Plate Tectonics

1. Plate Tectonics

2. Continental Drift Alfred Wegener’s theory: Continents were once a single land mass that drifted apart. • Fossils of the same plants and animals are found on different continents • Called this supercontinent Pangaea, Greek for “all Earth” • 245 Million years ago • Split – Laurasia & Gondwana about 180 million years ago http://members.enchantedlearning.com/subjects/astronomy/planets/earth/Continents.shtml

3. Evidence of Continental Drift • Fossils – various fossil remains of the same prehistoric organisms have been found in places now separated by oceans.

4. Evidence (cont.) • Landforms: Coal fields and glacial remains from some continents seem to line up with the edges of other continents.

5. Evidence (cont.) • Climate: Fossils and rocks associated with different climates than present can be found in the layers of earth, and resemble those of other continents from a similar time period.

6. Plate Tectonics (cont.) • Plates are pieces of the lithosphere that move around on top of the asthenosphere. • Fit together like jigsaw puzzles (or at least they did at one time)

7. What causes plates to move? • Heat from the earth’s core (higher temperature and pressure) causes convection currents in the mantle. • As fluid is heated in the lower mantle it becomes less dense, therefore, it rises toward the upper mantle where there is less heat. • As the fluid cools it becomes more dense and subject to gravity, causing it to move back into the lower mantle.

8. Sea Floor Spreading • At the mid-ocean ridge, molten material (magma) rises from the mantle and erupts. Forms the largest mountain chains in the world, completely under water! • Magma rises to the surface and solidifies and new crust forms. • Older Crust is pushed farther away from the ridge. • Process continually adds new material to the ocean floor. Video

9. Mid-ocean ridges

10. Mid-ocean Ridges

11. Evidence at mid-ocean ridges • Molten rock - The shape that basalt makes when it cools rapidly (underwater). Seeing this all over the ocean shows that eruptions have occurred over and over. “pillow lava”

12. Drilling • Scientists took core samples from different sides of the mid-ocean ridge and compared their age. • The Glomar challenge found that the further you got from the ridge the older the rocks were. • This was enough evidence to prove sea-floor spreading and gave Wegener his mechanism for the continental drift theory! Glomar Challenger

13. Magnetism- evidence shows that earth’s magnetic poles have reversed themselves. • Last happened 780,000 years ago (so magnetic north was actually south). • The rock that makes up the ocean floor actually lies in patterns of magnetized “strips”, which hold a record of reversals in earth’s magnetic field.

14. Plates move in all directions

15. Types of Plate Boundaries • Plate boundaries define the edges of different pieces of lithosphere. Faults – breaks in the earth’s crust where rocks have slipped past each other, form at these boundaries. The 3 types of plate boundaries are: Convergent, Divergent, and Transform. http://pubs.usgs.gov/gip/dynamic/understanding.html

16. Convergent Boundary One plate dives under another in a process called subduction. Oceanic crust (more dense) subducts under continental crust (less dense). The subducting plate sinks back into the mantle over tens of millions of years. This creates a deep ocean trench. Earthquakes and Volcanoes are common at these boundaries. http://pubs.usgs.gov/gip/dynamic/understanding.html & http://www.geology.com

17. The Pacific rim is commonly referred to as “The Ring of Fire” • Subduction zones • 452 volcanoes, over 75% of the worlds volcanoes in this region. • 90% of the world’s earthquakes occur here as well.

18. When two continental plates collide (both low density granite rock), the crust buckles and forms mountain ranges. This is how the Himalayas formed, and the Grand Tetons. http://pubs.usgs.gov/gip/dynamic/understanding.html & http://www.geology.com

19. Indian and Eurasian Plates converged to form the Himalayas. Eurasian Plate Indian Plate

20. Rock folding can be observed at converging continental plates

21. When two oceanic plates collide the more dense plate will subduct, creating a trench and volcanic island arcs.

22. Example: Aleutian Island Volcanic Arc – subduction of Pacific plate beneath N.A plate. trench

23. Divergent Boundary - oceanicNew crust is generated as the plates pull away from each other Results in the formation of new oceanic crust at the mid-ocean ridge. Also associated with volcanic activity. http://www.geology.com

24. Divergent Boundary – ContinentalDiverging continental plates result in the formation of rift valleys. Eventually, when the rift is wide and deep enough, ocean will fill the space between. http://www.geology.com

25. Arabian and African Plates diverging as the Red Sea floods the space between. Arabian Plate Red Sea African Plate

26. Transform Boundary – 2 plates slip past each other, moving in opposite directions. Earthquakes occur frequently along these boundaries. www.geology.com

28. Volcanic hotspots – magma deep within the mantle melts through the crust like a blow torch. As plates move over top, so does the volcanic activity.

30. Yellowstone hotspots • Scientists have traced Yellowstone's origin to a hot spot in the mantle, one of a few dozen such hot spots on Earth. Buoyant material from a hot spot rises through the upper mantle, bringing heat from the Earth's interior closer to the surface. • The Yellowstone hot spot has interacted with the North American plate for perhaps as long as 17 million years, causing widespread outpourings of basalt that bury about 200,000 square miles in Washington, Oregon, California, Nevada, and Idaho under stacks of lava flows half a mile or more thick. Some of the basaltic melt, or magma, produced by the hot spot accumulates near the base of the plate, where its heat melts rocks from the Earth's lower crust. These melts, in turn, rise closer to the surface to form large reservoirs of potentially explosive rhyolite magma. Catastrophic eruptions have partly emptied some of these reservoirs, causing their roofs to collapse. The resulting craters, some of which are more than 30 miles (50 kilometers) across, are known as volcanic calderas. Because the plate was moving an inch or so per year southwestward over the hot spot for millions of years as the calderas formed, groups of calderas are strung out like beads on a string across parts of Idaho and Wyoming. • The thermal energy of the presumed Yellowstone hotspot fuels more than 10,000 hot pools and springs, geysers (like Old Faithful), and bubbling mudpots (pools of boiling mud).

32. Remember, ALL of this occurs in Geologic time!!! Our lifetimes do not compare… not even close. • Millions to tens of millions of years. • Plates move and mountain ranges rise by only mm to cm each year.

33. Plate Boundaries Mid-ocean ridge Rift Valley Trench Volcanoes Subduction Mt. Rainier Mt. Everest Convergent San Andres Fault Transform