Geol Lectures 8 & 9
Geol Lectures 8 & 9 Geol101
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This 155 page Class Notes was uploaded by Shelby Green on Sunday February 21, 2016. The Class Notes belongs to Geol101 at Clemson University taught by Dr. Coulson in Spring 2016. Since its upload, it has received 42 views. For similar materials see Physical Geology in Geology at Clemson University.
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Date Created: 02/21/16
Announcements Geology in the News • Tin cans found within the walls of collapsed buildings after Taiwan earthquake Lecture 8- Geologic Time • PT 1- Relative Dating Techniques • PT 2- Absolute Dating Techniques • PT 3- Case Study: Carbon-14 Dating Geologic Time • Why do we care? Dating Methods • 2 approaches: • Relative-just trying to put together a sequence to know what came 1 , 2 , and 3 rd – Qualitative method (ex: dinosaurs went extinct before humans were on earth) – Essentially free (cost effective) and easier • Absolute-putting numbers on actual events – (ex) dinosaurs went extinct X# million years ago – Can be expensive and more difficult – May not be all the important PT 1- Relative Dating • Fossils: – Any evidence of past life forms • Footprint, plant impression, sea shell, bones, etc Relative Dating • Stratigraphy: subset of geology (predates geology) study up strata or layers of rocks – Fossils mainly found in sedimentary rock Stratigraphy • Unconformities: Breaks or gaps in time record – Not all the time periods are represented in the layers of rocks (ex: layers on the right) – Ex on left not likely 8 Ma 9 Ma 8 Ma 10 Ma 10 Ma Unconformities • Why is there a gap? • 1) Run out of sediment deposit so no other layers are formed during that time Unconformities • Why is there a gap? • 2) Run out of accommodation space – Basin becomes full Unconformities • Why is there a gap? • 3) Start eroding sediment: – eroding sediment faster than it is being deposited • Losing layers that used to be there Types of Unconformities • 3 types, classified by comparing strata above & below the gap • 1) Disconformity-sedimentary rock on one side of gap and a diff type of sedimentary rock on the other shale Siltstone sandstone Types of Unconformities • 2) Nonconformity- sedimentary rock one side of the gap and on the other side there is either igneous or metamorphic rock – Study tip: Nonconformity means they’re not the same shale basalt See file 8c Types of Unconformities • 3) Angular Unconformity- • Layers underneath gap come up and strike gap at an angle • Several steps involved – Takes a long time Unconformities • Problems w/ unconformities: • 1) Identification: hard to find gap Unconformities • Problems continued • 2) Duration: hard to find out much time is last Next Topic: Stratigraphic Principles • 1) Principle of Original Horizontality: layers of sediment originally form as a horizontal line Stratigraphic Principles • 2) Principle of Superposition: when looking at a stack of layers, the one on the bottom is the oldest and the layers above get younger youngest In-between oldest See file 8b Stratigraphic Principles • 3: Principle of Cross-Cutting Relationship: whatever did the cutting is youngest The fault (black line) is younger than all the rock layers it cuts through What is the Youngest Feature? Answer: E Stratigraphic Principles • 4) Principle Faunal Succession: fossils seen in different layers are always going to occur in a set pattern – The older rocks will contain the oldest fossils and vise versa Woolly mammoth T. rex Archaeopteryx Dimetrodon Faunal Succsession • Correlation: linking to places together to see how old the rocks are in relation to one other • Q: which area has the oldest rocks? South Carolina Texas Faunal Succsession • Very powerful tool • Correlation found via faunal succession Woolly mammoth T. rex T. rex Archaeopteryx Archaeopteryx Dimetrodon Correlation with Fossils • Not all fossils are great for correlation – Little inconspicuous fossils tend to be better for correlation • Want to identify short spans of time: – more precision = stronger correlation Correlation with Fossils • Index (or guide) fossils: ideal fossil to use for correlation • 4 qualifications to be index fossil: • 1) Were numerous – A lot of them don’t get fossilized, so