Chapter 8 The Rock RecordChapter 9 The Mesozoic and Cenozoic Eras

8.1 Determining Relative Age

• Hutton-Theorized that the same forces that changed the landscape also changes the landscape in the past.

• Hutton’s principle of Uniformitarianism• While processes are the same, the rates of the

processes may vary

8.1 Determining Relative Age

• Hutton noticed it look a long time , possibly millions of years, to change geologic features.

• Relative age indicates that one layer is older or younger than another layer but does not indicate the rock’s exact age.

• Law of Superposition-used to determine the relative age of rock-states that layers above other layers are younger.– The layer on the top is the youngest.

8.1 Determining Relative Age

• Principle of Original Horizontality-Normally we can tell the age of rock because it creates horizontal layers.

• There are exceptions what “mess up” the layers of the earth– IE earthquakes, erosion/weathering,– Also known as uncomformities

8.1 Determining Relative Age

• Unconformities Pg 189– Nonconformities-Caused by crustal movements

that push the earth’s surface up– Angular Unconformity-Layers of the Earth are

folded or tilted and then eroded– Disconformity-Layers are uplifted and are eroded.

Large time gap exists.

8.1 Determining Relative Age

• Crosscutting Relationship pg 190– Disturbed by faults or intrusions– The fault or igneous intrusion is always younger

than all the rocks it cutes through above and below the unconformity. (The layers have to exist to create the “crosscut” relationship.

8.1 Determining Relative Age

8.1 Determining Relative Age

8.1 Determining Relative Age

8.2 Determining Absolute Age

• Numerical Age• Ways to find the age of things– Rates of Erosion, deposition (ie 30 cm are deposited

over 1,000 years), varve count,– Radiometric Dating compares the relative

percentages of a radioactive isotope and a stable isotope. (Pg 194)

– Half-life-the time required for half of a sample of a radioactive isotope to break down by radioactive decay to form a daughter isotope.

8.2 Determining Absolute Age

• Look at Page 195, Review the chart and look specifically at the effective dating range.

• Remember some Radiometric dating methods are only useful during specific time periods. – Carbon Dating is used in younger rock material,

useful as all living things contain carbon, when things die, the radioactive carbon breaks down to C-14.

8.3 The Fossil Record

• Also all fossils are found in sedimentary rock. • Fossils provide important clues to the

environmental changes and how organisms have changed.

• Fossilization-Only occurs if dead organisms are buried quickly or protected from decay.

• Ways Fossils form: mummification, amber, Freezing, Petrification

8.3 The Fossil Record

• Types of Fossils: Imprints, molds and casts, coprolites, and gastroliths.

• Trace Fossils-showed past animal movement• Index Fossils-Fossils that are used to

determine the absolute age of the rock• Index Fossils and Absolute Age- Geologists use

this information to locate rock layers that are likely to contain oil and natural gas deposits.

9.1 Geologic Time

• Evidence of change is recorded in the rock layers of Earth’s crust.

• The Geologic Column-an ordered arrangement of rock layers that is based on the relative ages of the rocks and in which the oldest rocks are at the bottom.

• Look at Pg 211• Scientists compare similar rocks with unknown ages to

the Geologic Column to identify relative ages of the rock (when radiometric dating methods don’t work)

9.1 Geologic Time

• Divisions of geologic time Pg 213