EESA02H3 Lecture 7-9.docx

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Department
Environmental Science
Course
EESA06H3
Professor
Zachariah Campbell
Semester
Winter

Description
Lecture 7: Geologic Time, Fossils, and Evolution - earth is 4.56 billion years old - how do we know this?  a brief history i. 11 century: Persian geologist Avicenna (mountains were built over time) - mountain formed after a long series of events predating humans ii. 11 century: Chinese naturalist Shen Kuo - concept of geologic or “deep time” - contrary to popular belief that the earth was young iii. ancient Greeks recognized fossils embedded in rocks now on tops of mountains - marine fossils how did they get there? iv. western geologic theory pre 1800’s - heavily based in Christian religion - the earth was created in 7 days - climate is fixed - people found these sediments till, sand and gravel attributes to = dilivium (biblical floods) - calculated how old the earth was based on the old testament e.g. 1658 archbishop usher of Ireland gave an age of the earth of about 5600 years based on the study of old testament v. theory of catastrophism: earth stays he same but a huge disaster effects a large area of the world - meteorite impact, earthquake, eruption - Georges Cuvier the idea that earth has been affected in the past by sudden, short-lived violent events, possibly worldwide v. ** James Hutton ** - found tilted rocks (not flat) but the rocks on top were flat (expected) called this an unconformity 1. recognized unconformities within layers of rocks and the igneous origin of granite (granite cooled from molten magma) before people thought all rocks came out of the ocean 2. proposed concept of uniformitarianism (present is the key to the past), all of these changes are constant past and present rather than a catastrophe that happens now and than (SMALL CHANGE CONSTANTLY ALTERING EARTH) 3. came into conflict with Abram Werner and diluvialism and catastrophism 4. thought that the earth was extremely old 5. Charles Darwin, Charles Lyell was a supporter 1. early ideas of stratigraphy (study of rock layers) 2. fossils 3. relative age of rocks - James Hutton’s theory on unconformity got him thinking about how one event could cause the rocks to be tilted on the bottom and flat on the top. Instead he came up with an alternate theory: i. deposition of older strata (Silurian) ii. deformation of strata in mountain building event, the rocks were deep and hot and folded iii. erosion to produce surface of unconformity, removal of top layer from wind, ice, rain (makes an unconformity) due to erosion iv. deposition of younger strata/sediments (Devonian) v. uplift, tilting, erosion - John Playflair, geologist who was interested in the concept of deep time and noticed that the older primary strata was deformed than the second layer was unconformity: missing information gap in the geologic record and the secondary strata was flat - Unconformities in Ontario: Marmora THE GREAT UNCONFORMITY i. iron rich rocks on the bottom (Precambrian, 1 Ga, billion) ii. red line in the unconformity surface iii. on top is younger rocks (Paleozoic, 450 Ma, 450 million years old)  completely different aged rocks sitting on top of each other no in between , there probably was rocks that were ages in between but they are gone because of erosion - another great unconformity in Ontario i. angular unconformity 500 million missing years - the igneous origin of granite (Charles lyell) i. limestone (sedimentary) was there first deep underground ii. hot magma intruded in between the layers of limestone proving granite was liquid - the industrial revolution i. great demand for resources and geologists - same thing was happening in Canada because they wanted to build a railway from coast to coast which is now known as Canada i. need for coal (eastern provinces, Maritimes, nova scotia) ii. formed in situ (in place) from plant debris growing in swamps iii. confirmed by discovery of in situ lithified tree trunks in coal seams proves how the layer of coal was formed ** layer 1 rocks are under the layer 2 rocks** when we see an unconformity we see the contact between the layer 1 and layer 2 rocks what are fossils and what can we learn from them? - a fossil may be any evidence of life in the geologic past - i. Body fossils = direct evidence e.g. bones, shells - ii. Trace fossils = indirect evidence e.g. tracks, burrows, resting, coproliths - fossilization is very unusual because decomposition usually occurs - usually an animal or plant is eaten or decomposes - hard parts: bones, teeth, horns, shells or wood may survive - soft-part preservation is extremely uncommon  instances when this can occur is due to permafrost, amber, dehydration, peat exquisite fossils found in fossil lagerstatten - finer the grain size of the sediments the better for fossils - the use of index fossils for identifying relative age of strata and gaps (disconformities) - some species go extinct so if you have find that species in a rock