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Image copyright 1997 by the American Geophysical Union
As the Earth cools, hot material from the deep interior rises to the surface. Hot material is depicted in red in this drawing, under an ocean shown in blue green.
The hotter material elevates the nearby layers, and the cooler, denser lithosphere (in yellow in the drawing) slides away from the elevated region. The drawing shows that the cool crust slides at a rate of about 4 cm per year. The elevated region where new material is coming forth is called a “spreading ridge”. Most of the spreading ridges of today are to be found in the central portion of the world’s oceans.
The large version of this drawing shows a spreading ridge at the left and a slab of lithosphere being subducted at the bottom right. Near the subducting slab, remelting, and volcano formation are taking place.
source: windows to the universe