P337\Section 4

Geologic Time Scale

• 1a How long ago did the Earth form?

• 4.6 billion years ago

• Summarize the process by which scientists determined the age of Earth.

• Scientists studies rock layers and fossils worldwide and placed Earth’s rock layers in order by relative age. Later, radioactive dating helped them determine the absolute ages of the divisions.

• What are the basic units into which the geologic time scale is divided?

• The geologic time scale is divided into eras and periods.

• Study figure 14. (p336) Which major division of time was the longest?

• Precambrian• When did it begin?• 4.6 BYA• When did it end?• 544 MYA

• Place the following in the correct order from earliest to latest: Tertiary, Jurassic, Quaternary, Triassic, and Cretaceous.

• Triassic, Jurassic, Cretaceous, Tertiary, and Quaternary

Section 5 Early Earth

p. 341e

• 1a How long ago did the Earth form?• The Earth formed about 4.6 billion years ago

• Summarize the process by which scientists determined the age of the Earth.

• Scientists used radioactive dating of rocks from Earth and moon to determine the age of the Earth.

• What physical features formed during Earth’s first several hundred million years?

• Oceans, the atmosphere, and continents were formed.

• How did volcanic eruptions and comets change early Earth?

• Volcanic eruptions released carbon dioxide into the atmosphere, and comets released water vapor.

•What caused water erosion to begin on Earth’s surface?

• Rain

• What do scientists think were the first organisms to evolve on Earth?

• Organisms similar to our present day bacteria were probably the first life forms.

• How would Earth’s atmosphere be different if organisms capable of photosynthesis had not evolved?

• The Earth’s atmosphere would have little or no oxygen. A by-product of photosynthesis is oxygen.