a bigger population increases chances of fossils Correlation with Fossils • 2) widespread – The more places it lives in, the more places that can be correlated together Correlation with Fossils • 3) Went extinct quickly – “Die as past as you can.” – Precision = stronger correlation • Cockroaches are bad index fossils because they’ve been around for like ever Correlation with Fossils • 4) Are easy to identify – To insure your comparing the same fossils Other Correlation Tools • Same places don’t have index fossils to use for correlation so there are other techniques that don’t rely on fossils • Lithostratigraphy: correlations based on rock types – Decent at starting correlation but too many underlying factors to finalize correlation sandstone sandstone breccia sandstone breccia Other Correlation Tools • Sequence Stratigraphy: correlation based on pattern/sequence of unconformities – Beneficial to oil and gas industry b/c costal areas are ideal for sequence stratigraphy Other Correlation Tools • Chemostratigraphy: chemically correlate areas – Ex- Iridium anomaly at the Cretaceous-Tertiary boundary •must have been a causehave a massive amount of iridium so that Other Correlation Tools • Magnetostratigraphy: magnetic signature that gets recorded in rock layers – A little rough on the eyes • Like trying to take two bar codes and slide them around until you can find matching black and white strips Geologic Time Scale • Originally built via stratigraphy • Fossils were key for defining boundaries – 4 billion years not accounted for in the right diagram Geologic Time Units • Eons • Only 3-4 recognized Eons • 1) Hadean: 1 billion years of the earths existence • 4.5 Ga – 4.0 Ga (disputed) – Density stratification period – Earth’s earliest atmospheres/oceans form Eon • 2) Archean: continent building • 4 Ga – 2.5 Ga (about 1/2 billion years long) Eons • 3) Proterozoic – Oxygen build up in the atmosphere for the first time • 2.5 Ga – 550 Ma Eons • 4) Phanerozoic: current eon we’re living in – Fossil record is at its best • Divided into 3 eras Phanerozoic Eras • 4a: Paleozoic (550 Ma – 200 Ma) translates to“ancient life” • Cambrian Explosion-a sudden burst of new organisms on the scene Phanerozoic Eras • 4b) Mesozoic – Dinosaurs • 200 Ma – 65 Ma Phanerozoic Eras • 4c) Cenozoic – Mammoths becomes dominate species – Ice age • 65 Ma - now Review- Things to Know • Relative vs absolute dating • Unconformities • Stratigraphic principles • Index fossils • Eons & eras Announcements Geology in the News • New info on why quakes occur deep in subduction zones • Water released from mineral called lawsonite reduces friction and enables the fault to move despite the high pressure environment Lecture 8, PT 2- Absolute Ages Two approaches • Non-radiometric: involve zero radiation • Radiometric: involve radioactive materials Non-radiometric Methods • 1) Varves: type of sediment deposits – Thin layers that alternate light and dark bands – Only form under certain conditions: • Lake deposits if the lake is cold enough to freeze over near the top – Light color normal deposit during warmer months – Dark color sediment cant get in lake while its ice over, so organic rich molecules floating in the water get deposited • Counting bands will tell you how long the climate in the area has met the proper conditions Varves • Uses Varves • Restrictions: – Only form under those conditions – Only form if nothing disturbs the sediment Non-radiometric Methods • 2) Dendrochronology: – Counting the growth rings in trees • Each ring one year • Dark rings: Tree had stopped growing Dendrochronology • Uses: learn about area and its environment – But trees only live so long… • So find trees that overlap/correlate in age, and the time period can be extended further back Dendrochronology • Restrictions: – Some species of trees don’t form rings yearly • Sometimes 3 or 4 rings a year • Some 1 every 2 years – Forest fires can stop the trees from growing – Climate an affect if the tree grows quickly or slowly Radiometric Dating • Use of radioactive materials for dating specimens Radiometric Dating • Isotopes: atoms of exact same element with different number of neutrons present Grey = protons green = neutrons Radioactive Isotopes • Radioactive isotopes: neutrons are