geographic poles) shifts slowly (on the order of a few thousand years to make a circuit) as the magnetic poles rotate around the geographic poles. Second, on the order of every half million years, the polarity of the magnetic poles appears to rapidly switch so that the north and south magnetic poles reverse. The reason for this switch is the subject of debate.

The polarity reversal of the magnetic poles can be determined from studying the residual magnetism of rocks, primarily igneous rocks containing iron. The igneous rocks have the earth's magnetic field orientation frozen in as residual magnetism when they cool below about 500oC (their Curie points). This residual magnetism can be determine from an igneous rock and the rock's age can be determined by radioactive (radiometric) dating. By doing this for rocks from stacked lava flows, the polarity of the earth's magnetic field as a function of geologic time has been determined.

The polarity of the residual magnetism is said to be "normal" if the orientation is in the same general direction as today earth's magnetic field and "reversed" if the orientation is in the opposite direction. Note that the movements of plates (through plate tectonics) around the earth's surface may have changed the apparent orientation of the earth's magnetic field from the time the rock was formed. Plate tectonics makes determining the polarity more difficult in rocks older than Mesozoic. We generally know the movement of the continents in the Mesozoic and Cenozoic, so the change in orientation from plate movement can be subtracted out.

The magnetic reversals are used to date the sea floor. The oldest sea floor is Mesozoic in age. If an area of the sea floor has a residual magnetism parallel to the present earth's magnetic field, a positive (stronger) magnetic field is measured for the earth directly over that sea floor and vice versa. A stronger magnetic field is called a positive anomaly; whereas, a weaker magnetic field is called a negative anomaly.

Symmetrical Magnetic Anomalies to Mid-Oceanic Ridges

The symmetrical magnetic anomaly pattern of positive and negative anomalies, relative to the mid-oceanic ridges, can be related to the orientation of the earth's magnetic field at the time the sea floor was created. The basaltic sea floor originally formed at the ridge, picking up a residual magnetism corresponding to the orientation of the earth's magnetic field. The sea floor then moved away from the ridge as new sea floor was created. This movement away from the ridge has created the present symmetrical pattern of magnetic anomalies. Since the ages of changes in magnetic polarity are known from radiometric dating on land, the magnetic anomaly pattern can be used to date the sea floor.

Polar Wandering

We can determine, from the residual magnetism of igneous rocks, the location of the two poles of the earth's magnetic field at the time of formation of the rock. The movements of the plates have produced an apparent movement in the positions of the magnetic poles. We know the poles haven't actually moved because the apparent positions of the magnetic poles are different for rocks of the same age that are located on different plates. The only explanation is that the plates are moving independent of each other.

Sea floor spreading explains the rift valleys (grabens and normal faulting) associated with the midoceanic ridges, the presence of strike-slip faults (transform faults connecting other plate boundaries), and the occurrence of pillow basalts in oceanic regions. The spreading is of the order of 5 cm/year for the Pacific sea floor and less for the Atlantic sea floor. Presently there are 8 large plates and several smaller plates.

The occurrence of shallow and deep earthquakes in the Benioff zones was explained by subduction in plate tectonics. The associated andesite volcanism in island arcs and on continental margins was explained as due to partial melting of the sea floor in the subduction zones. The arcuate shape of the volcanos is related to the arcuate shape of the trenches which is due to the spherical shape of the earth, e.g., try pressing down on a ping pong ball and see the arcuate outline of the depression). The great age of the continents and the general age trend to increasing age in the center of the continents was due to the inability to subduct the lighter continental crust and the gradual accretion of additional material to the continents along their margins. Hot spots or mantle plumes or thermal plumes are used to explain nearly immobile points in the mantle which serve as the source of magma for volcanoes erupting on the surface of the earth and then carried away from the magma source These hot spots create aseismic ridges or chains of volcanoes in which the only active volcano is over the hot spot, e.g., the big island of Hawaii in the Hawaiian Island chain.

Rifting appears to begin with a mantle plume, producing a three armed rifting center, a triple junction. The three arms may consist of a spreading zone, a subduction zone or a transform fault. In general, one of the arms become inactive and the other two arms connect up with active arms from other mantle plumes. The result is a linear zone of rifting which can begin under a continent or under an ocean. The failed rift arms are frequently the sites for river valleys, e.g., the Mississippi River and the Amazon River.

The rifting of a continent probably involves a nearly stationary continent, rather than an actively moving one over the asthenosphere. Doming occurs along with rifting as a continent splits. East Africa is nearly stationary and appears to be about to begin splitting along the East African Rift connecting to the Red Sea rift and the Gulf of Aden rift (triple junction). In this case all three arms of the triple junction are remaining active.

Continental rifting in the early stages results in basaltic magma with normal faulting (plateau or flood basalts and shield volcanoes and a rift valley). The early seaway is shallow, producing nonmarine clastics (red beds), followed by evaporites as the seaway enters the rifting region, followed by the development of passive continental margins as the continent separates. The passive continental margins accumulate large deposits of marine sediments.

