1. Continental Drift Seafloor Spreading Theory of Plate Tectonics

2. Continental Drift Evidence for Continental Drift
Pangea: Alfred Wegener (1912) suggested that all the continents were joined together at some time in the past
Continental Drift was suggested that this huge land mass (called Pangea) began to break apart about 200 mya
Idea wasn’t accepted as he was unable to explain how they drifted apart
Long after his death (1930) was there enough evidence to accept Continental Drift

3. Pangea Videos

4. Clues to support Continental Drift
Fossil clues: fossils found (Mesosaurus) in S America and Africa that were freshwater land animals that couldn’t have swum sea water to both sides, therefore lived together before the split
Widespread Plants: Glossopteris fossil found on Africa, Australia, India, Antarctica, S America suggested that these plants thrived once when continents were attached

5. Clues to support Continental Drift
Rock clues: similar rock structures found on different continents
Climate Clues: glacier deposits found in tropical climates and tropical plants found on island in Arctic Ocean

6. How could continents drift?
Wegener’s radical idea of continental drift provided no evidence as to how the continents plowed through higher density ocean floor material
His proposal suggested it was caused by Earth’s rotational spin, but no evidence supported that
Only after discovery of seafloor spreading was his idea re-introduced as explanation for continental drift

7. Seafloor Spreading
Mapping the Ocean Floor: sound and radar waves were used to map the ocean floor in 1940’s and 50’s
Noticed that in middle of oceans were huge mountain ridges
This discovery caused scientists to ask why?

8. Seafloor Moves Harry Hess suggested that the seafloor moves
Proposed that hot, less dense material below Earth’s crust rises toward the surface at mid ocean ridges , lows sideways carrying the seafloor away from te ridge in both directions

9. Evidence for Spreading
Research ship Glomar Challenger gathered data in 1968 by drilling ocean floor
Found younger rock are located at the mid-oceanic ridges and older rock found farther away
Mid-oceanic ridges have molten rock, heat, exotic life-forms

10. Evidence for Spreading
Magnetic clues: when molten rock cools, it cools with magnetic attractions to the north and south poles
Found that magnetic north has changed several times in earth’s history and the magnetic poles are preserved in the rock at mid-oceanic ridges
On each side of the mid-ocean ridges portrays these magnetic changes
Magnetometer detects the magnetic field reversals on both side of ridges indicating that the ridges are moving apart when new rock is formed

11. Seafloor spreading and Magnetic reversals

12. Theory of Plate Tectonics
Plate Movement: plate tectonics combine the knowledge of continental drift and seafloor spreading in the Theory of Plate Tectonics
Earths crust and upper mantle are broke into plates
These plates ride on top a plastic-like layer of the mantle much like a raft on water

13. Composition of Earth’s Plates
Lithosphere is the outer layer of earth’s crust
Made of less dense material than plastic-like layer below it (asthenosphere)

14. Plate Boundaries
When plates move they interact with each other in many ways
They can move toward, pull away or slide past each other.
When plates interact, the result of their movement is seen at the plate boundaries

15. Plate Boundaries
Plates Move Apart: Divergent boundaries are areas where plates move away from each other
Mid-Atlantic Ridge is divergent boundary between North American Plate and the Eurasian and African Plate
Great Rift Valley in eastern Africa is result of continental plate being pulled apart

16. Plate Boundaries
Plates Move together: Convergent boundaries are areas where plates move together
As crust is added in one area, crust disappears in other areas when seafloor cools, becomes denser and sinks (convergent boundaries)
When oceanic crusts converge with less dense continental crust, the denser oceanic plate sinks under the continental plate in subduction zones
Deep-sea trenches are associated with convergent boundaries

18. Plate Boundaries
As ocean plates sink under continental plates, they warm and melt and magma forms. This less dense magma rises to surfaces and form volcanoes
Cascade Mt ranges are result of convergent boundaries and subduction zones
A subduction zone can form from two oceanic plates that collide where the older, denser plate sinks (Mariana Islands are chain of volcanic islands )
When 2 continental plates collide, usually they crumple up forming mountain ranges (earthquakes common, but no volcanoes) (Himalayan Mts)

19. Plate Boundaries
When plates slide past each other: transform boundaries are result where two plates slide past each other
They move in opposite direction and usually earthquakes are associated with transform boundaries
San Andreas fault is an example

20. Causes of Plate Tectonics
Question about what was force that would cause plates to move were finally answered by convection currents
Convection currents in earth’s crust are currents caused by warm less dense magma rising
Some magma reaches the surface at ridges, some is moved away from rising magma and drags the plates with them. They cool and settle back to earths center where it moves and is heated up and cycle repeats

21. Causes of Plate Tectonics

22. Causes of Plate Tectonics

23. Features Caused by Plate Tectonics
Interactions of plates build mountains, create ocean basins and cause volcanoes
Earthquakes are associated with these interactions
Tension forces result in earth crust being pulled apart and form normal faults
Causes large blocks of crust to break, tilt, or slide down and large fault-block mountains form
Rift valleys and mid-oceanic ridges are associated with divergent boundaries where tension forces pull crust apart and normal faults occur

24. Tension forces and Normal faults
Rock layers above the fault move down when compared with rock layers below the fault

25. Compression Forces cause Reverse Faults
Where plates meet, compression force cause massive folding and faulting into mountain ranges (Himalaya) by reverse faulting
Rock layers above the fault surface is moved up relative to rock layers below the fault

26. Reverse and normal faulting
Reverse fault:
Compression forces
Normal fault:
Tension forces

27. Strike-slip Faults
Caused at transform boundaries where 2 plates move past each other (usually in opposite directions, but can be where both plates are moving in same direction but at different speeds)
These are called strike-slip faults

28. New technology and Plate tectonics
With GPS and modern technology we have been able to record crustal movement on plates as low as 1 cm per year
Pacific plate is moving at rate of 2.5 cm per year average