weighed unstable • Radioactive Decay-radioactive atoms will break down to become stable – Release energy (radiation) • Radiation-emitted energy Radioactive Decay • Parent: before decay • Daughter: after decay • Can have multiple daughter isotopes Radioactive Decay • Decay Series (aka chain)-multiple steps/daughters of decay to become stable stable Next Topic: Radiometric Dating • Misconception- Watching atoms ‘pop’ /decay • The rate of radioactive decay is constant. – No way to predict when atom will decay • Ex) which popcorn kernel will be first to pop? – Rate of decay is known: • overall it will take 3 minutes for popcorn to pop See file 8d Units of Radiometric Dating • Half-life: the time it takes for half the current atoms to decay • Parent/2 % of parent material left (1.0 = 100%) Time measured in half-lives Half-Lives • Each radioactive isotope has its own half-life – 23Th = 24.1 days 210 – Pb = 22.3 yrs – 23U = 4.4 billion yrs Half-Lives • Half-life does not vary with any environmental factor! Requirements • Must always think about your specimen • 1- Radioactive isotopes MUST be present in your specimen to radiometric date Requirements • 2- Need measurable amounts of both the parent & daughter atoms in your specimen • Why? Just one or the other will not allow measurement of the passage of time – Measure before decay process is over or after decay process starts Requirements • No parent = clock stopped running – Decay process is over and the atom is stable Requirements • No daughter = clock hasn’t started Requirements • 3- Can only go so far back in time • Usually 5 is max number of isotopes • Problem: the parent eventually runs out – After 1 HL = 50% parent left – 2 = 25% – 3 = 12.5% – 4 = 6.25% – 5 = 3.125% • Ex:cant carbon date dinosaur bones b/c carbon half life is too short to measure back X million years ago. The carbon that was in bone is long gone Requirements • 4) Closed system: not interactive with its surroundings – Nothing added or subtracted • Don’t want parents or daughter isotopes to be added or subtracted Requirements • Problems w/ open system: • Add parent or lose daughter- looks like little time has passed Requirements • Lose parent or add daughter- looks like lots of time has passed PT 3- Case Study: Carbon 14 • 14C 14N + particle + energy – When carbon decays to nitrogen14 • Half Life of C14 =5,730 years • Can track through 10 half-lives • Extremely sensitive • Carbon has 3 isotopes: carbon14 is to only radioactive one Forming Carbon 14 • Q: Earth is older than 57,000 years…. why 14 haven’t we run out of C ? 0 14 14 + • Cosmogenic ray + n + N C + p • This reaction makes C14 *Some isotopes have gone extinct tho Forming Carbon 14 • Global distribution: atmospheric reaction so happens all around the planet • Steady state: two reactions seen are in a steady stage (amount decaying=amount produced) How Do We Know It Works? • Radiometric techniques are developed by first dating things that we already know the age of. – Ex- Egyptian mummies What Can We Date with C-14? • Cannot date everything – Can only go back so many years – Can only carbon date certain materials • Doesn’t work for rocks/minerals • Works for fossils (ex: mammoths) Why Can We Date Fossils? 14 14 • In atmosphere: C + O = C2 2 • 14CO incorporated into food chain 2 • The plant has carbon14 from the 14carbon dioxide its filtering, the bunny eats the plant and is exposed to C14, and same with the fox. • When the animal dies, it becomes a closed system because he’s not adding anymore parent atoms from eating • Since rocks don’t eat, the don’t have C14 Calculating the Age Decay Equation • Age = [ln(N /N ) / -0.693]*half-life f 0 • N fN =0% C in the sample relative to amount found in living tissue – i.e., how much parent is left • Ex: fossil still has 10% of its C14 • Age = [ln(0.10) / -0.693) * 5,730 • Age = 18,940 years old How old is this fossil? 