Guide for. Reading i Why is the geologic time scale used to show Earth's history?
... Scientists first developed the geologic time scale by studying rock layers.
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The Geologic Time Scale Guide for Reading i Whyis the geologic time scaleusedto showEarth's history? a Whatarethe different unitsof thegeologic time scale?
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onths, years,or even centuriesaren't very helpful for thinking about Earth'slong history.Becausethe time span of Earth's past is so great, geologists use the geologic time scale to show Earth's history. The geologic time scale is a record of the life forms and geologiceventsin Earth'shistory. Scientistsfirst developedthe geologictime scaleby studyingrock layers and index fossilsworldwide. With this information, scientistsplaced Earth'srock layersin order by relativeage.Later,radioactivedatinghelped determinethe absoluteageof the divisionsin the geologictime scale.As geologistsstudiedthe fossilrecord,they found major changesin life forms at differenttimes.They usedthesechangesto mark whereone unit of geologictime endsand the next begins.Therefore,the divisionsof the geologictime scaledependon the eventsin the history of life on Earth. Geologictime beginswith a long span of time calledPrecambrian Time.This span,which coversabout87 percentof Earth'shistory,ended 570 million yearsago.After Precambrian Time, the basic units of the geologic time scaleare eras,periods, and epochs. Geologists dividethe time betweenPrecambrian Time and the present into three long units called eras. They are the PaleozoicEra, the MesozoicEra,and the CenozoicEra. The PaleozoicEra beganabout 570 million yearsago and lastedfor 325 million years.Many animals that lived during the Paleozoicwere animalswithout backbones,or invertebrates.The MesozoicEra began about 245 rnillion yearsago and lastedabout 180 million years.People often call the Mesozoicthe Age of Dinosaurs,though dinosaurswere only one group of organismsthat lived during this time. The Cenozoic Era beganabout 65 million yearsago and continuesto the presentday. The Cenozoicis sometimescalledthe Age of Mammals. Erasare subdividedinto units of geologictime calledperiods. Geologic periodsrangein lengthfrom tensof millionsof yearsto lessthantwo million years.The Paleozoic Eraincludessixperiods:the Cambrian,the Ordovician, the Silurian, the Devonian,the Carboniferous,and the Permian.The MesozoicEra includesthree periods:the Tiiassic,the Iurassic,and the Cretaceous. The CenozoicEra includestwo periods:the Tertiaryand the Quaternary.The namesof the periodscome from placesaround the world wheregeologists first describedthe rocksand fossilsof eachperiod. Geologists furthersubdividethe periodsof theCenozoicErainto epochs. Why are epochsusedin the time scale?The fossilrecordof the Cenozoicis much more completethan the fossilrecordof earliereras.Thereare a lot more eventsto placein sequence, and usingepochsmakesthis taskeasier.
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The Geologic Time Scale I Understanding Main ldeas Put thefollowing in orderfrom oldestto mostrecentby writing a numberin the blank besideeach. l. MesozoicEra 2. PrecambrianTime 3. CenozoicEra 4. PaleozoicEra to makeit true. Rewritethefollowingsentence subdivideepochsinto periodsand periodsinto eras. 5. Geologists
Answerthefollowing questions0n a separatesheetof paper. 6. Why is the geologictime scaleusedto show Earth'shistory? 7. What methodsdid geologistsusewhen they first developedthe geologictime scale? 8. How did geologistsdecidewhereone division of the geologictime scaleends and the next begins? ct
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Fill in the blank to completeeachstatement. 9. Animals without backbonesare called 10.The recordof life forms and geologiceventsin Earth'shistoryis calledthe
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