Evidence of ancient subduction zones occurs in the formation of melanges in the accretionary wedge which separates the trench from the forearc basin lying in front of the active volcanoes. On continents, behind the zone of volcanoes, deep-water marine flysch is deposited in the foredeep basin, followed by shallow, non-marine mollase deposits. Ophiolites, representing ancient sea floor, are frequently exposed in regions where two continents have sutured together following the destruction of an intervening ancient ocean (in a subduction zone). In the process of suturing, the crust is thickened by thrusting and uplifted.

The coexistence of adjacnet terranes, formed in different regions by different processes, can be due to movement into present position along transform faults, e.g., Alaska and British Columbia. Terraines may also coexist due to suturing of island arcs to continents at converging boundaries, e.g., Sonoma County in California.

Where Mountains Form at Converging Boundaries

The process of mountain building is called orogenesis and a particular episode of mountain building is called orogeny. Mountain building occurs near subduction zones and where two continents are sutured together, following the destruction of an intervening ocean by subduction. The suturing is the result of the continental crust being too light to be subducted when continental crust converges against continental crust. Mountain building results in a thickening of the continental crust. Because of crustal underthrusting, the crust is doubled in thickness when two continents are sutured together. Pieces of ancient oceanic crust are often preserved as ophiolites, emplaced in the acretionary wedge when the subduction zone was active. Mountains associated with suturing will consist of folded sediments that have been pushed upward along the suture line and have then slid by gravity away from the suture line, e.g., the Alps and Himalayas. These sheets of folded sediment are called nappes. Erosion of these mountains leads to block faulting (isostatic adjustment) as the crust rises due to the loss of overlying weight, exposing granite and metamorphic rock which is eroded into mountains, e.g., the Appalachians.

Features Associated with a Subduction Zone

Along a continental or island arc margin behind a subduction zone, there will be an igneous arc of composite volcanoes associated with partial melting of the oceanic crust, e.g., the Andes. The crust is thickened as the result of emplacement of plutons in the igneous arc and the extrusion of volcanics. On either side of the igneous arc will be a zone of metamorphism due to heat and compression. The mountain building can be due to compression from the subducting plate combined with gravity spreading once the crust has been thickened to the point that it is unstable vertically. Eventual erosion of the volcanoes will lead to block faulting (isostatic adjustment) as the crust rises due to the loss of overlying weight, exposing granite and metamorphic rock which is then eroded into mountains, e.g., the Sierra Nevada and the Rockies.

Towards the continental interior will be a fold and thrust belt bordering the foredeep or foreland or backarc basin. The foredeep or foreland basin will first fill with deep-water marine flysch (black shale and turbidite) deposits and then with fresh-water (alluvial fans, flood plains, etc,) molasse deposits. Thick molasse deposits are sometimes referred to as a clastic wedge. The thrusting in the fold and thrust belt is along an inclined basal thrust surface called a decollement. The decollement slopes upward towards the interior of the continent. Much of the movement along the decollement may be due to gravity spreading in the igneous arc and metamorphic zone.

Towards the subduction zone, is the forearc basin, filled with debris from erosion of the igneous arc and metamorphic zone. The accretionary wedge separates the forearc basin from the subduction zone. The accretionary wedge contains a melange of material derived from the subduction zone. Ophiolites are preserved in the accretionary wedge.



Chapter 6, The Primordial Earth: Hadean and Archean Eons (p. 196-239)

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Archean Eon - from the beginning of the earth, about 4.6 b.y. ago to 2.5 b.y. ago. The end of the Archean is thought to represent the time of the beginning of modern plate tectonics on the earth's surface.

Archean time is part of the Precambrian time. Precambrian time refers to everthing before the Cambrian Period which started about 540 m.y. ago. No rocks are present in the earliest age of the earth - about 4.6 to 4 b.y. ago and sometimes this is called tha Hadean Eon - however, in this course the Hadean is included in the Archean Eon (4.6 b.y. to 2.5 b.y.). The Archean rocks are exposed in parts of the Precambrian shield areas in the centers of each continent. The Precambrian shield consists of exposed Precambrian rocks within the craton of each continent. The craton includes both the Precambrian shield and those areas covering the Precambrian rocks that have been undeformed since the beginning of the Paleozoic era.

Archean Rocks and Fossils

Most of the Archean rocks are crystalline rocks, either igneous or metamorphic. They are generally divided into two groups:

even-grained, high-grade metamorphic rocks known as granulites (consisting of the metamorphic rock granulite and the igneous rock granite) and thought to be the core of volcanic rocks and

volcanic and sedimentary rock (derived from forearc and backarc basins associated with subduction zones) known as greenstone belts.

Because of their great age, most sedimentary Archean rocks will have been metamorphosed or otherwise destroyed.

