Lecture-15-F10 - EAS 111 General Announcements 10/25/10 •...

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Unformatted text preview: EAS 111 General Announcements 10/25/10 • Schedule Reminder • Today, Geologic Time II – Plan Ahead! • Lecture Exam II – 11/1/10 (Monday) – review next time, 10/27/09 (Wednesday) • Lab Exam II – During your regular lab time, staring 11/2/09 (same week as Lecture exam) – That means review lab materials THIS WEEK, after the geologic time lab. – Lecture review sheets are posted. – Office Hours Wed 1-2 (12?-2) Earth Science News The Oldest Intact Rock Ever Found? •4.28 Billion! •Previous was 4.08 Ga •Oldest mineral grain is 4.36 Ga •Shore of Hudson Bay •Implies a widespread solid crust earlier than we had thought •Plate tectonics active by 3.8 Ga Earth and Atmospheric Sciences 111 Fall 2010 Geologic Time II -Correlation and the Fossil Record -Radiometric Dating (Absolute Dating) •Putting it all together -Lecture 15 Geologic Time Scale 09.08.a1 Geologic Time: How It Works • Relative Dating – Understand what happened in which order • Correlation – Connect the pieces of the geologic record together to construct a coherent and complete history for all areas of the planet • Absolute Dating – Determine the numerical age of an event or rock using laboratory methods A C F A B C D F V L B F G C G N A R D R R K S L D G Correlation • Connecting records at one place to simultaneous events at another – Can be done with the fossil record – the most widespread and best tool – can also be done with widespread rocks from large, rapid events – major volcanic eruptions, meteor impact ejecta, etc. – is necessary to reconstruct the fragmentary and widely distributed geologic record into a coherent history How Are Fossils Preserved? Shells/hard parts Bones Replacement Cast or mold Amber Thin carbon film Impressions Constructed feature 09.05.a How Fossils Vary with Age Mammals and Mammals grasses grasses Dinosaurs Dinosaurs and flowering plants plants Crinoids, coral, Crinoids, clams, certain fish, plants, insects, and amphibians amphibians Simple Simple creatures and fossils, such as stromatolites stromatolites 09.06.a Boundary based Boundary on mass extinction (dinos and others) Boundary based Boundary on major mass extinction called the Great Dying Great Boundary based Boundary on widespread appearance of hard-shelled organisms organisms Geologic Time Scale Older subdivisions of Cenozoic of 09.08.a1 How Fossils Change in Sequence of How Sedimentary Rocks Sedimentary Can use overlap of two fossils to constrain age Some spanned long times Some species lived short time Some span so give narrow age range span Fossils change up section Fossils in systematic way in Faunal Succession 09.07.a1 There are some mis-matches, like between Cof theFsame These two sections of rock contain many and on the These left and A and F on the right. This surface is andashed fossils. Match the two sections, envisioning unconformity. What does this signify? lines connecting places where the two sections A.a correlate (i.e., represent the same time). buried layer of meteorite impact debris correlate B.a buried layer of oil shale C.a buried erosional surface D.a special kind of buried fossil 09.07.b1-2 Comparing Partly Overlapping Sections Comparing Observe how these rock sections correlate and how the thicknesses of some units vary 09.07.b3-4 Using both sections, we can reconstruct a more complete section Absolute Ages • Producing a numerical age for rocks and events – Typically done with a wide variety of radioactive materials in many different rock types – Can also be done by matching well-dated magnetic reversal patterns – Can also be done by stable isotopes, especially those of oxygen in ice and water – Many other methods as well, especially with surface exposures, or now-buried surfaces. Common Radioactive Decay Series Potassium (K) Potassium decays to argon (Ar) decays Rubidium (Rb) decays Rubidium to strontium (Sr) to Dating a rock by several methods Carbon-14 (C) decays Carbon-14 to nitrogen (N) to 09.02.c Thorium (Th) and uranium Thorium (U) decay to isotopes of lead (Pb) lead How Does Dating by Radioactive Decay Work? 09.02.a Before decay, Before unstable parent atoms parent Example Example for 1000 atoms atoms Half the parent atoms Half decayed to daughter atoms (time = half life) atoms Before Any Before Decay Decay After a second half After life, only ¼ parent atoms remain atoms After Two After Half-Lives Half-Lives 250 750 After One After Half-Life Half-Life 500 500 Atoms of Atoms Parent Parent Atoms of Atoms Daughter Daughter 1,000 0 CPS: So how old is this rock? In a lab you measure that you have 100 unstable parent atoms left from an original concentration of 800. The half life of this decay series system is 5 million years. How old is this rock? A.5 million years B.10 million years C.15 million years D.20 million years E.we can’t tell 800 -> 400 -> 200 -> 100 = 3 half lives 3 h.l. x 5 million years each 15 million years! What Can Isotopic Ages Tell Us? 09.02.c Age of eruption Age of solidification Age of meta. event When rock cooled Age of source Age of sediment of Age of recent Age sediment sediment Where Age of Earth Comes From Age of Age meteorites meteorites Dated Moon Dated rocks rocks Oldest dates on Oldest Earth rocks Earth 4.55 billion 4.5 billion 3.9 to 4.3 b.y. (rock) 3.9 to 4.4 b.y. (grains) to Data from astronomy on age of Data Solar System and Universe Solar 09.09.d Correlating Correlating Units and Events Events Observe these Observe two sections, noting how each layer correlates or doesn’t from one section to another another The radiometric ages for the Can use: basalt units don’t match exactly. • Rock This Rock type because: is probably A. They are actually • Position in sequence Position unrelated events • Fossils Fossils B. This is just experimental errorNumeric agesmethod in the dating • Numeric C. It took 2 million years for Magnetic sequence Magnetic the •lava to flow between these locations D. The burial process has altered the radioactive material 09.12.a1 Assigning Numeric Ages to Timescale Date a volcanic layer Date a dike or clasts 09.08.b Bracket fossilbearing bed by bearing dating volcanic units dating Using Timescale to Assign Numeric Ages Use fossils and timescale Use to assign numeric age to Bracket using fossil ages from timescale timescale 09.08.c Bracket age using fossil Bracket and numeric ages Geologic Time: How It Works • Relative Dating – Understand what happened in which order • Correlation – Connect the pieces of the geologic record together to construct a coherent and complete history for all areas of the planet • Absolute Dating – Determine the numerical age of an event or rock using laboratory methods Geologic Time Scale 09.08.a1 EAS 111: Assignments for 10/2711/1/10 • Study for Exam II and Lab Exam II •Review sheets and Before You Leave this Page boxes EAS 111: Assignments for 11/3/10 • Exploring Geology • 13.1 section C only • 13.5, 13.6, 13.12, 13.11, 13.10 (this order will make the most sense I think) • The basics of global climate and climate change through geologic and human time scales – Watch for additional readings for 11/3/10 ...
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