14 • Mammoth tusk w/ 7% of the C still present • Age =(ln.07/-.693)*5,730=21,987.8 Assumptions for Carbon-14 • #1- System remains closed after death • Is this a good assumption? – No – Extra tests required to ensure specimen was in a closed system Assumptions for Carbon-14 • #2- Amt of C in living tissue doesn’t vary through time – i.e., the starting amount is always the same • There are 2 problems with this assumption… Checking the Assumptions • 1 problem- 1C production varies over time • Variations are small & on short timescales – Dating a mammoth tusk, fluctuation makes estimate off by 3 years = no big deal • Source of C14 varies based on solar cycles – The sun’s activity can fluctuate the radioactivity on earth Checking the Assumptions nd • 2 problem- Fossil fuel burning has changed the relative amount of the carbon isotopes in the atmosphere – Ratio in atmosphere of how much carbon in the atmosphere is now, and was back then – Doesn’t affect anything b/c method has been changed Resolving the Problems • Note- Finding a problem with a method does not instantly invalidate the method! • These two ‘problems’ were solved long ago Limit of Carbon 14 Dating • Some organisms don’t get carbon from the atmosphere or food chain • Ex: use dissolved CO i2 water that contains little-no C Ex- Zombie Clams • Headline: “Scientists claim living clam died 30,000 years ago” • Can’t date just anything! Think about your specimen! Review- Things to Know • Varves • Dendrochronology • Radioactivity & isotopes • Half lives • Requirements for radiometric dating • Carbon-14 dating Announcements • Exam 2 Geology in the News Lecture 9- Climatology • PT 1- Climate Basics • PT 2- Controls on Climate • PT 3- Recording Climate PT 1- Climate Basics • Why do we care? • Climate- • Weather- System Interactions • Interactions among all these components are complicated to untangle • Feedbacks- Positive Feedback • Every time you complete the cycle, you repeat the cycle When A increases… B changes... which causes A to increase again…. • Ex: temp melt ice temp more Positive Feedback • ‘A’ can DECREASE every time you come around the cycle too! • Ex: lower the temp = more ice forms = temp drops again • Hard to break out of Negative Feedback • When you complete the cycle, the next time through you do the OPPOSITE of what you did the first time – 1- When A increases, B decreases – 2- When B decreases, A decreases – Next time through the loop: – 3- When A decreases, B increases – 4- When B increases, A increases (back to step 1) Negative Feedback • Ex: you do poorly on exam 1, so you start to study more. You then do better on the exam 2, so you start to slack off, and thus do poorly on the exam 3. • Creates a stabilizing see-saw effect *Study Tip* • Be able to tell the difference between positive and negative feedbacks PT 2- What Controls Climate? • Insolation- • Several things affect how much insolation earth receives 2a- Variations in Insolation • Aphelion- – 152 million km • Perihelion- – 147 million km Variations in Insolation • Milankovich cycles- • 1- Eccentricity- • ca 100,000 yrs Milankovich Cycles • #2 Obliquity (Tilt)- • ca 41k yrs See file 9b Obliquity • Obliquity is why we have opposite seasons in the N and S hemispheres Obliquity • Seasonal Contrast- • High Obliquity angle = high seasonal contrast • Low Obliquity angle = low seasonal contrast Milankovich Cycles • #3 Precession- http://starchild.gsfc.nasa.gov/Videos/StarChild/precession.mov Precession • Affects what time of year you experience each season NH winter NH summer NH summer NH winter 2b- The Atmosphere • Albedo Atmospheric Gases • Atmo- Composition: – Nitrogen- 78% – Oxygen- 21% – Greenhouse gases (GGs) See file 9b2 Greenhouse Effect • Works b/c wavelength is changed Greenhouse Effect • They’re present in such small amounts.... are they really a big deal? Insolation Changes w/ Latitude • Why are the poles colder than the tropics? Latitudinal Insolation Distribution • Must redistribute the energy Atmospheric Heat Transport • Hadley cells- 2 2 Atmospheric Heat Transport • Other cells continue moving heat poleward Case Study: El Nino Events • Affect global weather patterns • Caused by changes in wind strength in the equatorial Pacific Ocean Normal Conditions • West Pacific Warm Pool- Normal Winter: • 1- Trade winds push warm water W • 2- The ‘void’ left is filled by cool water upwelling in the E Normal Winter: • 3- WPWP creates low atmospheric pressure • 4- Low pressure systems creates lots of rain (just like the Hadley cell near the equator) Normal Winter: • So in the western Pacific during NORMAL conditions it’s warm & wet • In the eastern Pacific it’s cool & dry El Nino