layer you know approximately when it lived, and correlate that rock layer with rock layers around the world - the best index fossils are (geologic) short lived and lived in a large area (wide geographic range).. why? i. if you had a creature that lived for a long time it wouldn't be useful to figure out how old a rock is because you would find it, it many rock layers ii. law of faunal succession: you can use the fossils to tell how old the rocks are in comparison to one another iii. volcanic ash can also be used to give relative ages 4. absolute age - Arthur Holmes: figured out how to take radioactive isotopes that occur naturally in rocks to tell how old the rock was (isotopic age dating/radiometric age dating) th absolute ages – impossible for early geologists (until the 20 century) - early geologists were able to determine only the relative ages of the various parts of the earth. For this they were using several principles: uniformitarianism, original horizontality, superposition, original continuity, cross-cutting (helps you decide what happened first and last) if you want to cut across it the thing that you are cutting but have existed first - first thing that happens is you have the sea, sediments are deposited there by rivers and they pile up - law of continuity: sediments lay down flat over a large area - igneous intrusion comes in (magma is younger, how do you know? 1. It cuts across the other layers 2. Contact metamorphism, magma is so hot that is makes the rocks around the edges) - tilted and eroded because they are buried deep down, plate tectonics pushed them together causing them to fold - making an unconformity surface - dike, magma that cuts across the layers before, the dike is the youngest because it cuts across everything 5. the geologic timescale - the construction of the geologic time scale was initially bases on relative age determinations of sedimentary rock units by using the principles of stratigraphy and on correlations of widely separated units using fossils. Until the 20 century only the relative ages were known ** why did physicist not know the earth was old because they didn't know anything about radioactive decay so they didn't know that more heat was being generated inside the planet - radioactive decay makes the process of cooling a lot longer add more heat - radioactive isotopes are unstable, they undergo radioactive decay, which converts them into a different element - radioactivity: the spontaneous emission of radiation, generally alpha or beta particles often accompanied by gamma rays, from the nucleus of an unstable isotope - radioactive dating of the earth: daughter products from the parents at characteristic rates of decay (half lives) - ratio of daughter products to remaining parent material tells how much time has elapsed - radioactive decay is predictable - carbon 14  nitrogen 14 half life of 5730  helpful for dating things archeology wise, cant use it to date things that are very old or that doesn't have carbon in it - uranium 238  lead 206 half life of 4.5 billion years  used to date volcanic rock, couldn't use it to date the dead sea scrolls or dinosaur bones - usually they use three or four age dating so that they have agreement - isotope: different versions of an element having the same number of protons but different number of neutrons (same number of protons as the regular element but a different number of neutrons) - zircons are found in metamorphic rock (gneiss) and can help with age dating because they have growth rings so they can tell use about events such as:  being heated, cooled, under went pressure - the concept of half life (divide by 2): the amount of time necessary to reduce the number of parent atoms by 50% from the original number - parents decay to daughters: i. start off with 100 parents after half life you have 50 parents left, the other 50 broke down into daughters, ii. after another half life you have 25 parent atoms and you have 75 daughters - radiocarbon age dating is used to date organic material as old as 75,000 (75 ka) years (not very old). Corrections are needed for changing solar flux and variations in amount of C-14 produced in atmosphere  it would be good for dating the dead sea scrolls, trees, woolly mammoth, human remains - Precambrian eon takes up most of geologic time: hadean, archean, Proterozoic - Phanerozoic eon is not: Paleozoic, Mesozoic (dinosaurs), Cenozoic 4,600,000,000 million years ago is the same as 4.6 billion years ago 65 million years ago is when dinosaurs were living - Layer 1 (basement rocks): - Precambrian in age - Generally more than a billion years old - Gneiss (metamorphic) and (squished) metasedementary, subjected to a lot of pressure - Formed when the core (craton) of north America was being formed by old collision - Plate tectonics causing mountains to form: layer 1 is the product of the collisions - The black line on top of layer 1 (erosion surface) is the great unconformity -
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