How Volcanoes Work

Image Described in Caption
 Two of the Tharsis Shield Volcanoes -- These shield volcanoes are 10 to 100 times the size of those on Earth. They probably form by voluminous eruptions of basaltic magma from a stationary mantle-plume source. Without the lateral motion of plate tectonics, these crust remains stationary over the source, which allows the shield volcanoes to grow to enormous sizes. Courtesy of NASA/JPL