How To Understand The History Of The Earth

Transcription

1 HISTORICAL GEOLOGY UNIT 5 Lecture and Study Guide THE PRECAMBRIAN AND LIFE HISTORY Chapter 19 (Revised 1/15) UNIT 5: VIDEO HIT HOMEWORK; Write a minimum of two paragraphs with minimum of three sentences each for the unit video site For any Unit Web Hits and Unit Web Videos, go to the DMC HOME website; in Search box type Geology, select Vernon Kramer, scroll down to GEOL 1404 or 1304, select Syllabus, select Video Web Hits, click on icon for web site OR: go to DMC Home website, select Degrees, Certificates, Courses, scroll down to Natural Sciences and select Geology, select Faculty Listings, select Walter Vernon Kramer, find Geol 1404 or 1304, select Syllabus, and there you can find the video hits, click on icon of interest for video site [IF NONE OF THE WEB SITES COME UP, YOUR COMPUTER PROBABLY NEEDS TO BE REBOOTED (RESTARTED) What Is The Precambrian? - The name Precambrian does not refer to an Eon, nor Epoch or Period. - Precambrian: Informal name for the earliest chapter in Earth s history - Refers to all rocks beneath Cambrian-aged rocks, and to all time before the Cambrian Period - The Precambrian began when the Earth originated +4.6 BY ago and ended 542 million years ago when marine, multi-cellular animals with shells appeared. - This represents 88% of Earth s history or about 4.0 BY. Precambrian Exposures - Only 15%-20% of Earth s Precambrian rock surfaces are exposed. The remaining rocks are buried by younger rocks and sediments. - Most of the Precambrian surfaces are composed of igneous and metamorphic rocks. - These types of rocks do not yield much evidence of geologic events of Earth s past history. - Precambrian sedimentary rocks are generally folded, faulted and slightly to heavily metamorphosed. - Very few fossils are found in the Precambrian; most are single-cell organisms. - Most Precambrian rock exposures are found in remote and harsh environments. This includes the tops of mountains, bottom of deep canyons, in the Arctic regions and tropical rain forests. Thus these Precambrian exposures are difficult to find and study. - The Precambrian is subdivided into three eons: Hadean, Archean, Proterozoic. Locations of Precambrian Rocks - The largest North American exposure is found around the Hudson Bay, Canada. - Precambrian rocks in the US: Lake Superior, core of Appalachian Mts., St. Francis, MO, Black Hills, SD, core of the Rocky Mts., southern parts of NM and AZ - In Texas: Llano area, Central TX, Carrizo Mt. near Van Horn, TX, and the Franklin Mts. at El Paso, TX - Drill deep enough on the continents and you will probably encounter continental granitic crust. Geologic Precambrian Terms - At one time it was thought that the Precambrian lasted less than 100 million years - Basement rocks: usual name for Precambrian rocks - Shield: exposed portion of the basement of a continent that is composed of Precambrian rocks - Platform: buried or covered surfaces of a shield - Forms the foundation upon which other sediments are deposited - Craton: the nucleus of a continent made up of a shield and a platform - Cratons are not composed of uniform series of rock types. 1

2 Canadian Shield and platform - Canadian Shield: The exposed portion of the North American craton Hadean Eon - It would be during this time that Earth formed and was totally melted - Within a couple of hundred million years, the moon was formed - Generally, the only rocks available for the beginning of this time are meteorites - During this Hadean time, the Earth became layered (differentiated) with a core. An atmosphere and the oceans were beginning to develop by outgassing. - At 4.4 BY, innumerable meteorite impacts would keep tearing apart the Earth s crust of basalt (oceanic crust) - Soon water would begin covering the Earth, even as the meteorite impacts continue. - Up to 2008, we thought there was no rock record on Earth between 4.56 BY to 4.0 BY OR so we thought! - Supposedly, the Earth s surface was totally melted but we have a reported discovery in 2008 of 4.28 BY old rocks in Nuvvuagittuq greenstone in Ungava, Quebec, Canada (oldest rocks on planet Earth?). Some other radiometric methods give a 3.8 BY or Proterozoic date. 2

