The Geologic Time Scale

1. The Geologic Time Scale 10.15.07 / 10.16.07

2. Correlation • Using rock formations and fossil types to relate geologic materials from different regions

4. Essential Question • How do scientists use the geologic time scale to understand and describe natural history?

5. What is the geologic time scale? • It is a graphic model of Earth’s history divided into units based on geologic events found in the fossil record. • First developed in the 19th century as a relative time scale using relative dating principles • Later modified, in the 20th century, to include absolute ages

6. Geologic Time Scale

8. Mass extinctions • Geologic time is divided according to types of life • These divisions often occur from mass extinctions, examples of which include: • Cretaceous-Tertiary extinction killed dinosaurs • Triassic-Jurassic extinction (22% of marine families) • Permian-Triassic extinction (95% of all species)

9. Precambrian Time • 4.6 billion – 540 million years ago • Comprises about 88% of Earth’s history • Divided into three eons • The further back in time, the less we know about that time • Life forms had soft bodies so there are few fossils • Many rocks from this time have been recycled • Surviving rocks are often deeply buried • Grand Canyon reveals some Precambrian rocks

10. Precambrian life • Early atmosphere had an abundance of CO2 resulting from cooling magma • First life was anaerobic (not requiring oxygen) bacteria • Bacteria and early plants released oxygen into the atmosphere • Fossils exist for life as far back as 3.5 billion years

11. Paleozoic Era • Paleozoic means “ancient life” • About 540 million years ago, many varied types of fossils appear • Paleozoic life had shells, bones and teeth, and was therefore more likely to become fossilized • Invertebrates, fish, amphibians and land plants all live in this time • Ends with mass extinction (90% marine, 70% land life dies)

12. Mesozoic Era • “Middle life” • Age of reptiles (dinosaurs) • Dryer environment led to proliferation of trees and land animals • Ends with mass extinction • Impact hypothesis • Asteroid impact may have caused extinction • Iridium layer around the world provides evidence

13. Is this the K-T Asteroid? • 6-8 mile wide asteroid • Left 120 mile wide crater

14. Cenozoic Era • “Recent life” • Mammals take over in colder climate • Mammals develop in four ways: • Increased size • Increased brain power • Specialized teeth for specific diets • Specialized limbs for specific living environments

15. Cenozoic extinctions • Over the last 8-20,000 years, many large species of mammals have gone extinct (ex. mastodons and mammoths, saber-toothed cats, giant sloths, etc) • Most likely explanation is that human hunting of these animals and their food sources led to extinctions

16. Life across the geologic time scale First life fish reptiles mammals humans

17. Summary activity • Briefly describe the major distinctions between the Precambrian time and each of the three most recent eras. • The geologic time scale has been updated as new information has become available. Why did the early versions of the time scale not include approximate dates for each segment of time?