Fossils are generally limited to stromatolites, made by cyanobacteria, also called blue green algae. These photosynthesis bacteria began producing the oxygen for the atmosphere.

Ages of the Universe and the Solar System

Our solar system is about 4.6 b.y. old as shown by the ages of most meteorites which date back to that time.

stony or chrondite (rocky) equivalent to the earth's mantle.

iron (metallic) equivalent to the earth's core.

stony-iron (mixture) equivalent to a mixture of the earth's mantle and core.

carbonaceous chondrites (carbon-rich stony meteorites in which the carbon molecules contain amino acids that are different isomers (different structure) from similar amino acids produced by Earth's organisms. They also have different 13C/12C and 18O/16O ratios.

Another indication of the age of the solar system is that lunar rocks have been dated as old as 4.6 b.y. Ages of lunar rocks indicate heacy meteorite bombardment in the solar system occurred during the first 600 m.y., i.e., 4.6 to 3.9 b.y. ago.

The oldest rocks dated on earth are about 3.96 b.y in age in Canada. These are younger than lunar rocks and meteorites because the earth's crust is continually being destroyed by erosion and weathering, and the radiometric clocks are reset by metamorphism and igneous activity. Note that zircon grains in Archean sedimentary rocks, derived by erosion of older rocks, have been dated as old as 4.4 b.y. in Western Australia.

The universe must be older than 4.6 b.y. as shown by the Doppler effect of light waves. This is an increase in wavelength of waves reaching an observer if the object sending the waves is moving away from the observer. The effect is proportional to the speed of the object and can be used to calculate a time of 15 to 20 b.y. ago when the matter in the universe was at the same location. A subsequent explosion, the big bang, may have begun the expansion of the universe to the present day.

The Planets

All the planets rotate around the sun in the same direction and all, with the exception of Pluto, revolve around the sun in nearly the same plane and all, except for Pluto and Venus, rotate around their axis in the same direction. Theses observations suggest the planets formed at the same time from the same rotating dust cloud. Pluto may not be a true planet but an asteroid and Venus's change in rotation around its axis may be due to an early collision when Venus was first forming.

      Composition	  Density    Temp. Range   Atm.        Magnetic
                                    C                          Field

Mercury  rocky               5.7       -175=>425    trace         weak 
(has cratered surface)


Venus    rocky               5.2         =>475    CO2, H2SO4
(has cratered surface)                               high 
                                                   pressure

Earth    rocky               5.5                     N2,O2       strong


Mars     rocky               3.9                      CO2         none
ice caps of H2O  CO2                                  low 
(has cratered surface)                              pressure

asteroid belt


Jupiter                      1.35                H2, He, CH4, NH3


Saturn                       0.70                    H2, He       present
(has iron core)

Uranus                       1.3        -185       H2, CH4, NH3


Neptune                      1.6                     H2, He


Pluto                        2.1  

Origin of the Solar System

solar nebular theory - One possible origin begins with a collapsing sun which heats up to form heavy elements in the process by fusion, followed by the sun exploding as a supernova. The explosion produces a dense cloud of cosmic dust, a nebular. Because the parent star was rotating as it exploded, the resulting cloud of dust rotated. In the explosion, the lighter elements were thrown further out from the center of the cloud.

Other theories simply begin with the preseence of cosmic dust cloud which collapses due to its proximity to a supernova event.

The cloud condensed as particles stuck together and begins to rotate more rapidly as it contracts in order to preserve angular momentum. Less dense materials, e.g., gaseous elements, were expelled towards the edge of the rotating cloud and beyond into space. With increasing rotation speed, the cloud flattened into a disc which segregated into rings. The material in each ring coalesced into a protoplanet with the young sun forming in the center. The coalescence process involves gravity as the protoplanets and sun grows in size. The accumulation leads to an increase in temperature due to heat released by friction as particles are incorporated into the planets and sun, eventually leading to molten planets and a temperature high enough in the sun to support nuclear fission and fusion.

An alternative theory has the sun and planets forming from separate dust clouds in which the planetary dust cloud was captured by the sun's gravitational pull.

The origin of the earth's moon is thought to be related to the earth as the result of a planetary collision of the earth with another object. Moon rocks show a close relationship with earth rocks, having similar 18O/16O ratios, which are not generally similar with or between meteorites. Ancient moon rocks have a weak residual magnetic field, indicating a molten core was present to explain the presence of a magnetic field. Presently, the moon lacks a magnetic field so the core is no longer molten.

Excesses (relative to meteorites from outside the solar system) in amounts of Xenon 129 and Plutonium 244 in some meteorites indicate the formation of these meteorites occurred within a 100 m.y. of the formation of the sun. Excess xenon 129 formed from iodine 129 which was created during the formation of heavy elements by the collapsing sun and preferentially incorporated into meteorites. The half life of iodine 129 is only 17 m.y., so the meteorites formed prior to the decay of all the iodine 129. Thus, the entire solar system may have formed in this time interval of about 100 m.y.

