Explosion depths for phreatomagmatic eruptions

Subsurface phreatomagmatic explosions can result from the interaction of ascending magma with groundwater. Experiments over a wide range of energies show that for a given energy there is a depth below which an explosion will be contained within the subsurface (not erupt), and there is a corresponding shallower depth that will optimize ejecta dispersal. We combine these relationships with constraints on the energies of phreatomagmatic explosions at maar-diatreme volcanoes and show that most eruptions are likely sourced by explosions in the uppermost ~200m, and even shallower ones (<100m) are likely to dominate deposition onto tephra rings. Most explosions below ~200m will not erupt but contribute to formation of, and to the vertical mixing of materials within, a diatreme (vent structure), with only rare very high energy explosions between ~200 and 500m erupting. Similar constraints likely apply at other volcanoes that experience phreatomagmatic explosions. Key Points Subsurface explosive tests constrain depth of containment for given energy Intrusion sizes, ballistics, and rock fracture constrain phreatomagmatic energies Most eruptions sourced by <200 m deep explosions