High Quality Content by WIKIPEDIA articles! A volcanic crater is a circular depression in the ground caused by volcanic activity. It is typically a basin, circular in form within which occurs a vent (or vents) from which magma erupts as gases, lava, and ejecta. A crater can be of large dimensions, and sometimes of great depth. During certain types of climactic eruptions, the volcano`s magma chamber may empty enough for an area above it to subside, forming what may appear to be a crater but is actually known as a caldera.