Chapter 5Chapter 5Section 1 Section 1

Geologic HistoryGeologic History

• ObjectiveObjective

• ContrastContrast relative dating relative dating with absolute dating.with absolute dating.

• Relative DatingRelative Dating• Scientists use Scientists use differentdifferent

methods to determine the methods to determine the aaggee of objects in sedimentary of objects in sedimentary rocks. rocks.

• One method, One method, RelativeRelative datindating, g, examines a fossil’s pexamines a fossil’s positionosition within rock within rock lalayyersers to estimate to estimate its age.its age.

• The The bottombottom layers of rock layers of rock are usually the are usually the oldestoldest, and , and the the totop layers are usually p layers are usually the ythe younounggestest. .

• Scientists can use the Scientists can use the orderorder of rock layers to determine of rock layers to determine the the relativerelative age of objects age of objects within the layers.within the layers.

• For example, fossils in For example, fossils in the the bottombottom layers are layers are usually usually olderolder than fossils than fossils in the in the totop layers.p layers.

• The Geologic ColumnThe Geologic Column• To make relative dating To make relative dating

easier, geasier, geoeollooggistsists combine data from all of combine data from all of the the knownknown rock rock sequences around the sequences around the world.world.

• From this information, From this information, geologists created the geologists created the ggeoeollooggicic columncolumn — an ideal — an ideal sequence of rock layers that sequence of rock layers that contains contains allall of the known of the known fossilsfossils and and rockrock formations formations on Earth, arranging layers on Earth, arranging layers from oldest to youngest.from oldest to youngest.

• PaleontologyPaleontology• PPaalleoneonttoolloogy is the science gy is the science

involved with the study of involved with the study of past life. Scientists who study past life. Scientists who study past life are called past life are called ppaalleoneonttoollooggistsists..

• Paleontologists collect Paleontologists collect datadata by studying by studying fossilsfossils..

• Section 2 Section 2 Looking at FossilsLooking at Fossils

• ObjectiveObjective

• ExExpplainlain how fossils can be how fossils can be used to determine the used to determine the history of changes in history of changes in environments and environments and organisms.organisms.

• The The remainsremains or physical or physical evidenceevidence of an organism of an organism ppreservedreserved by geologic by geologic processes is called a processes is called a fossilfossil. .

• TraceTrace fossilsfossils are any are any naturally preserved naturally preserved evidence of animal activity.evidence of animal activity.

• A A cavitcavity in rock where a y in rock where a plant or animal was plant or animal was buriedburied is called a is called a moldmold. .

• A A castcast is an object that is is an object that is created when created when sedimentsediment fills fills a mold and becomes a mold and becomes rockrock..

• History of Changing OrganismsHistory of Changing Organisms

• Scientists study the Scientists study the relationshirelationshippss between fossils to interpret how between fossils to interpret how life has life has chanchanggeded over time. over time.

• Since the fossil record is Since the fossil record is incomincomppletelete, paleontologists look , paleontologists look for for similaritiessimilarities between fossils between fossils over time to try to track changeover time to try to track change.

• Using Fossils to Date RocksUsing Fossils to Date Rocks

• Scientists have found that Scientists have found that pparticulararticular types of fossils types of fossils appear only in certain appear only in certain lalayyersers of rock. of rock.

• By dating rock layers By dating rock layers aboveabove and and belowbelow these fossils, these fossils, scientists can determine the scientists can determine the timetime span in which the span in which the organism organism livedlived. .

• If the organism lived for a If the organism lived for a relatively relatively shortshort period of time, period of time, its fossils would show up in its fossils would show up in limitedlimited layers. layers.