Chapter 10 Section 1

1. Chapter 10 Section 1

2. Continental Drift Objectives • Summarize Wegner’s hypothesis of continental drift • Describe the process of sea floor spreading • Identify how paleomagnetism provides support for the idea of sea floor spreading • Explain how sea-floor spreading provides a mechanism for continental drift

3. Continental Drift • One of the most exciting recent theories in Earth Science began with observations made over 400 years ago • As early explorers sailed the globe they collected information on the continents coastlines • Map makers used this information to make the first reliable world maps

4. Continental Drift In 1912 a German scientist named Alfred Wegner proposed the hypothesis of continental drift He hypothesized that the continents all formed one land mass called a super continent, at one time and then began to break apart about 250 million years ago, during the Mesozoic Era

5. Continental Drift • In addition to the similarities of the coastlines Wegner proposed that fossil evidence should exist between areas that once were connected • A identical fossil of Mesoasaurus was discovered in both South America and Western Africa

6. Continental Drift • The theory of continental drift is also supported by the age, type, and shapes of the rocks found on the edges of divergent continents

7. Continental Drift • Changes in climate patterns also suggest that continents are not where they once were • South Africa displays evidence of glacial activity • Fossils of tropical plants found on Antarctica

8. Continental Drift • Despite the evidence Wegner had many scientists at the time rejected his hypothesis of continental drift • He spent the rest of his life trying to find the mechanism for continental drift, but died before the discovery occurred

9. Continental Drift • Evidence for Wegner’s theory came two decades after his death • Scientists who set out to map the Mid-Atlantic Ridge made some discoveries • Amount of sediment near the ridge is less than further away • Radiometric dating of rocks showed rocks get older as the move away from the ridge • Ocean rocks are much younger than continental rocks

10. Continental Drift • In the 1950’s geologist Henry Hess proposed that the mechanism for Wegner’s hypothesis was sea floor spreading • At the center of the valley is a rift were new magma is rising and cooling creating new sea floor

11. Continental Drift • If you have ever used a compass you know Earth has a magnetic pull • As magma cools the iron rich minerals in the magma align to Earth’s magnetic field and it is permanent (paleomagnetism)

12. Continental Drift • Earth’s magnetic field has not always pointed north • Rocks with normal polarity have the same dating periods as other normal polarity rocks • The same holds true for rocks with reverse polarity (south aliened)

13. Continental Drift • The stripped pattern on one side of the mid ocean ridge mirrors the pattern on the other side of the ridge • Age of the ocean rocks are symmetrical

14. Continental Drift • Rocks on land have magnetic symmetry also • Similar layers and age in rocks • Further proof verifying Wegener's hypothesis