Continental Drift

1. Continental Drift Alfred Wegener

2. Continental Drift

3. The theory • In 1912, Alfred Wegener and Frank Taylor first proposed the theory that 200 million years ago the Earth had only one giant continent, from which today's continents broke apart and drifted into their current locations. • To support their theory:They used the fit of the continents, the distribution of fossils, a similar sequence of rocks at numerous locations, and ancient climates.

4. The Evidence:the jigsaw • The bulge of Africa fits the shape of the coast of North America while Brazil fits along the coast of Africa beneath the bulge. • Pangaea: Original landmass

5. The Evidence: rocks match • A geologic column from Africa and South America matched when the end of the continents joined.

6. The Evidence: Ice match • Glacial striations on rocks in South America match up to glacial “scratches” in Africa.

7. The Evidence: Fossils • The fossil of a fern (tropical plant) found in Antarctica. • Wegener believed that the climate has not changed but the continents moved toward a different climate zone.

8. Sea Floor Spreading HarryHess

9. The Theory • In 1962 Hess thought it possible that the continents were in motion. He believed that molten magma from beneath the earth's crust could ooze up between the plates in the Great Global Rift. As this hot magma cooled in the ocean water, it would expand and push the plates on either side of it.http://www.pbs.org/wgbh/aso/databank/entries/do53ri.html

10. The Theory cont. • Hess proved Wegener's basic idea right and clarified the mechanism that broke the once-joined continents into the seven with which we are familiar. The continents are attached to the plates and do not move independently of them. But the plates themselves shift and change shape, carrying the continents along.

11. The Evidence Molten Material • In the 1960’s scientists found evidence of new material erupting along the mid ocean ridge. http://pubs.usgs.gov/publications/text/topomap.html Drilling Samples • The farther away from the ridge older rocks were found. The youngest rocks were always closer to the ridge.

12. Subduction Deep –Ocean Trench • Oceanic crust (basalt: more dense) is forced back into the mantle.

13. Plate Tectonics • J. Tuzo Wilson

14. The Theory • Wilson combined what was known about sea floor spreading and continental drift into a single theory. • Wilson stated that the pieces of Earth’s lithosphere are in constant, slow motion, driven by the convection currents in the asthenosphere.

15. Convection currents in mantle

16. Plate Boundaries • Transform boundary: two plates slip past each other, moving in opposite directions. Crust is neither created or destroyed. • Divergent boundary: two plates move apart. - mid ocean ridge - rift valley: occurs on land

17. Convergent boundary: two plates come together. When two plates collide the density of the plates determines which plate will sink under the other.