Origin of the Earth's Structure, Atmosphere, and Oceans and of the Moon's Surface

Concentric Layering of the Earth and the Development of the Oceans, Atmosphere, and Crust

Homogeneous accretion versus heterogeneous accretion

Homogeneous - condensed material of both heavy and light elements which were segregated gravitationally once the heat released by condensing, melted the material. The dense elements sank to the form the core, and less-dense elements floated nearer the surface to form the mantle. Radioactive elements followed the light elements because they formed compounds with these elements.

Heterogeneous - a dense core condensed first followed by condensations of less dense silicates - alternative theory to homogeneous condensation.

atmosphere - degassing of the earth, primarily from molten material during early formation of earth. Degassing continues with volcano emission. Present atmosphere is 78% N2 and 21% O2 with minor amounts of H2O and CO2. The O2 is the result of photosynthesis of plants and the early atmosphere was deficient in O2.

oceans - degassing of H2O from molten material during early stage of earth formation. Upon cooling of the planet's surface, the water condensed to form oceans. Dissolved salts are now in steady state with inputs balancing outputs in the oceans which acocunts for their constant composition.

oceanic crust - first crust to form. Basaltic magma formed from partial melting of ultramafic rock (peridotite) making up the mantle. Initially, there probably was no crust on the surface and no convection in the mantle. Partial melting of mantle rock below the surface formed less mafic magmas which rose by buoyancy to the surface to crystalize as basalt overlying gabbro (oceanic crust). Later, when mantle convection began, the oceanic crust formed from magma moving upward in spreading ridges.

continental crust - second crust to form. Felsic rock formed from partial melting of the underlying oceanic crust where it was thick. This magma rose by buoyancy to the surface to form continental crust. Later, when mantle convection began, partial melting of oceanic crust in subduction zones formed magma which rose by buoyancy to form andesitic volcanoes with roots of granite (continental crust).

mantle - ultramafic silicates, forming during gravitational segregation of lighter elements from heavy elements - has subsequently solidified by cooling from convection currents.

outer core - liquid iron with minor sulfur and nickel- formed during gravitational segregation of heavy elements from lighter elements - is still liquid - perhaps it is solidifying by increasing the size of the inner core.

inner core - solid iron with minor sulfur and nickel - formed during gravitational segregation of heavy elements from lighter elements - has subsequently solidified by cooling from convection currents.

The lunar rocks from the maria craters date at between 3.9 and 4.6 b.y., indicating great meteorite showers occurred during this time. Note that an asteroid impact melts the surface rock and resets the radiometric clock, causing the age dates on the rock to give the age of the impact. The composition of the earth's crust may have been altered by these meteorite showers. The actual impacts have been removed by erosion and other earth-surface processes.

More on Archean Rocks

Greenstone belts provide more information than Archean granulites which are rocks subjected to intense metamorphism. The greenstone belts typically occur as pods or synclines within granulites in which ultramafic volcanics (komatites) grade upward into mafic volcanics (basalt with pillow structures) into felsic volcanics (andesite to rhyolite) into sediments. The sediments are not commonly those of continental-shelf sequences, e.g., carbonates (limestones) or delta sequences with cross-bedded sandstones (quartz sandstones and feldspar-rich sandstones or arkoses). Instead, they are typically marine, e.g., mudstones (marine shales) and turbidites (greywackes and conglomerates), cherts, and banded-iron deposits with few indications of terrestrial or fresh water deposition. The presence of mostly deep-water sediments are explained by the presence of small protocontinents of felsic crust and the absence of large continents and their associated shelves during Archean time.

Archean Continents

The first large continent apparently formed in South Africa, about 3 b.y. ago. Other large continents were formed between 2.1 to 2.7 b.y. ago as indicated by dating of metamorphism related to intrusion of magma in cratons. The end of the Archean time at 2.5 b.y. marks the beginning of the widespread occurrence of continental crust together with modern plate tectonics.

Banded Iron Deposits

Banded iron formations (iron-rich layers alternating with cherts) are common during the Precambrian, particularly during the Proterozoic. They also occur during the Archean in greenstone belts and make up the oldest dated rocks. The lack of oxygen (no higher plants to make it by photosynthesis) in the Precambrian atmosphere may help explain these iron formations which are not forming today during Phanerozoic time. Oxygen tends to prevent accumulations of iron in aqueous solution, by causing precipitation of iron-rich minerals.

Archean Life

Graphite in banded-iron formations has 13C/12C ratios similar to those in organic tissue. Bacteria are the only life forms represented in Archean fossils. These include both animal-like and plant-like bacteria. Stromatolites, thought to have been formed by cyanobacteria (of which modern species undergo photosynthesis), occur in rocks dated as old as 3.5 b.y. They occur as layered structures of organic-rich calcium-carbonate sediment alternating with layers of calcium-carbonate sediment. Stromatolites grow in shallow seas on continental shelves and are thus rare in Archean rocks because of the scarcity of continental shelves around the small protocontinents existing during this time. They were much more abundant during the Proterozoic and become less abundant during the Phanerozoic when competitors evolved to limit their growth.

