Geology - Geologic Time - Quick Guides | Geology

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Study GuideGeology–Geologic Time1. Geologic CorrelaƟonsGeologists often need tomatch rock layers from different locationsto figure out their age andhistory. This process is calledgeologic correlation. Because Earth’s rock record israrely completeor fully exposed, geologists rely on several clues to decide whether rocks in different places formed atthe same time.1.1 Physical ConƟnuity: Matching Rocks by LocaƟon and Layers1.1.1 Tracing Rock LayersThe simplest way to correlate rocks is throughphysical continuity. If a rock layer or boundary can beseen continuouslyfrom one place to another, it is easy to prove that the rocks are the same.However, this is rarely possible over long distances. Erosion, vegetation, and burial often break uprock exposures. As a result, geologists usually mustcompare rock sequences in detailrather thantrace them directly.1.1.2 Comparing Rock SequencesGeologists examine:•Therock type•Theorder of layers•The rocks foundabove andbelowthe layer being studiedFor example, if you find this sequence in two different places:•Red shale•Sandstone•Green shale•Limestone•Chert

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Study Guideand the order is the same in both locations—even 20 kilometers apart—it is reasonable to concludethat thegreen shale layerformed at the same time and under similar conditions in both places.1.1.3 Marker BedsSometimes, a rock layer has aunique appearancethat makes it easy to recognize. These speciallayers are calledmarker beds. Even if only parts of a sedimentary sequence are exposed, markerbeds help geologists:•Match layers across distances•Reconstruct thecomplete geologic historyof an area1.2 The Fossil Record: Using Life to Track Time1.2.1 Fossils and TimeAnother powerful way to correlate rocks is by studyingfossils. The study of fossils is calledpaleontology.Because plant and animal species existed only during certain periods of Earth’s history:•Rocks with thesame fossilsformed during thesame time interval•Fossils act as naturaltime markers1.2.2 Index FossilsSome fossils are especially useful.Index fossils:•Lived during ashort, well-defined time span•Werewidespread•Are easy to recognizeIf an index fossil is found in a rock layer, geologists can narrow downwhen that rock formed.

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Study Guide1.2.3 Fossil AssemblagesAfossil assemblageis a group of different fossil species found together. Assemblages are oftenmore reliable than single fossilsbecause they:•Provide a clearer age range•Help identify theenvironment of deposition(such as ocean, reef, or swamp)Using fossil assemblages, geologists can correlate rock layersacross continents and evenworldwide.1.3 The Geologic Time Scale1.3.1 Dividing Earth’s HistoryBy combining fossil evidence from around the world, geologists created thestandard geologic timescale. This scale divides Earth’s history into:•Eons•Eras•Periods•EpochsEach division represents major changes in life, climate, or tectonic activity.

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Study Guide

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Study Guide1.3.2 The PrecambrianWhen the time scale was first developed, scientists believed that the earliest fossil life appearedduring theCambrian Period, which began about570 million years ago.All timebefore the Cambrian—from about4.5 billion to 570 million years ago—is called thePrecambrian.Because fossils from this time are rare, the Precambrian is divided mainly using:•Radiometric age dating•Major tectonic and crustal events2. Absolute AgeGeologists first built thegeologic time scaleusingrelative dating, which compares the order of rocklayers and events. However, knowingwhich rock is olderis different from knowinghow old it actuallyis. Finding theactual numerical agesof rocks became possible only after scientists discoveredradioactivity.Thanks to radiometric dating, geologists can now assign real dates to major divisions of Earth’shistory, such as:•The boundary between thePrecambrianandPaleozoic: about570 million years ago•TheMesozoic Era(the “Age of Dinosaurs”): from about245 millionto66 million years ago2.1 RadioacƟvity and Atomic ClocksWhat Is RadioacƟve Decay?Some chemical elements naturally break down over time. This process is calledradioactive decay,and it happens at aconstant and predictable rate.Radioactive decay begins once atoms are locked intocrystalline mineral structuresas a rockforms.

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Study GuideIsotopes ExplainedMany elements exist in different forms calledisotopes. Isotopes are atoms of the same element thathave:•The same number of protons•Different numbers of neutronsFor example:•Uranium-238 (U-238)has three more neutrons thanUranium-235 (U-235)Some isotopes are unstable and decay over time.2.2 Parent and Daughter ProductsAs a radioactive isotope decays, it transforms into new elements calleddaughter products.For example:•Uranium-238decays through a series of steps•Eventually, it becomeslead-206, which is stableBecause the decay rate is constant, geologists can calculate how long decay has been happening bymeasuring theratio of parent isotope to daughter productin a mineral.2.3 Common Radiometric DaƟng SystemsSeveral decay systems are used to determine absolute age, including:•Carbon-14→Nitrogen-14•Potassium-40→Argon-40•Rubidium-87→Strontium-87•Thorium-232→Lead-208•Uranium-235→Lead-207•Uranium-238→Lead-206Each system is useful fordifferent rock types and age ranges.

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