The eruption of Krakatoa unleashed a tsunami, a large seismic sea wave, that swept the coasts of Java and Sumatra and drowned more than 36,000 people. A volcano can grow with frightening speed and often affects territory far beyond the area on which the cone forms. When volcanoes are born in the sea, the eruptions may be more violent than those on land because the contact between molten rock and seawater produces steam. Volcanoes also create craters and calderas. Craters are formed either by the massive collapse of material during volcanic activity, by unusually violent explosions, or later by erosion during dormancy.
Calderas are large, basin-shaped depressions.
Most of them are formed after a magma chamber drains and no longer supports the overlying cone, which then collapses inward to create the basin. One of the most famous examples is the still-active Kilauea caldera in Hawaii. The Earth's Surface Types of Volcanoes Volcanoes are usually classified by shape and size. These are determined by such factors as the volume and type of volcanic material ejected, the sequence and variety of eruptions, and the environment. Among the most common types are shield volcanoes, stratovolcanoes, and cinder cones.