Week 6 Notes
Week 6 Notes 80176 - GEOL 1010 - 001
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80176 - GEOL 1010 - 001
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This 10 page Class Notes was uploaded by Sarah Canterbury on Friday February 19, 2016. The Class Notes belongs to 80176 - GEOL 1010 - 001 at Clemson University taught by Alan B Coulson in Fall 2015. Since its upload, it has received 20 views. For similar materials see Physical Geology in Environmental Science at Clemson University.
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Date Created: 02/19/16
Lecture 10—2/16/16 Geology in the News Tin cans found embedded in the walls after Taiwan earthquake Shows that buildings weren’t made from reinforced concrete like they were supposed to Part 1- Geologic Time Why do we care? When did certain things occur? Even non-scientists wonder Dating Methods 2 approaches Relative: put things into a sequence of events Which came first and which came last No hard numbers Before, during, or after? Absolute: a precise number of when something occurred No longer comparing the date Why do relative dating? Seems outdated and we are accustomed to knowing numbers for anything Absolute dating is very expensive and requires difficult to use lab equipment Sometimes don’t even need the absolute date Relative dating is relatively free Relative Dating Fossils: any evidence of past life on earth Really found just in sedimentary rock People have been finding fossils Stratigraphy: study of strata (layers of sand or sedimentary rock); people started studying strata so they could better understand why the fossils were found there Ideal: find layers and layers representing Unconformities: breaks or gaps where time went by, but you don’t have any rock to represent it Why? 1. run out of sediment: if you don’t have sediment, you won’t have evidence 2. run out of accommodation space: if the basin is where sediment is deposited is full, you can’t put more sediment there 3. start eroding sediment: sediments that have been deposited are getting eroded faster than they’re being deposited Types of Unconformities 3 types, classified by comparing data above and below the gap 1. Disconformity: when there is one type of sedimentary rock on one side, and a different type of sedimentary rock on the other side 2. Non: sedimentary rock on one side and a different type of rock on the other side (not sedimentary 3. Angular Disconformity: rocks below unconformity are tilted up at an angle, while rocks above the disconformity are horizontal Several steps involved Long time Layers are formed, then folded, then eroded, then more layers form on top horizontally Problems 1. Identification: it’s very difficult to spot an unconformity 2. Duration: don’t know how much time was lost Does gap represent 100 yrs or 100 million yrs? Part 2- Stratigraphic Principles 1. P. of Original Horizontality: when you first make sedimentary layers, they’re going to be horizontal Makes sense: gravity won’t let them form at an angle without anything to support it When there are layers at different angles, it’s due to something that happened later on 2. P. of Superposition: when looking at a stack of layers, oldest is on the bottom and youngest is on the top; in between is a progression of time 3. P. of Cross-Cutting: whatever did the cutting is the youngest When a fault line intersects or cuts through rock layers, the fault line is younger than the rock it cuts through 4. P. of Faunal Succession: when going through a bunch of strata, fossils will be found in a very specific order Younger fossils will be found in younger rock, and older fossils will be found in older rock Useful for comparing different layers in different areas Correlation with fossils Not all fossils are great for correlation Want to identify short spans of time Therefore, big fossils like dinosaurs and saber tooth cats aren’t very useful Want to find fossils that give evidence of something that is more specific Index fossils: any species that is really good for correlation 1. numerous: want a big population 2. widespread: global distribution would be awesome 3. went extinct quickly: want small units of time 4. are easy to identify: don’t want to get hung up on whether or not fossil was identified correctly Other correlation tools Fossils aren’t the only way to correlate Lithostratigraphy: correlate areas based on having the same rock types Can have many glitches depending on what type of rock is found Can have problems with unconformities Sequence Stratigraphy: correlation based on patterns of unconformity Works best with a lot of unconformities present, and near the shore (due to changes in sea level) Chemo stratigraphy: can use chemical signals from the rock Have a wide range of options Look for certain chemical signals such as a certain isotope, trace metals, elements Ex) iridium anomaly at the Cretaceous-Tertiary boundary Really only find iridium in high impact and in space, implies a meteorite Magnetostratigraphy: look at magnetic record within the rocks Correlate by looking for magnetic patterns that are similar between two areas Difficult- can be like comparing two bar codes until finding a match Part 3- Geologic Time Scale Originally built via stratigraphy Fossils were key for defining boundaries Eons are subdivided into eras, which are subdivided into periods, etc Geologic Time Units Eons Only 3 or 4 recognized 1-Hadean 4.5-4.0 Ga (disputed) Everything was in its molten state, when denser material Almost no material on the surface to study this 2-Archean 4.0-2.5 Ga First evidence of continental material Still didn’t have oxygen 3- Proterozoic 2.5 Ga- 550 Ma Oxygen is finally present 4- Phanerozoic When fossil record shows up 3 eras 1- Paleozoic (550Ma-200Ma): Cambrian Explosion, when you see a huge diversity of life 2- Mesozoic (200 - 65 Ma): Dinosaurs 3- Cebozoic (65 Ma - now): mammals are dominant Lecture 11 – 2/18/16 Geology in the News New info why quakes occur deep in subduction zones Scientists confused on how subduction zones can move under so much pressure Water released from a mineral called lawsonite enables the fault to move despite the high pressure environment Part 1- Absolute Ages Two approaches: - Non-radiometric - Radiometric Non-radiometric Methods 1) Varves: really thin alternating bands of light and dark sediment Need winter temperatures to get cold enough to get the top of the lake to freeze Light layers are what have been deposited during the summer Dark layers are what have been deposited during the winter One light band + one dark band = one year Uses: Tell you about the history of that area A stretch of no varve represents winters that weren’t cold enough to freeze lake Restrictions: Only tells you about that immediate area Can’t take that data and use it for the surrounding areas Easy for layers to get disturbed and mix together Organisms living in the sediment would disturb the varves 2) Dendrochronology: counting the growth rings inside of trees Dark area on ring is when that ring stops growing How many rings represent how many years Useful on a local scale, but hard to expand it to another area Uses: Find trees that overlap in age so that can go father back in time By overlapping trees, can go back almost Restrictions: Identify species because some species don’t have annual growth rings (some form one every few years or some form several times per year) Why do trees vary in growth rings? Different climates, some trees are susceptible to environmental stresses (not enough water, forest fires) Radiometric Dating Use of radioactive materials for dating specimens Isotopes: atoms of the same element, but with different numbers of neutrons Most isotopes are stable, but some are unstable Radioactive decay: atoms emitting particles and energy to stabilize that atom Radiation: combination of energy and particles that are being emitted Parent atom: unstable atom that you start with Daughter atom: atom that forms after radioactive decay of parent atom Sometimes the daughter is still unstable and will have to undergo radioactive decay again Decay Series (aka chain): having multiple radioactive daughter atoms until a stable one forms Misconception: watching atoms ‘pop’ The rate of radioactive decay is actually constant There’s no way to tell when a specific atom will go through decay, so you can’t focus on one particular atom and wait for it to decay Rate of decay of the isotope can help us figure out time it takes to decay Half-life: amount of time it takes for half of parent atoms to decay into daughter atoms Follows exponential curve- won’t reach zero Each radioactive isotope has its own half-life - U = 4.4 billion yrs (almost age of the earth!) 210 - Pb = 22.3 yrs If you’re measuring something that happened quickly, you’ll need an isotope with a small half-life, and vice versa Half-life does not vary with any environmental factor! Crucial because it’s what’s allowing us to measure time in our universe Requirements: Must always think about your specimen 1- Radioactive isotopes must be present in your specimen If it doesn’t contain radioactive isotopes, that we can’t use radioactive dating 2- Need measureable amounts of the parent & daughter in your specimen Without parent, radioactive decay has stopped Without daughter, radioactive decay hasn’t started yet 3- Can only go so far back in time Problem: the parent eventually runs out After one half-life, 50% is already gone After two, 25% is gone, etc. 4- Closed system Your material is not exchanging anything back and forth with its environment Problems with open system: Add parent or lose daughter, will look like little radioactive decay has happened, and vice versa Part 2- Case Study: Carbon 14 Only radioactive carbon isotope 1C 1N + particle + energy Half-life = 5730 years Track through 10 half-lives, which is an exception to the general rule that you can only track through 5 Can’t apply carbon dating to everythingQ: Earth is older than 57,000 years, so why haven’t we run out of C?4 Carbon is always forming on Earth Forming Carbon 14 Global distribution- carbon is found all over Earth Reactions are in a steady state: amount we are creating is the same as the amount we are taking down Commented [A1]: How do we know it works? Radiometric techniques are developed by first dating things that we already know the age of to Ex) Egyptian mummies- we have detailed historical records that state when certain people died, so we know how old the mummy is. So we test to see if carbon dating works by testing it for things we already know the age of Can’t carbon date everything Carbon dating is horrible for dating rocks, but great for dating organic material such as fossils Why can we date fossils? 14 14 In atmosphere: C + O2 = CO2 14CO2 incorporated into food chain Calculating the age Age = [ln(N /N 0)/-0.693]*half-life N f/N0 = % of 14C in the sample relative to amount of food in living tissue - How much parent is left Assumptions for Carbon-14 1- System remains closed after death Is this a good assumption?- No, but there are ways to double-check this assumption 2- Amount of 14C in living tissue doesn’t vary Problem 1: production of carbon varies over time Variations are small and on short timescales Over a long time scale, the amount of 14C will be the same Problem 2: fossil fuel burning has changed the relative amount of the carbon isotopes in the atmosphere The method has been corrected to compensate for changes Finding a problem with a method doesn’t invalidate a method Limit of Carbon-14 Dating Some organisms don’t get carbon from the atmosphere or the food chain, which can change the dating Can’t just date anything! Think about your specimen!
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