Continental Drift to Plate Tectonics

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Continental Drift

1915

*A theory proposed by Alfred Wegner in 1915
*Continents were once joined together forming a supercontinent named Pangae meaning 'all land'.
*Evidence for Continental Drift include:
-fossils found on adjacent continents, also the animals could not have made the long swim between the continents.
-rocks on different continents show the same age and type
-climate, there are glacial deposits found in regions in the southern continents (South America, Southern Africa, Antarctica, India, and Australia) and coal fields in the eastern U.S., Europe, and Siberia would suggest that the continents were located in tropical climates with swampy areas.
*Wegner's hypothesis rejected because he could not explain how the continents would have moved.

Convection Current Theory

1929

*Proposed by British geologist, Arthur Holmes.
*Proposed that convection currents are the force behind continental drift.
*A convection current is the flow of material in the mantle of the Earth due to unequal heating.
*Theory was not accepted at the time but was later supported.

Discovery of Mid-Ocean Ridges

1950

*The Navy used sonar to map the ocean floor.
*Discovered chains of mountains underwater throughout all of Earth's oceans.
*Named them Mid-Ocean Ridges.
*Began recognizing magnetic variations across the ocean floor

Polarity

1963

*Discovered by two British geologists, Fred Vine and Drummond Mathews.
*Linked together the Navy's discovery of different strengths of magnetism in ocean rocks.
*Stronger magnetism showed normal polarity while weaker magnetism showed reverse polarity.
*Proved Hess's theory right by showing new crust was continuously being produced at mid-ocean ridges.

Seafloor Spreading

1963 - 1968

*Proposed by Harry Hess in 1963.
*Proposed that molten material comes through the crack in mid ocean ridges which causes the seafloor to spread until some of it sinks into the mantle of the Earth at trenches.
*Evidence for Seafloor Spreading is:
-Using a mini-submarine (Alvin) scientists saw the first pictures of the ocean floor and molten material on mid-ocean ridges.
-Found that there are magnetic strip patterns in rocks on the ocean floor. This also identified reverse polarity in the Earth's south and north poles.
-Earthquake patterns support subduction around trenches showing shallow earthquakes occur in and around the trenches while deeper earthquakes occur farther away.
-In 1968, the Glomar Challenge drilled holes in the ocean floor and found that the farther rocks were away from the ridge, the older they were. This evidence supported that newer crust formed at the center of the ridge.

Glomar Challenge Findings

1968

*A vessel that set sail to study the mid-ocean ridges between South America and Africa.
*Drilled holes into the ocean to uncover that the rocks were younger the closer they were to the mid-ocean ridge, and older the farther they were from the ridge.
-The oldest were found along the edge of the surrounding continents.
*Helped to prove Hess's theory of sea-floor spreading by showing new crust was coming up from the mid-ocean ridges.

Theory of Plate Tectonics

1968

*Proposed by Canadian geologist J. Tuzo Wilson.
*Combined evidence from all previous discoveries.
*In the Theory of Plate Tectonics, the Earth's lithospheric plates move relative to each other rater slowly and are moved by convection currents in the mantle.