3 Oldest Earth Particle - The Jack Hills of Western Australia have meta-sedimentary rocks which contain zircon crystals. - These zircon crystals were first developed within an igneous rock that was later weathered by hydrological processes and even later incorporated into a sedimentary rock that was finally metamorphosed. - Dated at BY, one zircon crystal represents the oldest particle on planet Earth. Oxygen isotopes within these crystals indicate that the Earth had a water ocean at the time of their development! Archean Eon - Represents period of 4.0 BY 2.5 BY - Only in one area has there been found Archean rocks ay 4.0 BY (proven for sure), the Acasta gneiss in Northern Canada - About a dozen areas of Early Archean rocks aged 3.8 BY 3.9 BY, have been found within the shields of all major continents. - By the way, the Acasta gneiss proves there was continental crust at the beginning of the Archean Eon (one zircon from there was dated at 4.2 BY) Early Archean Environments - Earth s outward heat flow was 3 5 times today s rate because of the meteorite impacts and radioactive decay. - The moon was much closer than today causing the Earth s crust to rise and fall by 200 feet every day. - There was no free oxygen in the atmosphere. The oceans were getting deeper but were subjected to 1,000 foot tides (because the moon was so close)! - The sun was 30% dimmer. - Meteorites continued to pelt the Earth until 3.8 BY age. - Plate tectonics were already active at this time. 3

4 Archean Earth Archean Provinces - Before 4.0 BY, innumerable meteorite impact would continue tearing apart Earth s oceanic crust of basalt - With the pounding by the meteorites, this would help break up the oceanic crust and convergentsubduction zones were inevitable - Before 4.0 BY, we probably had only oceanic crust being subducted under oceanic crusts - When oceanic crust is subducted under oceanic crust, island arcs that create continental crust (geologic province) are created - Typical for an island arc, a volcano rises from the deep ocean floor with associated pillow lavas, lava flows, and volcanic ash. - After the volcano reaches the surface, erosion will produce sands and muds that slide down deep into the ocean - Later, bodies of granite will form and invade all of these rocks - - Java Indonesia is a good example of a typical island arc island - Most of the early lavas would have formed underwater, creating a phenomena called pillow lavas - Pillow lava: lava that is extruded into water and forms ellipsoidal masses - Sediments would have been washed off the island in a series of submarine slides called turbidity currents - After sliding out to the ocean floor, this sediment will settle out in a very distinctive pattern called graded bedding Graded bedding - Graded bedding: a sandstone called graywacke that shows a decrease in grain size from the bottom to the top of the bed and composed of abundant clay and volcanic rock fragments - Graywackes always form in deep waters either beyond a continental shelf or off an oceanic island - Since we find mostly graded bedding in the Archean, there were no continental shelves like today - NEXT STEP TO CREATE A LARGER LAND MASS: another island arc on another oceanic crust will collide with our previous island arc, yielding a much larger landmass - This collision would create a large mass of metamorphic rocks at their contact boundaries 4

