Define the following terms at the top of your notes for Ch 21! Era Eon Period Epoch Correlation Radiometric dating Radioactive decay Index fossils

Fossils and the Rock Record Chapter 21

The Geologic Time Scale – Ch 21.1 Geologists study characteristics of rocks and the fossils within them to: Interpret the environment the rocks were deposited in Reconstruct Earth’s history Possibly predict events or conditions in the future

The Rock Record Geologists have divided the history of Earth into time units based upon the fossils contained in them Geologic Time Scale  see p. 554

Geologic Time The oldest division of time on the scale is at the bottom, just as it would be in a rock sample in nature, and the youngest division of time is at the top The time scale is divided into units: Eons  longest unit of time, measured in billons of yrs Eras  measured in hundreds of millions of yrs Periods  measured in tens of millions of yrs Epochs  measured in millions of yrs

Geologic Time Eon Longest unit of time, measured in billions of years Era Defined by the differences in life-forms found in rocks Named based on the relative ages of these life-forms “Paleo” means old (Paleozoic Era) “Meso” means middle (Mesozoic Era) “Ceno” means recent (Cenozoic Era)

Geologic Time Period Defined by the life-forms that were abundant or became extinct during the time in which the specific rocks were deposited Some were named for the geographic region in which the rocks of that age were first observed, studied, and described The Jurassic Period was named for the rocks that were described in the Jura Mountains in Europe

Geologic Time Epochs The fossil record of the Cenozoic Era is relatively complete since there has been less time for weathering and erosion to remove evidence of this part of Earth’s history This makes it easier to use different groups of organisms to distinguish the various epochs

Relative Dating – Ch 21.2 Several theories came about of how old the Earth was. The major one was: Uniformitarianism  states that processes occurring today have been occurring since Earth formed Mountain building Erosion Earthquakes Sea-level changes

Principles for determining Relative Age Places the ages of rocks and the events that formed them in order, but without exact dates Done by comparing one event or rock layer to another Geologists use several ideas to help them figure out the order in which these events happen

Principles for determining Relative Age Original horizontality States sedimentary rocks are deposited in horizontal layers Superposition States that an undisturbed rock sequence has oldest rocks at the bottom and youngest rocks at the top

Principles for determining Relative Age Cross-cutting relationships States that an intrusion or fault is younger than the rock it cuts across Inclusions  where overlying rock layers contain particles of rock material from the layer beneath it The older bottom layer is eroded and becomes incorporated into the newly deposited top layer

Other means of determining Relative Age Earth is constantly changing as a result of processes, so it is difficult to find an undisturbed sequence of rock layers Other features are used to help figure out the order in which they showed up and what might have happened there

Gaps in the Rock Record Because of changes from erosion and movement of rocks from geologic processes Unconformities  buried erosional surface that results in a gap in the rock record

Putting the Pieces together Geologists look at all this information from one place, but it doesn’t necessarily tell them a lot Geologists compare similar rocks in different areas to figure out if they are about the same age This is called correlation They look for distinctive or unique fossils and mineral features in rock layers

Absolute Dating of Rocks – Ch 21.3 Determines the actual age of a rock using radioactive isotopes These particles change in a predictable pattern over time, allowing scientists to determine how long the isotopes have been decaying (and therefore the age of the sample from which the isotopes came) This process is called radiometric dating Determines the ratio of parent nuclei to daughter nuclei in a given sample of rock or fossil

Other ways to determine age Tree rings  counting the number of annual tree rings in a cross section of the tree The widths of tree rings are directly related to the climactic conditions during growth periods Sediments deposited in lakes Light colored sediments during warm seasons Dark colored sediments during cold seasons

Remains – Ch 21.4 Fossils contained in the rocks can also help geologists determine the ages of the rocks Some fossils (index fossils) can be used by geologists to determine the ages of rocks since they are easily recognized, abundant, and widely distributed geographically AND lived during a short period of time

Other fossils Mold Trace fossil When the original shell parts of an organism within a sedimentary rock are weathered and eroded, minerals can fill in the spaces and make a rock version of the shell of an animal Trace fossil Indirect evidence providing evidence on how an organism lived, how it moved, or how it obtained food Footprints Burrows

Why study fossils? Allows scientists to interpret and describe Earth’s history Fossils from different geologic time periods describe how organisms have changed through time, ancient environmental conditions, show patterns and cycles that can be used to predict future phenomena

Questions?