1. Unit 9: Evolution
9.1 The Age of Earth

2. The Age of Earth
How do we date events in Earth’s history?

3. Geologic Time Scale
The geologic time scale is based on both relative and absolute dating.
Comparing layers of rock
Using radioactive decay to determine the age of organisms.

4. Geologic Time Scale
Using absolute dating, scientists have determined that Earth is 4.6 billon years old
Geologists and paleontologists have build a timeline of Earth’s history called the geologic time scale
The major divisions of geologic time are eons, eras, and periods.

5. The Age of Earth
How has the Earth changed over time?

6. Physical Forces Climate and Temperature
“Heat wave” of the Mesozoic Era, Earth’s average temperatures were only 6°C to 12°C higher than they were during the twentieth century.
During the ice ages, world temperatures were only about 5°C cooler than they are now.
These relatively small temperature shifts changed the shape of life on Earth.

7. Physical Forces Geological
Geological forces have transformed life on Earth, producing new mountain ranges and moving continents.
Volcanic forces have altered landscapes and even formed entire islands.
Local climates are shaped by the interaction of wind and ocean currents with geological features such as mountains and islands.

8. Physical Forces Theory of Plate Tectonics
The theory of plate tectonics explains how solid continental “plates” move slowly above Earth’s molten core—a process called continental drift.
Early in Earth’s history, all land was part of one big “supercontinent,” Pangaea.
Later, these supercontinents have split apart and reformed.

10. Biological Forces
The first life on Earth were likely unicellular prokaryotes.
The activities of organisms have affected global environments.
For example, Earth’s early oceans contained large amounts of soluble iron and little oxygen.
During the Proterozoic Eon, however, photosynthetic organisms produced oxygen gas and also removed large amounts of carbon dioxide from the atmosphere.
The removal of carbon dioxide reduced the greenhouse effect and cooled the globe. The iron content of the oceans fell as iron ions reacted with oxygen to form solid deposits.