Bacteria fall in the kingdom Monera and are "prokaryotes", meaning that they lack a cell nucleus, DNA in chromosomes, and other cell organelles, that are present in the advanced cells called "eukaryotes" which did not exist in the Archean.

Amino acids are the building blocks of proteins needed by living organisms. There are 22 naturally-occurring amino acids. An amino acid contains the group:

H

|

-C-COOH

|

NH2

Proteins are formed by linking amino acids between the NH2 of one amino acid with the COOH groups of another amino acid. Each linkage is accompanied by the release of one molecule of water H2O. Amino acids are found in carbonaceous chondrites (carbon-rich stony meteorites) as well as on earth. Electrical sparks or ultraviolet light in a gas phase of ammonia (NH3), methane (CH4), hydrogen (H2), and steam (H2O) can produce amino acids. Other gas phases can be used such as carbon monoxide (CO), nitrogen (N2) and hydrogen (H2). Could this type of reaction have anything to do with the beginning of life on earth? ATP (adenosine triphosphate) is used by modern cells as the source of energy to build organic compounds. Instead of manufacturing ATP, it can be produced from simple gases. Could the early cells have obtained ATP from their environment rather than manufacturing it.

Note that bacteria can obtain energy to store as ATP in two ways:

Chemosynthesis - the breakdown of simple chemical compounds within a cell

fermenting bacteria - fermentation - breakdown of sugars to form alcohol and carbon dioxide and wate. These bacteria cannot tolerate oxygen and presently live out of contact with the earth's atmosphere.

methane-producing bacteria - Archaebacteria use organic compounds to form methane and sugars. These bacteria cannot tolerate oxygen and presently live out of contact with the earth's atmosphere.

sulfate-reducing bacteria - reduce sulfate to breakdown sugars to form sulfide and carbon dioxide and water. These bacteria cannot tolerate oxygen and presently live out of contact with the earth's atmosphere.

aerobic respiration - aerobic animals, animal-like cells, and animal-like bacteria reduce oxygen to breakdown sugars to form carbon dioxide and water - however, little oxygen was available in Archean time and there may not have been bacteria using aerobic respiration.

photosynthesis - use of light energy to form chemical compounds within a cell

photosynthesis - The common earth-surface process utilized by plants, plant-like cells, and plant-like bacteria in which light interacts with the green pigment chlorophyll. An Archean example is cyanobacteria (blue-green algae) which use carbon dioxide and water with chlorophyll and light energy to form oxygen and sugars. The buildup of oxygen began with photosynthesis of cyanobacteria.

sulfide-oxidizing bacteria - anaerobic photosynthetic purple and green bacteria which use carbon dioxide and hydrogen sulfide and light energy to form sulfur and sugars. These bacteria cannot tolerate oxygen and live out of contact with the earth's atrmosphere.


Chapter 6 - Review Questions - Archean Eon



Chapter 7, The Proterozoic Eon of Precambrian Time (p. 240-265)

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Proterozoic Eon, 2.5 b.y. to 540 m.y.

Characteristics of Proterozoic Eon

During the Proterozoic Eon, the modern style of orogeny (mountain building), which is related to plate tectonics, began. The first modern-style orogeny was the Wopmay orogeny of Canada which occurred between 2.1 and 1.8 b.y. ago along the western margin of the present-day Archean age microplates forming the core of the Canadian Shield. In this area, continental crust had rifted two continental masses apart and subsequently, a subduction zone developed along the boundary. The igneous belt, metamorphic belt, and a fold and thrust belt (containing the backarc (also known as the foredeep or foreland) basin sediments) are preserved in the rock record. Stromatolites are preserved in the preorogenic shallow marine sediments of the foredeep basin.

Proterozoic Depositional Patterns

During the Proterozoic Eon deposition of sediment shifted from being primarily deep-water marine (in the Archean Eon) to shallow-water as the shelf areas along continents developed.

Review of Mountain-Building Features

As described briefly below, mountain building occurs along plate margins where plates are converging.

Andesitic volcanoes form along the edge of an un subducted plate near a subduction zone in which oceanic crust is being destroyed, e.g., Cascades, Sierras, Rockies (difficult example), Aleutians, Japanese Islands, Philippine Islands, and Andes. Once all oceanic crust is destroyed in a subduction zone, continental crust will collide with continental crust, between the two plates. Because continental crust cannot be subducted (due too its low density), the two plates then fuse together and compress any sediment along the margins into mountains of folded sediment, e.g., Alps, Urals, Appalachians, and Himalayas. In both types of mountain building, subsequent erosion will result in the continental crust (granite) being uplifted by block faulting due to isostasy.

