This is a drawing of how a hot spot under the crust builds land on the surface.
Click on image for full size
Image copyright 1997 by the American Geophysical Union. Further electronic distribution is not allowed.
This drawing shows another way that islands are made. A rising hot bubble of material finds it's way into the crust of the Earth from the deep interior, and begins to erupt. 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, with a volcano in the middle.
A similar process built the Tharsis Ridge of Marsand many volcanic rises on Venus. The volcanoes of Mars which were built this way became *very* large.
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