<P> A volcanic crater is a roughly circular depression in the ground caused by volcanic activity . It is typically a bowl - shaped feature within which occurs a vent or vents . During volcanic eruptions, molten magma and volcanic gases rise from an underground magma chamber, through a tube - shaped conduit, until they reach the crater's vent, from where the gases escape into the atmosphere and the magma is erupted as lava . A volcanic crater can be of large dimensions, and sometimes of great depth . During certain types of explosive eruptions, a volcano's magma chamber may empty enough for an area above it to subside, forming a type of larger depression known as a caldera . </P> <P> In most volcanoes, the crater is situated at the top of a mountain formed from the erupted volcanic deposits such as lava flows and tephra . Volcanoes that terminate in such a summit crater are usually of a conical form . Other volcanic craters may be found on the flanks of volcanoes, and these are commonly referred to as flank craters . Some volcanic craters may fill either fully or partially with rain and / or melted snow, forming a crater lake . </P> <P> A crater may be breached during an eruption, either by explosions or by lava, or through later erosion . Breached craters have a much lower rim on one side . </P> <P> Some volcanoes, such as maars, consist of a crater alone, with scarcely any mountain at all . These volcanic explosion craters are formed when magma rises through water - saturated rocks, which causes a phreatic eruption . Volcanic craters from phreatic eruptions often occur on plains away from other obvious volcanoes . Not all volcanoes form craters . </P>

What does it mean when a volcano crater collapses