What type of rocks would we expect from mountain building?

Described below is a general vertical upward sequence of shallow-water to deep-water marine to shallow-water marine into fresh-water sediments - all in the backarc (foredeep or foreland) basin - and a lateral sequence of rocks going (away from the craton) from folded sediments (fold and thrust belt) to metamorphosed sediments, to igneous rocks. These sequences are associated with Phanerozoic mountain building such as the Wopmay orogeny of Canada.

A depositional basin exists between the volcanoes forming along the edge of a continent and the stable craton. The vertical trend in sediments in this basin grade upward from the (oldest) shallow-shelf marine shelf deposits which are preorogenic (sandstones and carbonates) to deep-water marine deposits (flysch: mudstones and turbidites) and then back to the (youngest) shallow-water marine and fresh-water deposits (molasse). The trend reflects the downwarping of a shallow basin by the stresses associated with mountain building (occurring further offshore), followed by filling the deep-water basin with sediments from the newly-formed mountains and from the craton. These sediments have been folded because of the compression associated with mountain building. In addition, the sediments closest to the mountain-building belt are metamorphosed because of the generally higher compression and the higher temperatures due to the presence of magmas associated with the volcanoes.

The lateral rock sequence going from the craton to the area of mountain building is a belt of folded and thrusted sediments, a belt of metamorphosed and folded sediments, to a belt of igneous rock. The first two belts are formed in the depositional basin described above and the third belt consists of rock making-up the volcanoes.

Proterozoic Glacial Deposits

Early Proterozoic glaciation occurred about 2.3 b.y. ago, as shown by glacially-derived features (tillites, dropstones, laminated mudstones with varves due to deposition in glacial lakes in the Gowganda Formation in southern Canada. Similar age tillites are also found in Wyoming, Finland, southern Africa, and India. Thus the glaciation was world-wide.

Late Proterozoic glaciation occurred in several pulses between 850 and 600 m.y. ago, as shown by tillites on all major continents except Antarctica. A mass extinction occurred with the glaciation at 600 m.y. which affected the acritarchs and other species and may have been due to a cooling of the surface waters of the oceans.

Atmospheric Oxygen

Between 2 and 3 b.y. ago atmospheric oxygen was thought to have been only about 2% of present-day concentrations, reaching moderate (15%) concentrations at around 2 b.y. ago. Cyanobacteria and other organisms undergoing photosynthesis and producing oxygen, did not become abundant until about 2.3 b.y. ago, although cyanobacteria actually evolved about 3.5 b.y. ago. Sulfate-reducing bacteria, sulfide-oxidizing or purple-green bacteria, methane-producing bacteria, and fermenting bacteria cannot survive in oxygenated environments. These bacteria have become less common since Archean time, being restricted to anaerobic (without oxygen) environments, out-of-contact with the atmosphere.

The generally low levels of atmospheric oxygen in the Precambrian time are needed to explain the banded iron formations which commonly formed between 2.5 and 1.8 b.y. ago, as well as the occurrence of uranium minerals with uranium in a reduced form U4+. The common occurrence of pyrite (having both reduced iron and reduced sulfur) in Precambrian sediments, is another indicator of low levels of atmospheric oxygen. Pyrite is not stable in an oxygenated environment and decomposes. The banded iron formations consist of alternating bands of chert with layers that are usually composed of iron-rich minerals, with iron in a reduced state (Fe2+). Iron and uranium are sinks for oxygen (combine with it) and would not normally be deposited in reduced forms under earth-surface conditions, unless the oxygen in the atmosphere was at low concentrations. In addition, uranium is actually very soluble in its oxidized form and would have been dissolved quickly if deposited in an oxidizing environment.

Red beds, containing oxidized iron as Fe3+ in hematite, Fe2O3, appear in rocks younger than 2.3 b.y. ago, as the result of a gradual increase in oxygen content of the atmosphere. In addition, not all of the iron in banded iron formations is in the reduced state. Some of the minerals contain iron in both reduced and oxidized forms (e.g., magnetite, Fe3O4)) indicating some oxygen in the atmosphere.

Before the concentration of oxygen in the atmosphere could be built up to present-day levels by photosynthesis, various oxygen sinks had to be filled. Carbon monoxide (emitted by volcanoes) had to be oxidized to carbon dioxide in the atmosphere, sulfide minerals exposed at the earth's surface had to be oxidized to sulfates, and reduced iron, uranium, and other metals exposed to the atmosphere had to be oxidized to metal oxides, e.g., rust. A lot of the initial oxygen produced by photosynthesis was used up in these reactions. Once levels of oxygen were built-up in the atmosphere, they stopped increasing and reached a steady state concentration due to the evolution of organisms, using oxygen in respiration. In essence, photosynthesis became balanced by respiration, producing a balance between atmospheric oxygen and carbon dioxide.

