DEPTH CONTROLS OF WATER/MAGMA INTERACTION IN MAAR VOLCANISM

Field and laboratory measurements can be used to identify the aquifer rocks involved in maar volcano eruptions. These measurements help constrain the depth below the surface of the water/magma interaction that cause maar volcanism. Commonly applied measures include the abundance and type of lithic fragments in the tephra, the depth of the crater, well logs, and regional stratigraphy. While most maars show evidence for interaction within a few hundred meters below the earth’s surface within the zone of saturation, maar craters associated with diatreme eruption suggest that interaction can occur at much greater depths in the earth’s crust. Because aquifer pore pressure increases with depth, many researchers have assumed that once pressure exceeds water’s critical pressure, interaction ceases to be explosive because steam is not formed. This assumption is incorrect according to results of theoretical and experimental work, which show the potential for dynamic interaction at confining pressures up to and perhaps exceeding 100 MPa. Even for geopressured aquifers (confined under lithostatic stress), these results suggest that interaction can initiate at maximum depths >4 km, reaching depths perhaps exceeding 10 km. For interaction at great depth, considerations of hydrofracture and host rock brecciation, aquifer thermal and pressure gradients, and seismic response are important in evaluating eruption dynamics. Especially for interactions at great depth, eruptive venting may be delayed or absent. Detailed evaluation of multiphase thermal and fluid dynamics provides methods of formulating hypotheses about the depth (pressure) controls of water/magma interaction. These hypotheses are tested by results of experiments and field observations.