Close-Up--Plate Tectonics Theory

Midocean Ridges

The midocean ridges are rifts in the ocean floor through which very hot magma wells up from the mantle. This creates new oceanic plates and these move away from the ridge. As new magma wells up and intrudes into the seafloor, the older crust is moved out away from the midocean ridge like a conveyor belt. As well, the heat from the mantle causes the ridge to swell upward and this plus the outpouring of lava create an undersea mountain ridge that is higher than the cooler rock on either side of it. That is, the cooled crust settles down. Hence, an immense ridge system like a huge mountain belt, runs through all the oceans and makes a continuous mountain range some 40,000 miles long. When magma solidifies, it freezes and locks in a magnetic polarity corresponding to that of the Earth's magnetic field. Over long periods of time, the Earth's polarity changes (this is called a magnetic reversal); that is, the magnetic north pole becomes the magnetic south pole, and so the polarity of the newly formed crust changes as well. The conveyor beltlike action of plates moving away from the ridge produces a series of strips of rock magnetized in opposite senses, with the magnetic stripes parallel and symmetrical to the ridges. On either side of the ridge you will have a similar magnetic striping (and the rocks will be the same age), similar to two identical mirror image bar codes on either side of the ridge.

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