5 - The resultant metamorphic rocks would be metasediments, gneiss and greenstone - This was probably how the Acasta gneiss was formed +4.0 BY ago - FYI: during this Archean metamorphism the green mineral chlorite would form within the metamorphosed sediments and lavas - Greenstone: former volcanic and sedimentary rocks that are rich in chlorite - So convergent (subduction) zones help build up continents - Each island arc would be considered as a tectonic province - Tectonic Province: small, independent pieces of continental crust that are were joined together by plate tectonic collisions Geologic provinces - Each tectonic province has a core of igneous rocks that is surrounded by metamorphic rocks. These core areas are thus sutured (or joined) together. - Thus present day continents have cratons with various provinces that have been sutured together. - Archean greenstones (former island arcs) are aligned in zones called greenstone belts - Greenstone Belts: zones of rocks rich in chlorite - Komatiites: ultramafic volcanic rocks derived directly from the mantle. They are found only from lavas in the Early Archean Eon Geological Provinces - From 4.0 BY to the end of the Archean at 2.5 BY, the continental land masses (cratons) will grow considerably larger - Later, we will learn of life on Earth at 3.85 BY at Isua, Greenland and at Pilbara, Australia at 3.6 BY - All of the present day continents have cratons with various provinces that have been sutured together - It was only recently that we have learned about these provinces - Before the widespread use of radiometric age dating, the Precambrian basement rocks of Canada were mapped in detail according to rock type and not rock age - Although lots of valuable mineral deposits were found, it was impossible to correlate anything (no fossils) With the advent of radiometric age dating, a totally different geologic map was derived - It became apparent that there was a collection of core areas or provinces, each different from the other - It was finally realized that a continent is built around a collection of core areas or provinces Oldest Archean Rocks in North America - In the Minnesota River Valley is the Morton Gneiss which is dated at 3.5 BY, thus holds the record as the oldest US rock. - At the end of the Archean (2.5 BY), there a large land mass (part of North America) that will be called Laurentia 5

6 Early Proterozoic - First Ice Age - At the beginning of the Early Proterozoic, Earth experienced its first ice age of continental glaciation ( BY). - The glaciers would break off (calve) into the sea to create icebergs. - Within these icebergs would be large quantities of rocks and sands. - Tillite: sedimentary deposits that include rocks and boulders from icebergs (sometimes called dropstones). - These dropstone deposits show major glaciers near India and the Gowanda Tillite of Southern Ontario, Canada Proterozoic Eon - Ranged from 2.5 BY to 542 MY - We will examine this at its Early, Middle and Late Proterozoic times. - By this time, ten or more provinces had created the basement rocks or North America. Early Proterozoic Eon - Ranged from 2.5 BY to 1.6 BY - During the Early Proterozoic North America will grow larger on its southern flank by the addition of the Mojave Province, the Yavapai (Grand Canyon rocks) Province and the Mazatal or Pecos Province (West Texas rocks) - During the Proterozoic, the continents became large enough (with enough sediment) for continental shelves to development - For the first time, long lines of tall mountains that would provide enough sediment for the building of continental shelves. - With the development of the continental shelves, sedimentary deposits of quartz sandstone and dolostone strata became more abundant. Early Proterozoic Banded Iron Formations - Oceans were filled with abundant dissolved iron and probably gave the oceans a green color. - During the Early Proterozoic, bacteria would evolve and begin to produce oxygen as a waste product, which would combine with the dissolved iron to form a solid - - The presence of oxygen would result in vast iron formations being deposited in shallow marine waters. 6

7 Banded iron formation - Band Iron Formations (BIF): alternating layers of iron oxides and chert deposited in the sea (chert is a type of quartz) - The BIF are the principle source of iron ore around the world today. - After 2.0 BY there would be so much free oxygen (about 1%) that very minor dissolved iron would reach the ocean and there would be no more thick deposits of BIF Middle Proterozoic Eon - Ranges from 1.6 BY to 0.9 BY - The first supercontinent will soon begin to form here but before that happened we have the big rip! - North America began ripping apart, with the Mid-Continental Rift. (This rift zone centered near Missouri would fail yet we continue to have earthquakes today from that zone). - This rift caused basalt flows that reached thousands of feet of thickness (Keweenawan Group) - The Keweenawan basalts are famous because they contain enormous amounts of copper metal and silver metal! Rift & Grenville Rodinia Middle Proterozoic Grenville Province - During the Middle Proterozoic the southern and eastern Grenville province was added to the North American craton. - The Grenville was formed by continental collisions during the formation of the supercontinent Rodinia, the first supercontinent ( BY). - The central Texas (Llano area) Enchanted Rock granite mass was part of the 1.2 BY Grenville province. - Footnote: 900 MY ago, the Earth s rotation has slowed down from an 8 hour day to an 18.2 hour day or 480 day year. It will take from that time (900 MY) to today to slow the Earth s rotation to a 24 hour day (365 day year) 7

