Mar 2-6th Notes
Mar 2-6th Notes GEO 101
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This 3 page Class Notes was uploaded by Cassidy Reid on Monday March 9, 2015. The Class Notes belongs to GEO 101 at Washington State University taught by Wilkie in Spring2015. Since its upload, it has received 68 views. For similar materials see Introduction to Geology in Geology at Washington State University.
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Date Created: 03/09/15
Geo Notes March 2 l7th Chapter 9 Geologic time Telling time geologically Earth s history is concealed in rocks One of the goals of geology is unraveling earths history Earth s geological clock Hot do we tell time Absolute Dating determining event s actual time Relative dating putting rockevents in proper order Stratigraphy the description correlation and classi cation of strata sedimentary rocks Formation a series of rock layers in a region that has similar physical properties may contain the same fossils and is mapped as a unit Principles of Relative Dating 1 Principle of original horizontality sediments are deposited in horizontal beds lf rocks do not lie in horizontal beds they have been distributed by some kind of tectonic force Exception Crossbeds 2 Principle of superposition In an undisturbed sequence of rocks each layer is younger than the one beneath it and older than the one above it 3 Fanual Succession Fossils as timepieces Use of index fossils to correlate rocks formations from different locanns 4 Unconformities Markers of missing time deposition of rocks isn t continuous forever 1 Disconformity an unconformity in which the rocks above ands below the unconformity are parallel 2 Angular Unconformity grand canyon old erosion surface 3 Nonconformity sedimentary rocks in contact with crystalline igneous or metamorphic rocks dissimilar rock types in contact with each other 4 CrossCutting Relations Dike crosscuts the preexisting rock layers Therefore it is younger than the rock layers it cuts across Fault crosscuts all rock layers Therefor the fault is younger that the rock layers it cuts across Absolute Dating Radiometric Dating uses unstable radioactive isotopes to determine absolute age The radioactive isotope the parent isotope evolves into a decay product the daughter isotope at a certain rate Isotopes Atoms with identical of protons but different of neutrons Radiometric Dating The ratio of parent daughter determines the age of the rock What we are actually dating is a mineral that constrain the isotope of interest Halflife of a radioactive element is the time it takes for half of the parent to decay into daughter product March 4 2015 What kind of rocks can be dated with radioactive methods In igneous rocks the clock begins when the molten rock cools get age of crystallization formation No parent or daughter atoms added or removed once started the radioactive decay No resetting with metamorphism or get time of metamorphism For sedimentary rocks radiometric dating you would get the average age of all the grains which form the sedimentary rock For metaphoric event may be more than one rather than the initial age of crystallization formation What about Carbon14 What it is o If carbon12 is carried to the upper atmosphere high energy photons from the sun will damage it creating Carbon14 in the form of C02 Organisms are made of carbon so they will consume the carbon 14 o The carbon14 is radioactive and decays to nitrogen14 What s the problem here 0 We measure the amount of carbon14 left in the organism and compare it to the amount originally around in the atmosphere to determine the age Reasons why we don39t use C14 in rocks 1 Most rocks do not contain carbon 2 The halflife is only 5730 years so you can t date anything older than about 70000 years Carbon14 can be used to determine Ages of recent lava ow Ages of recent ash pumice falls Ages of recent landslides lahars Geologic Time Scale Can be though of as a relativeage calendar of the Earth S geologic history A key component to constructing the geologic timescale is the correlation of fossils between outcrops How do we get fossils Usually rapid burial is required Examples Soft mud Flood deposits Vocanic ash Skeletons and shells preserve the best hard parts hardparts usually undergoes chemical replacement Petri ed Wood Plant cells have been replaced by quartz Si02 many are buried in volcanic landslides or eruptions Trace Fossils Evidence that the organism was there although the organism is gone Casts molds of footprints Burrowsthe burrowing animals are gone but their burrows remain Index Fossil Used to identify the relative age of rock unit 0 Used in stratigraphic correlation of rock units What makes a good index fossil unique and easy to identify ived for a short period of time widely distributed Geologic Time Scale Can be though of as a relativeage calendar of Warth s geologic history 1 Time is divided into Eons Ers Periods and Epochs 2 The time between each division is NOT the same Precambian from birth of earth Paleozoic acient life Mesozoicage of middle life