A caldera-causing eruption is the most devastating type of volcanic eruption. It permanently alters the environment of the surrounding area.A caldera is not the same thing as a crater. Craters are formed by the outward explosion of rocks and other materials from a volcano. Calderas are formed by theinward collapse of a volcano. Craters are usually more circular than calderas. (Calderas may have parts of their sides missing because land collapses unevenly.) Craters are also usually much smaller than calderas, only extending to a maximum of one kilometer (less than a mile) in diameter.