8 Late Proterozoic Eon - Ranges from 0.9 BY to 545 MY - Many geological events will impact the Earth during this time, including the breakup of Rodinia - The Belt Supergroup of the Northern Rockies (composed mostly of sediments) was added to the North American Craton. - Also added was the Ocoee Supergroup of the Southern Appalachians which was composed mostly of sediments. Late Proterozoic Eon Ice Age Snowball Earth Theory - During the Late Proterozoic began a collection of the most severe ice ages of all times - There is a lot of conflicting data to regards to its timing: Marinoan Glaciation: MY or MY Strutian Glaciation: MY or MY - Glaciation is supported by striations and tillites - Some have suggested the Snowball Earth theory for some of this time. - Snowball Earth Theory: a time when the entire Earth was covered with ice - During this time the land masses recombined to form the supercontinent Pannotia (650 MY) - Other evidence suggests that most of the continents were moving past the South Pole (lots of glaciers) and finally moved off the South Pole (melting of glaciers). Thus the entire Earth did not have to freeze. Review of Making Continents Bigger During the Precambrian - Collisions of small continents make continents bigger which can later breakup into smaller pieces again. - Island arcs will also collide with continents and make the continents larger. - Sediments can be deposited as a continental shelf and make the continents larger. - We have two separated periods of continental glaciers (about 2.5 BY and 700 MY) - We had two supercontinents that form and then fragmented: Rodinia and Pannotia Pannotia The Origin of Life on Earth Living Matter - All living matter is composed of chemical compounds of the elements C, H, O, N (Carbon, Hydrogen, Oxygen, and Nitrogen). - Some organic compounds also require S and/or P (Sulfur or Phosphorous). - Living organisms are heterogeneous and are organized differently and tend to be made of different parts. - Non-living things are generally internally homogeneous. 8

9 Living and Non-living - A living organism undergoes metabolic processes: - 1) They can react to their environment and heal themselves when damaged. - 2) They can grow by taking nourishment from their surroundings and processing it into energy. - A living organism undergoes reproductive processes: - 3) Organisms reproduce passing along some of its characteristics. - 4) They have the capacity for genetic change. - The preceding distinctions are not always clear. Viruses - Is a virus a living organism? - A virus is composed of a bit of DNA or RNA in a protein capsule. - Outside a host cell, a virus can neither reproduce nor produce organic molecules. - A virus will behave as a living organism ONLY when it is inside a host cell. - Some scientists feel that viruses are not living organisms. Origin of Life Theories - Almost every religion or culture has their story about the creation of the Earth and Life. - Spontaneous generation of life: life appears from nowhere - Space dust contains water, ammonia, methane and amino acids. Thus some feel that primitive life on Earth arrived from meteorites or comets. - Some feel that life was planted on Earth by aliens. - S.E.T.I. Search for Extra-Terrestrial Intelligence Primordial Soup - Primordial Soup: organic compounds, formed by a variety of processes, from molecules that accumulate in the shallow oceans - Over millions of years, the accumulations of organic compounds would have formed in the shallow water (remember, there was no oxygen at this time), and somehow life would have been created from this mixture. Some Simple Organic Compounds From C, H, N and O - Amino acids or Monomers: the basic building blocks (chemical compounds) for more complex organic compounds Proteinoids - Proteinoids: protein-like structures of linked amino acids - Can be created (polymerized) from a mixture of amino acids when heated, dehydrated and concentrated - Some suggest that this process occurred early in Earth s history, when proteinoids would have been created in shallow seas. - Under some conditions, proteinoids are suddenly formed when amino acids come in contact with clay or pyrite. - Protobionts: man-synthesized proteinoids that have characteristics between chemical compounds and living organisms 9

