Plate Tectonics

1. Plate Tectonics Evolution of the Earth and Our Restless Planet: Chapter 33

2. Review: Interior

3. 33.1 The Theory of Continental Drift • Scientists in the early 20th century believed • Oceans and continents were fixed in place • The surface is a skin spread over a cooling interior • Cooling caused contraction of the skin= wrinkles and bumps aka mountains and valleys

4. Alfred Wagner Thought Differently! • He believed: • The continents are slowly moving • All the continents has once been attached in a supercontinent and this was called Pangaea “all land” • Pangaea split and the continents moved apart • The boundary of each continent is not at the shoreline but the continental shelf

5. Alfred Wegener cont. No one believed Wagner…..at least not yet!

6. Scientific Revolution • Discovery of the Earth’s Magnetic Field gave a window into the past • Paleomagnetism (magnetism from the geologic past…shows 3 major things) • The position of the magnetic north versus the geographic north pole • Direction to the magnetic pole at the time the rock was formed • Magnetic latitude of the rock’s location at the time it was formed • The movement of the magnetic pole over time(500 million years) suggests that either the pole moves around or the continents move

7. The Earths North and South pole have flipped many times • These leaves magnetic ‘stripes’ in rock containing iron minerals

8. Seafloor Spreading: H. H. Hess • 1950’s had detailed mapping of the ocean floor • Figured out • Some really deep parts are next to the continents and some shallower parts are out in the middle due to the mountains under the water • Why?

9. Finally Wagner's’ Ideas became the: Theory of Plate Tectonics The earth’s outer shell, the lithosphere, is divided into 8 large plates and a number of smaller ones Most earthquakes, volcanoes, and mountains occur along the borders

10. All Based on Motion • It turns out that the upper section of the mantle is stuck to the underside side of the crust to form what we call tectonic plates

11. Evidence of Tectonic Plates Our first evidence of tectonic motion is based on similar fossils and rock types on opposing sides of the ocean

12. Tectonic Plates Today plate boundaries are determined by examining the location of volcanoes and earthquakes. Volcanoes result from the friction (heat) of the plates motion. Earthquakes occur where plate rub against one another

13. Tectonic Plates Volcanoes

14. Tectonic Plates

15. Why do the Plates Move? • No single idea explains everything but we can identify several forces that contribute to the movement of the plates. • Slab pull • The sinking of the cooled dense oceanic plates pulls on the rest of the plate • Ridge rises • The material deposited on the top of the ridge slides downs from the rise pushing on the plate • Convection • Movement within the mantle could be part of the driving force behind the motion of the plates.

16. Why do the Plates Move?

17. Why are they spreading? • Why is the Atlantic still getting wider • The plates are pulled apart by convection currents in the mantle below • Caused by heat released from natural radioactive processes • At the mid Atlantic ridge molten rock from below rises up to fill the gap with new basaltic rock

18. Plate Boundaries • Convergent – plates move toward one another • Divergent – plates move away from each other • Transform-Fault – plate moves sideways from each other

19. Plate Boundaries

20. Convergent Plates

21. Convergent Plates Ocean-Continental Convergence Ocean-Ocean Convergence Continental-Continental Convergence

22. Divergent Plates Spreading centers on the ocean floor and on land (create rifts) These create the beginning of a new ocean basin

23. Transform Plates

24. Transform Plates San Andreas Fault

25. Mid-Plate Hotspots Hawaii

26. Pangaea

27. Pangaea

28. Pangaea The break up of Pangaea

29. Where are we going? We appear to be headed for another super continent as North America, South America, Asia and Australia converge in the ever shrinking Pacific Ocean