Continental drift and plate tectonics

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The discovery of plate tectonics:


Plate tectonics gives us the mechanism for continental drift.
[Continental drift was a revolutionary theory explaining that continents shift position on Earth's surface. The theory was proposed by geophysicist and meteorologist Alfred Wegener in 1912, but was rejected by mainstream science at the time. Scientists confirmed some of Wegener's ideas decades later, which are now part of the widely accepted theory of plate tectonics. . ./QUOTE]

Wegener formulated his theory of continental drift by comparing rock formations and fossil plants and animals on different continents as well as the shape of the continental shelf of the continents and fitted the continents together based on all that information.

What Wegener did not have was the mechanism—he did not know and could not guess what moved the continents together and apart. This is why his theory was not widely accepted.

Ocean floor is continually created in the mid ocean ridged all oceans have—lines of underwater volcanoes. Magnetic surveys of the ocean floor showed that the ocean floor was composed of strips of opposite polarity—so one strip would have the polarity with north being where the magnetic north pole is now and some with the north pole being where the magnetic south pole now is. So we know that the magnetic poles flip every so often.

Ocean floors, oceanic plates are thin. Continental plates are thick—hence we can drill down thousands of metres looking for oil and gas.

As ocean floor is constantly being created the outer edge of the plate has to go somewhere—the planet is not increasing in size. Continental plates can be compressed and that is how the Himalayas and the European Alps were formed. The MacDonnell Ranges of northern SA and the southern NT are remnants of a mountain range 4500m high.

But ocean plates can also turn under a continental plate, going very deep down. There is friction and resistance to the movement of this subducted plate so it does not move smoothly down—it moves in fits and starts and these cause major earthquakes.

Plate tectonics explains why there are zones of earthquakes and vulcanism—the edges of adjoining plates acting on each other.

The west coast of the US is the zone where two plates are moving past each other, again with friction causing stops and starts, earthquakes.
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