10 Microspheres Microspheres - Microsphere: simple organic molecules that show great organizational complexity - Some microspheres contain more than 200 linked amino acids. - Some microspheres have been found forming near hot springs of volcanic vents. - Some microspheres can grow and divide but are not considered living. - Some scientists feel that the original life on Earth passed through a microsphere phase. Facts about Microspheres - They contain amino acids. - They have a film-like outer wall, similar to a cell wall. - They can swell and shrink in size, according to their environment. - They can have streaming movement of internal particles. - They can replicate themselves by budding or binary fission. - They are not alive. -Defining life is not always easy! Experiments - Miller (1953) was able to create all 20 amino acids, using only methane, ammonia, water, hydrogen gas and a spark of electricity in a glass apparatus. - All of the above chemicals would have been available on Early Earth. DNA or RNA - Proteins: monomers or amino acids can be linked together in a specific sequence to form polymers. - Polymers: they form in groups to create nucleotides which in turn form nucleic acids (DNA or RNA) - Man has never been able to put all of this together (either from chemicals or dead cells) to create life. - Man has never seen life created from chemicals in nature. - But the first life on Earth began as single-cell organisms. Precambrian Fossil Record of Life Fossil Types - Until the 1950 s, the Precambrian was considered unfossiliferous. - Since then, hundreds of good Precambrian fossil sites have been discovered. Life Form Domains (Two) Domains: Prokaryotes Eukaryotes (Five) Kingdoms: Monera Protista, Fungi Plantae, Animalia (Bacteria) 10

11 Some say that the bacteria actually represents two domains: (Three) Domains: A) Monera B) Archaea Eukaryotes (Six) Kingdoms: Bacteria Extreme Bacteria Protista, Fungi Plantae, Animalia - There are three types of Precambrian fossils: 1) Chemical fossils; 2) Body fossils; 3) Stromatolites Precambrian Chemical Fossils - The most ancient evidence of life forms is present as a chemical fossil - Chemical fossil: fossil life detected by using non-radioactive isotopes of 12 C/ 13 C from carbon films - The element 12 C has a non-radioactive isotope with an extra neutron 13 C - Life that uses CO2 to live and selectively used the lighter 12 C rather tan 13 C from the atmosphere - Organically (living) derived carbon has a different 12 C/ 13 C ratio than inorganically derived carbon - Organically derived carbon is enriched in 12 C, the lighter of the carbon atoms and isotopes - Isua, Greenland rocks hold the title at 3.85 BY as being the Earth s oldest fossil. - It is a fossil of probable primitive bacteria (present as a carbon film on rocks). - No similar-aged body fossils have yet been found in the Isua rocks. Important Terms Associated With Organisms - Aerobic: with oxygen - Anaerobic: without oxygen - Autotroph: an organism that produces food inorganically - a) Photosynthetic autotroph: produces food using light - b) Chemical Autotroph: produces food suing external energy (heat, chemicals, etc.) - Represented by Archean bacteria that can live in F waters - Some can live with extreme radiation. - Heterotrophic: organism that must consume organic molecules for survival (must eat other forms of live or dead organisms) Cyanobacteria - Cyanobacteria: aquatic bacteria that uses photosynthesis - The earliest may have been anaerobic but today most are aerobic photosynthetic autotrophs. - Individuals are small and unicellular. - They can grow in colonies large enough to see. - They are a contributor to the origin of plants. - The chloroplasts with which plants make food is actually cyanobacteria living within the plant s cell. - They are often improperly called blue-green algae. 11

12 Precambrian body fossils (bacteria in chert) Precambrian Body Fossils - Almost all of the early fossils are microscopic, single-cell organisms. - Most are found in black chert. - Some larger organisms will appear toward the end of the Late Precambrian. Oldest Body Fossils - Some of the next-oldest body fossils discovered are filaments of cyanobacteria aged at BY, found within the Warrawoona Group in Australia (before the first 2.5 BY ice age). - Of the eleven discovered species, some are very similar to species living today. - The nearby Apex Chert at BY also yielded cyanobacteria fossils. Stromatolites Stromatolites - Stromatolites: cyanobacteria and other bacteria that form colonial associations. - Mound-like structures consisting of laminated carbonate rock produced by trapping of sediments by groups of organisms (cyanobacteria) - These can be found today, throughout the world including Shark Bay, Australia, the Bahamas, the Indian Ocean and at Yellowstone. - Stromatolite mounds have been found in Western Australia (3.5 BY), South Africa (3.4 BY), and these existed before the first ice age and before Rodinia - The Gunflint Chert, Ontario, Canada at 2.1 BY found not only cyanobacteria, but also many different types of bacteria. - During the time of Rodinia, stromatolites lived on rocks found in the present day Grand Canyon (1.1 BY) 12

