Earth Science

1. Earth Science Section 17.1 - Drifting Continents

2. Objectives • Identify the lines of evidence that led Wegener to suggest that Earth’s continents have moved. • Discuss how evidence of ancient climates supported continental drift. • Explain why continental drift was not accepted when it was first proposed.

3. Hypothesis (review vocabulary) • Continental drift • Pangaea

4. Earliest Observations • With the exception of events such as earthquakes, volcanic eruptions, and landslides, most of Earth’s surface appears to remain relatively unchanged during the course of a human lifetime. • On the geologic time scale, however, Earth’s surface has changed dramatically.

5. The Early Observers • Abraham Ortelius – Dutch cartogapher • Work done in the late 1500’s • First noticed “fit” of continents on each side of the Atlantic • Thought they went together like a puzzle • Thought North America/South America separated from Europe/Africa by: • Floods • Earthquakes

6. The Early Observers • Antonio Snider-Pellegrini created these sketches to show the “fit” of the continents.

7. The Early Observers • The first time that the idea of moving continents was proposed as a scientific hypothesis was in the early 1900s. • Alfred Wegener • First proposed the continents moved in 1912 • Named the process continental drift • Called the “supercontinent” Pangaea • Pangaea is from the Greek – “all the earth” • Suggested Pangaea broke up about 200 mya • Continents drifted into present position since then

8. Continental Drift • Wegener developed an idea that he called continental drift. • He proposed that Earth’s continents had once been joined in a single landmass, a supercontinent called Pangaea, that broke apart about 200 mya and sent the continents adrift.

9. Wegener • In addition to the fit of the continents, he used: • Climatic data • Rock data • Fossil data

10. Rock Data • When Pangaea broke up: • Geologic structures broke up • Mountain ranges • Rock formations • Therefore there should be the same rock types on either side of the Atlantic • Ex. – the rock formations are similar in the: • Appalachian Mountains – North America • Mountains in Greenland and Europe • All older than 200 mya • These formations supported Wegener’s idea that the continents had once been joined.

12. Fossil Data • Wegener gathered evidence of the existence of Pangaea from fossils. Similar fossils of animals and plants that once lived on or near land had been found on widely separated continents.

13. Fossil Data • Similar fossils of animals and plants found on different continents • Land dwelling animals could not have swum across the ocean • Cynognathus • Lystrasaurus • Same true for freshwater aquatic animals • Mesosaurus • These fossils are older than 200 mya

14. Fossil Data

15. Cynognathus Fossilized Heat Artist’s Depiction

16. Lystrasaurus Fossil Artist’s Depiction

17. Mesosaurus Fossil Artist’s Depiction

18. Climatic Evidence • Got clues about climate from the fossils present • Use Glassopteris – a fern grows in temperate climates • A temperate climate is warm without extreme heat or cold • Fossils found in: • South America • Antarctica • India • Wegener reasoned that the area separating these fossils was too large to have had a single climate. • These areas must have been close to each other and to the Equator, and the land masses must have been joined

19. Locations of Glossopteris

20. Glossopteris Fossil Artist’s Depiction

21. Coal Deposits • Found in Antarctica • Coal forms from the compaction and decomposition of accumulations of ancient swamp plants. • So Antarctica had once been a tropical climate • Antarctica must have been close to the Equator

22. Glacial Deposits • Glacial deposits nearly 300 million years old on several continents led Wegener to propose that these landmasses might have once been joined and covered with ice. • Wegener examined materials left by glaciers in: • Africa • India • South America • Australia • Deposits from 290 mya • This area must have been near the South Pole • The extent of the ice is shown in white

23. How did this happen? • Wegener Proposed: • South Pole shifted position or • Landmasses, once near the South Pole had drifted • He believed it was more likely the landmasses had drifted rather than the Earth’s Pole changed positions.

24. A Rejected Notion • Although Wegener had compiled an impressive collection of data, the hypothesis of continental drift was never accepted by the scientific community. • Most scientist in the early 1900’s thought the landmasses were fixed in position • Wegener could not explain what forces were acting to move the landmasses • He though the Earth’s rotation may have caused this • Other scientists showed this did not provide enough force • Wegener could not explain how the continents had drifted through the ocean floor, since it was thought to be solid, and the Earth’s mantle below the ocean floor was also thought to be solid

25. REJECTED!!!! • Only in the 1960’s did new technology become available to gather information to support Wegener. • WITH THIS NEW TECHNOLOGY THEY DISCOVERED MORE EVIDENCE ABOUT HOW THE CONTINENTS MOVE. • With this new evidence scientists began to reconsider wegener’s ideas. • The key was seafloor mapping and what it told us about how the continents were able to move.