Life of the Proterozoic Eon

At the beginning of the Proterozoic Eon, life consisted of single-celled prokaryotic producers of the kingdom Monera, such as cyanobacteria. By the middle of the Proterozoic Eon, life had evolved to produce single-celled eukaryotes in the kingdom Protoctista: first into protozoans, animal-like protoctists (more than 1.8 b.y. ago) and then into plant-like protoctists, such as acritarchs (about 1.4 b.y. ago). The acritarchs are thought to have been dinoflagellates. They underwent adaptive radiation between 900 and 700 m.y. ago before undergoing a mass extinction 600 m.y. ago at the time of the Late Proterozoic glaciation. Only a few spherical forms survived. Present-day representatives of acritarchs are dinoflagellates.

By the end of the Proterozoic Eon, multi-celled plant-like protoctists had evolved together with multi-celled plants of the kingdom Plantae and multi-celled animals of the kingdom Animalia, such as jellyfish and sea pens of the phylum Coelentera, segmented worms of phylum Annelida, and organisms with external skeletons and jointed appendages of phylum Arthropoda. The evolution of multicellular animals required high oxygen concentrations in the atmosphere and advanced nerve cells and occurred less than 1 b.y. ago (as evidenced from trace fossil evidence of burrows).

As noted earlier, stromatolites originated about 3.5 b.y. ago, became very abundant, as shallow water environments became more abundant, about 2.3 b.y. ago and exist today. They are not useful as index fossils because they tend to have different shapes in different environments. They are no longer abundant because multi-cellular animals evolved that feed on the cyanobacteria. They only survive today in shallow-water environments too harsh for these predators, e.g., tidal channels.

Eukaryotes, (cells containing nuclei, chromosomes, and other organelles) evolved from the prokaryotes in the Proterozoic about 1.8 b.y. ago, based on the first appearance of large cells with thick walls (cannot identify as to being plant-like or animal-like). The first appearance of large cells in the fossil record date back to 1.4 b.y. ago in the acritarchs. In general, prokaryotes have much smaller cell sizes than acritarchs.

Eukaryotes are thought to have evolved through the union of two or more prokaryotic cells. Protozoans, animal-like cells in the kingdom Protoctista, are thought to have evolved first. The evolutionary process consisted of one prokaryote devouring a smaller bacterium, in which the smaller cell survived, although altered, and became a mitochondrion or an organelle capable of using oxygen to break down organic compounds, i.e., respiration in the first animal-like cell. This scenario is supported by the observation that bacteria are known to inhabit single-cell organisms and perform respiration for their host.

The first plant-like cell in the kingdom Protoctista is thought to be the result of a protozoan consuming a cyanobacteria which became an intracellular body known as a chloroplast (where photosynthesis occurs). This possibility is supported by structural similarities between cyanobacteria and chloroplasts. The oldest plant-like protoctist fossils date back 1.4 b.y. (algae plankton called "acritarchs" and thought to be dinoflagellates); whereas, the oldest positively-identified fossils of protozoans date back only 0.8 b.y.. However, since the plant-like protoctists moat likely evolved from protozoans, the protozoans must have originated more than 1.8 b.y. ago.

Algae

Algae include plant-like organisms, ranging from cyanobacteria to single-cell and multi-cellular photosynthesis protoctists to multi-cellular plants that (unlike advanced land plants) lack multicellular reproductive structures to protect their eggs and embryos). Multicellular algae are present in the fossil record beginning about 0.9 b.y. ago.

Multicellular Animals

Multicellular animals evolved from protozoans by developing multicellular body forms and advanced nerve cells. Between 900 to 600 m.y. ago, a series of accumulations of 13C-rich carbonate rock deposits, provide evidence that not much oxidation of organic material was occurring. (This follows because organic matter is rich in 12C, which upon oxidation would also be incorporated into carbonate rock, preventing carbonate rock from being rich in 13C.) The lack of oxidation of organic matter allowed rapid build-up of oxygen in the earth's atmosphere, which then promoted the evolution of multicellular animals, depended upon oxidation for respiration.

Multicellular animals may have not developed rapidly until near the end of the Proterozoic for other reasons besides a low atmospheric oxygen content. Evolution took time to produce the nerve cells which coordinate muscle movement. Perhaps, these nerve cells only developed near the end of the Proterozoic.

The oldest multi-celled animal fossil records are trace fossils (burrows, trails, tracks) of soft-bodied animals in rocks less than 1 b.y. old. Imprints of jellyfish and sea pens have been found. Fossil skeletons were not preserved until near the end of the Proterozoic Eon, about 600 m.y. ago, because organisms with skeletons evolved after the non-skeleton forms. The multicellular animals present at the end of the Proterozoic Eon included coelenterates (jellyfish and sea pens in phylum Coelenterata), segmented worms (annelids in phylum Annelida), and arthropods in phylum Arthropoda. The late Proterozoic Ediacara fauna in Australia contains fossils of many soft-bodied animals.