13 Eukaryotes - The first fossil evidence of single-cell eukaryotes (non-bacteria) occurred at nearly 2.7 BY - Eukaryotes: are important in that they have more advanced cells than bacteria - The earliest are single-celled organisms. - These cells are larger than prokaryote cells. - Eukaryote cells have a nucleus and prokaryotes do not. - Eukaryotes possibly arose by different types of formerly free-living prokaryotes joining together (symbiotic relationships). - Eukaryote cells contain organelles such as either mitochondria or chloroplasts. Cell Parts - Organelle: specialized structures found within cells that carry out distinctive cellular functions - Mitochondria: organelle in a cell that oxidizes organic matter to provide energy for use in cellular metabolism - Chloroplast: organelle with which plants make food is actually cyanobacteria living within the plant s cell DNA -Parts of cells like mitochondria and chloroplasts have their own DNA that is different than the cell nuclear DNA. Ancient Eukaryotes - They could represent either single cell plants or animals. Acritarchs - Acritarchs: are cysts of algae. In time of drought algae will create a cellular wall called a cyst to protect itself. Some acritarchs, as old as 2.1 BY, have been found in Australia. - Early single-cell protist (eukaryote) fossils have been found at Bitter Spring Formation, Australia dated at BY (during Rodinia and after the first ice age). Multicellular -Eventually, individual cells would clump together and form multicellular animals (metazoans) and plants (metaphytes). - Plants would be the first to form multicellular organisms. - Until recently, we thought the first multicellular fossils of algae were developed 700 MY ago. - But we have found 2.1 BY multicellular algae in Michigan with the Negaunee Iron Formation (just after the first ice age). 13

14 Multicellular Animals (Metazoans) - We can only speculate on how life got beyond being multicellular photosynthetic eukaryote plants to multicellular animals that were consumers of plants. - Almost all eukaryote animals require aerobic conditions for survival. - The evolution of multicellular organisms requires some major changes in body structure. - Individual cells would need to clump together within a body sac. - Specialized cells are required for locomotion, reproduction and food gathering. - A major change in metabolism is also required. Oldest Metazoans - Until recently, we thought the oldest metazoans developed 600 MY ago. - But animal burrows, aged at 1.2 BY have been found in Australia and West Texas (during Rodinia) Precambrian burrows Super-continents - At 1.0 BY, the super-continent of Rodinia had been formed. At 750 MY, the continent began breaking apart. At 650 MY some of the continents came back together to form a short-termed continent Pannotia. At this time we would see the evolution of the Ediacaran animals. Ediacaran Fossils (670 MY 520 MY) - With Pannotia came an ice age and the appearance of the Ediacaran fauna. Ediacaran life - The Ediacaran fauna existed in shallow, cold water marine environments, between 760MY-520 MY - Ediacaran fossils represent large assemblages of soft-bodied animals (no hard parts) - They would be around for +100 MY and would represent Earth s first large organisms. - Ediacaran fossils can be found on all continents except for South America and Antarctica. - All of these are soft-bodied creatures with no jaws and there were no predators - Some grew as fronds that reached six feet in height and some worm-like creatures reached more than three feet in length. - Some Ediacaran fossils are possible ancestors for trilobites, worms, jellyfish and sea urchins. - It is not clear why many Ediacaran life forms disappeared. - These life forms probably evolved because of the stress of the cold water environments and mutated because they were isolated by ice - Some evidence exists that they were just out-competed by more advanced life forms. - Oxygen levels were increasing and maybe after all this time of existence, this was beyond the tolerance of the Ediacaran life forms The next unit is the Phanerozoic Eon and the Paleozoic Era 14