1. Determining Earth’s History

2. Geologic Time Scale
Recall that the Earth is about 4.6 billion years old
Geologists have divided the Earth’s history into units based largely on fossils contained within the rocks.
The geologic time scale is a record of Earth’s history from its origin 4.6 billion years ago (bya) to today.
Helps scientists find relationships between geological events, environmental conditions, and fossilized life forms

3. Geologic Time Scale The time scale is divided into units called:
Eons (largest)
Eras
Period
Epochs (smallest)
Mass extinction provides ways for geologists to subdivide time.
Mass extinction occurs when many groups of organisms disappear from the rock record at about the same time.

4. Interpreting Geology
Scientists have NOT always believed the Earth was 4.6 billion years old.
James Hutton was the first to try to explain the Earth’s surface in terms of geologic forces
set the foundation for uniformitarianism

5. Uniformitarianism
Also recall that uniformitarianism states that geologic processes occurring today have been occurring since the Earth formed.
Example: The waves crashing on the shores during the Jurassic Period are much like the waves crashing on the shores today.

6. Absolute Dating
A rock’s absolute age is the time that has passed since the rock formed.
Geologists use radiometric (radioactive) dating to determine absolute age of rocks by studying the radioactive isotopes in igneous rocks.
Fossils can also be used to determine absolute age.

7. Relative-age Dating
Geologists can learn about the past by studying the present rocks.
Relative age dating provides geologists with a method to study the order in which geologic events occurred.
This does NOT determine exactly when events occurred, but provides a clearer understanding of events in Earth’s history.

8. How is relative age dating like a hamper of clothes?