EXAM 2 STUDY GUIDE

EXAM 2 STUDY GUIDE - Relative Geologic Time 11:20:00 ←...

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Unformatted text preview: 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 • 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 • 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 • 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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This note was uploaded on 05/03/2008 for the course GLG 111 taught by Professor Kuentz during the Spring '05 term at Miami University.

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EXAM 2 STUDY GUIDE - Relative Geologic Time 11:20:00 ←...

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