EXAM 2 STUDY GUIDE

EXAM 2 STUDY GUIDE - Relative Geologic Time 31/03/2008...

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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 dont form that way), then some processes occurring over some time frame tilted them Superposition If one layer is on top of another, then came (its 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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EXAM 2 STUDY GUIDE - Relative Geologic Time 31/03/2008...

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