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

2. Fossils and the Rock Record
Chapter 21

3. 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

4. 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

5. 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

6. 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)

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. Questions?