Abstract

Diatremes are debris-filled structures beneath maars that result from many magma-water (phreatomagmatic) explosions during a monogenetic volcano’s lifetime. A long-standing model requires deepening explosions, due to water table drawdown, that eject progressively deeper-seated country rock from the explosion sites, while the overlying diatreme and its surface crater widen due to subsidence. A revised model is proposed wherein explosions can take place at any level within a diatreme at a given time, most effectively venting material from near-surface explosions. Deep-seated country rock lithics in tephra deposits record stepwise vertical mixing of material by upward-directed debris jets and downward subsidence, rather than direct ejection from deep explosions. Juvenile and lithic clasts erupted during a given explosion may have had a complex history within the diatreme and need not directly reflect fragmentation or brecciation during the explosion that ejects them.

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