Winter: • 1- Weakening/reversal of trade winds • Quasi-periodicity of 4-7 years • Why do the trade winds weaken? El Nino Winter: • 2- Allows WPWP to migrate eastward • 3- Eastern waters warm, so upwelling stops EN Winter: • 4- Southern Oscillation- • 5- Rain follows the low pressure area EN Winter: • So in the western Pacific during EL NINO conditions it’s cooler & drier • In the eastern Pacific it’s warmer & wetter See file 9c How Does It Affect Other Places? • These changes affect atmospheric circulation (Jet Stream) which results in widespread changes in weather – Ex: During El Nino: SC has wet & cool winter, dry & warm summer Review Things to Know • Why climate is difficult to study • Milankovich cycles • Hadley cells • El Nino system Announcements Geology in the News 2c- The Hydrosphere • Water has rel high heat capacity – Ex- Gulf Stream See file 9d Oceanic Heat Transport • Thermohaline Circulation Oceanic Heat Transport • Why does water sink in the north Atlantic? Oceanic Heat Transport • Slowing in recent decades • Negative feedback loop 2d- The Biosphere • Plants – draws down CO 2or photosynthesis – affects albedo • Animals – release CO2& methane Biosphere • Biological Pump • Interaction of biosphere, atmosphere, hydrosphere, and lithosphere 2e- The Cryosphere • Ice covers ca ~9% of land surface Cryosphere • Most land surfaces albedo ~ 15-25% • Snow/Ice albedo ~ 40-90% 2f- The Lithosphere • Tectonics affects climate in several ways • 1- Continental position Lithosphere & Climate • 2- Continental size Lithosphere & Climate • 3- Collision zone uplift • Rain shadows Lithosphere • 4- Land bridges PT 3- Recording Climate • Instrument & historical records only go back so far Recording Climate • Air trapped in glacial ice Ice Cores • Ice cores from Greenland & Antarctica > 2 miles (3300 m) • Some represent > 1 million years Recording Climate • Proxies- • Different proxies record different aspects of climate on different timescales **Study Hint** • Air bubbles in ice were NOT a proxy • In that case, you were directly studying samples of the ancient atmosphere, so there was no substitute involved Proxy Rules • Always consider the nature of the proxy • Ex: Tree ring formation Climate Proxies • Tree Ring Width Climate Proxies • Biogeography Climate Proxies • Stable Isotopes • Measure as ratio: ex: O/ O16 • Diff atomic weights = diff amounts of ea isotope get incorporated into molecules Climate Proxies • Key- the ratio in some materials changes with climate variables • Ex: at one temperature, you add a certain ration of O/ O to a growing shell…. • …But at a different temperature, you add a different ratio of O/ O6 Oxygen Isotopes • Ex: many invertebrate & plankton shells • Can provide quantitative paleotemperature data Oxygen Isotopes • Ex- O/ O in fish bone reflects water temperature when the fish was alive T = 111.4 – 4.3*(Df – Dw) • Df = ratio in fish bone • Dw = ratio in seawater (= 1.0) • T = water temp (Celsius) • Q: measured ratio in a fossil fish is 20.1, what was the water temp? Other Temp Proxies • Some Trace Metals in Shells • Ex- Mg/Ca • Q: why do we want multiple temp proxies? Stable Carbon Isotopes • 13C/ C • Both isotopes form CO in 2he atmosphere 13 12 – Ex: CO 2 CO 2 • Plants take in both types of CO for 2 photosynthesis Stable Carbon Isotopes 13 12 • The C/ C ratio in each plant depends on the plant’s photosynthetic style • C3 plants vs C4 plants *Study Hint* • The names ‘C3’ and ‘C4’ do NOT refer to carbon isotopes; they refer to types of plants Stable Carbon Isotopes 1CO 2 1CO 2 13CO 2 12CO 2 C3 Plants C4 plants Cooler, wetter climate Hotter, drier climate Ex: forest Ex: savannah/grasslands 13 12 13 12 C/ C = X C/ C = Y Stable Carbon Isotopes C3 Plants C4 plants Cooler, wetter climate Hotter, drier climate Ex: forest Ex: savannah/grasslands 13C/ C = X 13C/ C = Y Animals eat the plants and inherit the plant’s isotopic ratio 13C/ C = X 13C/ C = Y Stable Carbon Isotopes • The C/ C ratio in animals’ skeletons thus reflects the type of plants in the ecosystem • Provides clues to climate Review- Things to Know • How each ‘sphere’ (atmo, hydro, litho, etc) affects climate • Air bubbles in glacial ice • Climate proxies- how they work and what they tell you
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