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GEOL 1002 Week Two

by: Gwendolyn Cochran

GEOL 1002 Week Two GEOL 1002-10

Gwendolyn Cochran

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These notes cover the determinants of sedimentary rocks, how they form, and how the react to different outside forces. They also cover how to date rocks. It entails the details, tools, and rules of...
Historical Geology
Catherine A. Forster
Class Notes
Historical Geology
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This 2 page Class Notes was uploaded by Gwendolyn Cochran on Thursday January 28, 2016. The Class Notes belongs to GEOL 1002-10 at George Washington University taught by Catherine A. Forster in Spring 2016. Since its upload, it has received 28 views. For similar materials see Historical Geology in Geology at George Washington University.


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Date Created: 01/28/16
Week Two Sedimentary Rocks:  Example: Navajo sandstone (Jurassic) ● nearly pure, rounded quartz sand in enormous desert dune deposits ● note the cross beds  Example: Kayenta formations ● siltstone, sandstone, and limestone beds ● ripples at the top indicate running water ● many Jurassic fossils (dinos!!) as well as turtles and frogs The Kayenta formation lies underneath the Navajo sandstone! The kayenta is below, therefore is older. What happened to the environment from  Kayenta times to Navajo times? A loss of the water sources! It has gone from a very wet oasis to a dune  field. Sedimentary rocks can show us changes in climate over time! Types of sediments 1. Bed Load: heavy material that rolls or jumps along the bottom of a body of  water or ground. Think the pebbles that roll on the bottom of a stream 2. Suspended load: sand and rocks suspended in a current; the sand carried  in a current about the bottom of the body of water 3. Ripples: avalanches of suspended loads and bed loads that cause cross  patterns­ or ripple patterns­ over time because of sediments.  How to Date Rocks  We must know the timing of events, order of events, and rates at which  change happened to be able to understand the history of the earth by  studying rocks. We can study rocks through these tools: 1. Relative dating a. determines the order b. Using The Law of Superposition­ the oldests is a the bottom unless it has  been disturbed by: i. deformation: folding + titling of tectonic plates  ii. igneous intrusions: igneous rock can be injected into already existing rocks  through volcanic action   1. dikes­ cut across (an eruption from below all the layers) 2. sills­ go with the strata (lava flow across the top of a formation)  iii. Faults­ a complete break of the rock  iv. The Law of Cross Cutting Relationships: igneous intrusion or faults are  younger than the rock it cuts. Rock has to already be there in order to be  cut. 2.  Absolute Dating­ through radiometric dating T = 1/ λ ln[(d/P) + 1] a. This finds the actual numeric age of rocks b. Radioactive decay: different isotopes of different elements decay at  different rates: finding the ratio of parent to daughter isotopes and applying  the decay constant i. Isotope  ii. An isotope can change from unstable to stable; from parent (locked into the crystal structure of the rock) to daughter, through loss of protons or  neutron. Over time the ratio of daughter to parent isotope grows. They are  both locked into the crystal forever, but decay begins as soon as the crystal is formed c. Radioactive decay happens in a predictable and measurable rate. This is  called the decay constant= λ d. All decay is exponential i. Rb^87 ­ Sr^87  the half life is .693 BY ii. Fl^21 ­ Ne^21  the half life is 4.158 seconds  iii. K^40 ­ Ar^40   the half life is 1.25 billion years iv. U^235 ­ Pb^207 the half life is 704 million years  v. C^14 ­ N^14  the half life is 5730 years (too soon to date rocks) e. You cannot date metamorphic rock this way; and new tech to date  precipitated sediment is becoming more widely used; using Uranium.  However, this is still generally not used for sedimentary rocks 


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