Proterozoic Cratons

Continents can grow by

(1) suturing two continents together, following the destruction, in a subduction zone, of oceanic crust between them.

(2a) accretion by attachment of microplate (e.g., an island arc) to a large craton, following the destruction, in a subduction zone, of oceanic crust between the craton and the microplate. The microplates are called exotic terrains.

(2b) accretion by metamorphism of sediments, near a subduction zone, that are on the continental margin. These can also be oceanic sediments that were smeared onto the edge of the continental margin, as they were pushed into the subduction zone.

(2c) accretion through the introduction of magma in volcanoes overlying a subduction zone on the continental crust. The magma represents partially melted oceanic crust and melted oceanic sediments in the subduction zone.

(3) introduction of basaltic magma in failed rifts within a continent. Although, continents become smaller by rifting, if the rifting proceeds to split the continents.

Formation of Rodina

During the Proterozoic Eon, a large supercontinent developed called Rodina. The continent eventually included North America, Greenland, Scotland, Ireland, eastern Russia, Baltica (Scandinavia), and possibly Siberia along its northeastern margin. Also attached to the continent were Australia (along its northern margin), Antarctica (along its western margin, and Africa (along its southwestern margin). The final stage of the supercontinent creation was the Grenville Orogeny in which South America sutured to Laurentia in Late Proterozoic. This supercontinent subsequently broke apart at the end of the Proterozoic: separating into Laurentia (North America and Greenland), Baltica, other northern hemisphere continents, the southern continents (that later joined in the early Paleozoic to become Gondwanaland), and several microcontinents. Eurasia did not exist during the Proterozoic Eon but formed later from fragments rifted from Laurentia and Gondwanaland in the Phanerozoic Eon. Note that the super-continent which existed at the end of the Proterozoic Eon was reassembled at the end of the Paleozoic Era to form the supercontinent named Pangaea. When Pangaea split into two supercontinents during the Mesozoic, these were Laurasia and Gondwanaland.

Formation of Laurentia

Laurentia began to form 1.95 to 1.85 b.y. ago through the assembly together of at least six microplates of Archean age to form the Canadian Shield. Note that Laurentia included Greenland which contains Archean age rocks. South of the shield area, a series of Proterozoic island arcs developed (in the region of the present United States) that were accreted onto the craton between 1.8 and 1.6 b.y. ago.

About 1.5 b.y. ago, Laurentia may have become smaller in size through the rifting of one of the microplates, Siberia, away from the western margin of the craton.

A rift began to develop in central Laurentia between 1.2 and 1.0 b.y. ago. The rift extended south from the Great Lakes region to Kansas, forming the Mid-continental rift (also called the Keweenawan Rift) which failed but can be recognized by basaltic rocks south of the Canadian Shield. The copper-rich Keweenawan basalts were extruded at this time.

Between 1.2 and 1.0 b.y. ago, the western margin of South America and Laurentia sutured together along the present eastern margin of the United States, during the Grenville orogeny to complete the formation of the supercontinent Rodina. During this time, Ireland, Scotland, Scandinavia, and eastern Russia were part of Baltica already joined to Laurentia. Siberia was already joined to Laurentia. The rest of what was to become Gondwanaland formed a crescent wrapping around Laurentia. Some of the suturing of these southern continents to Laurentia may also have occurred during the time of the Grenville Orogeny. The reassembly of the southern hemisphere continents in the vicinity of Laurentia follows the Samfrau belt of mountains, between South America and Africa, between South Africa and Antarctica and between Australia and Antarctica. The Samfrau mountains were active during the time of the Grenville Orogeny indicating suturing together of the southern continents at the same time they were possibly being sutured to Laurentia. The North America east coast suturing with South America produced a range of mountains in the Grenville Province. Remnants of those mountains are exposed in the Adirondack Mountains in New York. Rocks formed during the Grenville orogeny are also exposed in the Blue Ridge Mountains in Virginia, and in the Llano Uplift in Texas.

Breakup of Rodina

The supercontinent Rodina rifted apart between 0.8 and 0.6 b.y. ago. Laurentia and Baltica split apart from the continents which became Gondwanaland. There were several microcontinents such as northern Ireland, Scotland, southern Ireland, and England. The Iapetus Ocean separated the other continents from Laurentia. The rifting separating Gondwanaland from the western margin of Laurentia produced a clastic wedge of eroded sediments in basins preserved in the Belt Supergroup.

Formation of Gondwanaland

The sequence of accretion of continents to form Gondwanaland at the end of the Proterozoic is unknown. Metamorphic belts in the Proterozoic follow the present-day outlines of the different continents which formed Gondwanaland. However, these metamorphic belts appeared to form in the interior of Gondwanaland and the different continents later broke apart along these belts. The origin of the metamorphic belts is unknown. They are not the result of the normal mountain-building processes of plate tectonics.



Chapter 7 - Review Questions