Plate Tectonics

In 1912 Alfred Wegener, a meteorologist, revived a theory that the continents of the eastern and western hemispheres were once joined. Wegener, along with his followers showed how the east coast of the Americas align with the westside of the Old World, a coincidence that Leonardo da Vinci commented upon. But no one could come up with a convincing way to push, shove, or drift whole continents for thousands of kilometers. It was thought that the deep granite continental masses must somehow plow right through the bedrock of the seafloors.

Calculations of energy required produced forbidding estimates, it seemed impossible despite the evidence at hand. In the past ten years a revelation swept through geology and vindicated Wegener, he didnt imagine the mechanism that caused continental drift. The theory of plate tectonics emerged. The Earths crust is divided into two types of terrain: Continents, which occupy about 40 percent of the Earths surface and non-continents, which is generally deep ocean. Continental crust is lighter, thicker, and much older. Some thats been found is more than 3 billion years old.

The oldest rock under the open sea is only 150 to 250 million years old and much younger. The seashore and the edge of the continent should not be confused. Much of the continents are submerged in shelves extending hundreds of kilometers offshore to a depth of several hundred meters where the continental slope begins. Theres a stiff outer rind below the continent and non- continent which is called the lithosphere and is about 100 kilometers thick. Below that is the plastic, the asthenosphere, the thickness difference between the continents and the non-continents is shown by how deep the continents ride into the lithosphere.

They are like icebergs with crust extending 35 to 40 kilometers deep, most of it is granite and granulate rocks. More than 75 percent of the continents lie between sea level and about two kilometers in height, while 75 percent of the seabottom lies at the rather great depth of three to six kilometers below sea level. After World War II it was discovered that sediments became progressively thicker away from the mid-Atlantic ridge. The reason was not clear it could be that the sea floor furthest from the ridge was oldest, or something was causing a faster sedimentation rate away from the ridge and near the continents.

The conclusive evidence that the whole lot of it – ocean floors, continents, islands and all – are constantly shifting around like ducks in a pond was assembled in the mid-1960s by a team of Cambridge University geologists. The theory of plate tectonics was rapidly assembled: As fast as sea floor comes out of ocean ridges it has to be consumed somewhere. The stiff sheet of lithosphere that appears in the ridges goes down in subduction zones usually marked by deep trenches in the ocean floor. Often the trenches are near continental margins.

The continents move and shift, crash into each other from opposite sides of subduction zones, even split apart when spreading zones open across them, but they never go down into the Earth. A common theory is connected to the observation the as the ocean floor moves away from the spreading zones, it drops to progressively greater depths. The Earth is divided into six major plates and a number of smaller ones. Each has three types of boundaries: spreading zones, subduction zones, and shear zones.

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