Maar-Diatreme Volcanoes, their Formation, and their Setting in Hard-rock or Soft-rock Environments

Maar-diatreme volcanoes mostly form when rising magma in basic to ultrabasic volcanic fields interacts explosively with groundwater. Less commonly, there also exist maars associated with intermediate to acid magmas. The formation and growth of the maar-diatreme volcano type, the second most common volcano type on continents and islands, is reviewed applying the phre- atomagmatic model of its formation. The site of explosions is the root zone which penetrates downward on its own feeder dyke. Because of repeatedly developing mass deficiencies in the root zone, the overlying cone-shaped diatreme and the maar crater are the consequent collapse/subsidence features. Prolonged downward penetration of the root zone leads to repeated collapse phases of both the diatreme and maar, thus both grow in size the longer the maar-diatreme volcano is active. Two contrasting environments exist with respect to groundwater availability for the phreatomagmatic explosions: the hard-rock environment which is a joint aquifer and the soft-rock environment which is a pore aquifer. In the hard-rock environment, the tephra of maar-diatreme volcanoes contains large volumes of rock clasts originally derived from the hard country rocks formerly occupy- ing the root zone resp. the diatreme and maar crater. In the soft-rock environment, the tephra contains large amounts of the indi- vidual minerals and pebbles from the sediments but hardly any rock clast consisting of indurated sediments. The two environments are frequently combined in areas where unconsolidated, water-saturated sediments overlie diagenetically indurated sediments and/or crystalline basement rocks or in areas where unconsolidated sediments contain interbedded solidified and jointed sills and lava flows. Maar-diatreme volcanoes have typically formed in areas characterized by rather normal groundwater conditions. In contrast, are- as characterized by highly permeable water-saturated rocks or pebble beds just below the Earth's surface give rise to tuff-rings or tuff-cones. Tuff-cones also form in the shallow sea or in lakes.