This is a drawing of how a hot spot under the crust builds land on the surface.
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Image copyright 1997 by the American Geophysical Union. Further electronic distribution is not allowed.
The Hawaiian Islands are an example of the way some volcanoes are made. A rising hot bubble of material finds it's way into the crust of the Earth from the deep interior, and erupts material unto the surface. This bubble or "plume" is called a "hot spot". Lava from the eruption turns to layers of rock and builds a volcanic "cone". Continual eruptions eventually build a whole island on the surface.
On Earth, the hot spot is still, and the portion of crust on which the islands are formed moves past the hot spot. In this drawing, the crustal plate moves to the left, past the stationary hot spot or plume. Each island which is created has a volcano associated with it. The oldest island is at the upper left, the youngest at the lower right. This way, a chain of islands are formed, each one younger than the last. The oldest islands wear down by erosion, become covered by ocean water, and eventually return to the sea.
A similar process built the Tharsis Ridge of Mars and many volcanic rises on Venus. On these planets, the crust did not move past the hot spot, but stayed in place. The volcanoes of Mars which were built this way became *very* large indeed.
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