EXAM 2 STUDY GUIDE - Relative Geologic Time How geologists...

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Relative Geologic Time 31/03/2008 11:20:00 How geologists think about time The big word: uniformitarianism Simply put: the present is the key to the past Examples of uniformitarian inferences Sediment deposition rates are observed on the order of mm/year Uplift rates (mm/year) Erosion rates (mm/year) Plate velocities (mm/year) Relative age inferences Assumptions/principle: o Sediments deposited horizontally o Younger sediments deposited above older sediments o Units that cross-cut are younger than those being cut o Units that include bits of other units are younger Original horizontality
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If sedimentary strata are inclined at an angle (since they don’t form that way),  then some processes occurring over some time frame tilted them Superposition If one layer is on top of another, then came (it’s younger) Not in the case of overturned folds some layers can be completely upside down Cross-cutting relationships If a unit is cut by a fault or igneous intrusion, then the unit must be older Inclusions If a unit contains pieces of another, then the “includer” must be younger than the  “includee” Sequence of events from oldest to youngest Deposition of abo formation Yeso formation Moenkopi formation Agua Zarco formation fault offsets the four sedimentary units  Erosion of the uppermost units on either side of the fault Emplacement of bandelier rhyolite (as hot ash flow) erosion Relative geologic time scale based on fossils preserved in sedimentary rocks
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Sedimentary rocks with similar fossils are grouped into stratographic  (systems/series) Time units were designated for the periods when particular fossil organisms  existed (eras/periods) Different time periods named after regions where diagnostic fossils were  discovered Has resulted in global correlation of stratographic units Principle of faunal/floral succession Because this systematic “succession” (and the principle of superposition),  sedimentary rocks can be “dated” by their characteristic fossil content Fossil index and global mapping strategies Absolute dating Absolute ages have been assigned to the geologic time scale developed form  fossils assemblages Based on the radioactive decay of some elements in mineral crystals Radioactive decay One element changes to another element (plus a high-energy particle) Elements that decay: uranium, potassium, rubidium
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Products that decay: lead, argon, strontium Minerals that can be dated contain the following radioactive elements: feldspar,  zircon, Biotite, muscovite, amphibole, others
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