A maar is a low-relief, broad volcanic crater formed by a series of shallow explosive eruptions. The explosions are usually caused by the heating and boiling of groundwater. The walls of maars are usually formed mostly by fragments of pre-existing rocks, and, after the end of the eruptive activity, they are often filled with water to form a lake. Interesting maar structures are found in the Eifel area, in Germany.
Viti maar at Askja, Iceland; Öskjuvatn caldera in background. Note person (P. Alean) on crater rim. Photo: J. Alean