maar Crater, often occupied by a shallow lake, produced by an explosive volcanic eruption. Normally it is surrounded by a low rampart or ring of ejected material. Typically, maars are formed by the explosive interaction of volcanic magma with groundwater. The ejected material is a mixture of country rock and highly fragmented ash.

maar A crater, often occupied by a shallow lake, which is produced by an explosive volcanic eruption. Normally it is surrounded by a low rampart or ring of ejected material. Typically, maars are formed by the explosive interaction of volcanic magma with groundwater. The ejected material is a mixture of country rock and highly fragmented ash.