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Plate Tectonics

Plate Tectonics. The Theory.

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Plate Tectonics

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  1. Plate Tectonics

  2. The Theory • The earth’s outer shell is called the lithosphere and is composed of a number of rigid plates. There are approximately 8 plates plus a few dozen smaller plates. The concept was born in the late 1960’s by combining 2 preexisting ideas…….continental drift and seafloor spreading.

  3. Sea Floor Spreading • This theory suggest that the sea floor moves away from the mid oceanic ridge as a result of mantle convection. According to the concept, the sea floor is moving like a conveyorbeltaway from the crest of the mid oceanic ridge to disappear finally by plunging beneath a continent or island arc. This ridge crest is called a spreading center.

  4. Convection Current Theory • A slow convection circulation may be set up by temperaturedifferences in the rock; large cells driven by the rising of hotrock and sinking of coldrock. Hot mantle rock rises beneath the ridgecrest and sinking rock descends into the mantle at the trenches

  5. Evidence of the Convection Current Theory • Ridges- there is rising material at the ridges which creates tensionalforces and high heat flows • Trenches-cold, dense rockplunges down in trenches creating compressional forces and low heat flows.

  6. Plate Boundaries • A. Divergence- Boundary between plates that are movingapart; sea floor spreading. • Characteristics- A rift valley is marked by highheatflow, basaltic volcanism, and tensional fractures. • Example: Atlantic Ocean

  7. Magnetic Reversals • Most magnetic anomalies are arranged in bands that lie parallel to the rift valley of the mid oceanic ridge (alternating positive and negative). The pattern is symmetrical about the ridge crest. The pattern of magnetic anomalies on one side of the ridge is a mirror image of the pattern on the other side.

  8. B. Convergence- boundary between plates that are movingtogether • a. Ocean-Ocean (subduction)subducting plate bends downward forming an oceanic trench; earthquakes occur. Island arcs are formed (volcanic) • Example: Pacific Ocean

  9. Convergence • b. Ocean-continent (subduction)-A dense oceanic plate bends under a continental plate forming a trench and a volcanicmt. range on the edge of the continent. Earthquakes and volcanic activity occur.

  10. Convergence • c. Continent-continent (collision)-Two landmasses with the samedensitycollide. Compressional forces form foldedmts. at the boundary.

  11. C. Transform Boundary • This occurs when one plate slideshorizontally past another single or group of faults. It is marked by shallow focus earthquakes

  12. Hot Spot Theory • Seamounts in the Pacific reveal a chain of volcanic structures extending from the Hawaiian Islands to Midway and then continuing north toward the Aleutian Trench • Scientist proposed that a hotspot exists within the mantle and emits magma on the seafloor. Presumably as the Pacific Plate moved over the hot spot, successive volcanoes emerged.

  13. Evidence • Dating methods revealed an increase in age with an increase in distance from Hawaii (youngest island) • Suiko Seamount is 65 million years old; Midway is 27 million years old and Hawaii is 1 million years.

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