At the Big Hole maar in central Oregon, rising basalt magma came in contact with abundant ground water at a depth of more than 200 m below the surface, causing phreatomagmatic eruptions, possibly 20,000 yrs ago. Late-stage subsidence along a ring fault accounts for the large crater cut into older rocks. Subsequent erosion of pyroclastic debris in the crater wall increased the diameter of the crater while decreasing its depth.

Investigation of the distribution pattern of large ejected blocks and application of a model of block acceleration in a dense rising two-phase system permits calculation of the apparent density and velocity of the fluid system. Values for the final strong eruptions are 0.01g/cm3 and 226 m/sec and for some of the earlier strong eruptions 0.01g/cm3 and 200 m/sec. The kinetic energy of some of the strongest eruptions was approximately 1.33 × 1021 ergs.

Vesiculated tuffs, that is, tuffs with smooth-walled bubbles between the particles, are present between 1 and 2.5 km from the center of the crater; they appear to be the deposits of base surges.

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