Maar

A broad, low-relief volcanic crater formed when rising magma explosively interacts with groundwater. Many maars later fill with water, forming lakes.

Phreatomagmatism: Volcanic activity driven by explosive interaction between magma and external water. This process produces the blasts that excavate a maar.
Volcanic crater: A bowl-shaped depression formed by volcanic explosions, collapse, or both. A maar is a distinctive kind of volcanic crater.
Ukinrek Maars: Two maars formed during a 1977 eruption in Alaska's Becharof Lake region. Their directly observed formation provides a modern example of maar-producing explosions.
Caldera: A large volcanic depression formed when the ground collapses into a partially emptied magma reservoir. Calderas result mainly from collapse, unlike the explosive excavation characteristic of maars.
Groundwater: Water held beneath Earth's surface in the pores and fractures of rock and sediment. It supplies the external water that can trigger explosive magma fragmentation.
Tephra: Any fragmental material ejected into the air by a volcanic eruption. Maar explosions eject tephra that accumulates around the crater.
Eifel maars: A group of volcanic explosion craters in Germany's Eifel region, many containing lakes. These classic examples gave maars their geological prominence and include well-preserved crater lakes.
Scoria cone: A steep-sided volcanic cone built mostly from loose, vesicular fragments of basaltic magma. Scoria cones grow upward from erupted fragments, whereas maars are broad craters excavated by explosions.
Magma: Molten or partially molten rock beneath Earth's surface, containing dissolved gases and crystals. Rising magma supplies heat and fragments that power maar-forming explosions.
Tuff: Rock formed from compacted or cemented volcanic ash and other fine ejecta. Consolidated deposits around maars can preserve the products of their explosive eruptions.

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