maar

A maar is a broad, low-relief volcanic crater that forms from a phreatomagmatic eruption caused by the interaction of groundwater with magma. The explosive steam-driven activity ejects ash, tephra, and blocks, leaving a wide, circular depression at the surface.

Formation and morphology: The eruption occurs when rising magma heats groundwater, producing rapid steam expansion that fragments rock and excavates a crater. The resulting vent commonly develops a ring of ejecta and a surrounding tuff ring; a deeper vent may create a diatreme beneath the crater. After activity ceases, the crater may fill with water to form a maar lake or remain dry.

Distribution and significance: Maars occur in volcanic fields where groundwater is present and magma is near the surface. They are common in continental rift zones and intraplate volcanic regions, with well-studied examples in the Eifel region of Germany and other parts of Europe, as well as in other continents.

Notable examples: Meerfelder Maar and Pulvermaar in Germany are among the best-described maars. Worldwide, maars vary in size from tens to hundreds of meters across.

Ecology and current use: Crater lakes hosted by maars often support diverse ecosystems and are popular sites for recreation and tourism. Maars also provide records of past eruption and hydrological conditions.