Spatter-dike reveals subterranean magma diversions: Consequences for small multivent basaltic eruptions

Small-volume, short-lived basaltic volcanoes, Earth's most common subaerially, are abundant and tractable in size, thus offering opportunities for learning how magma behaves in the critical transition from coherent magma in dikes to pyroclastic dispersions. Spatter-dikes at the Castle Butte Trading Post volcanic complex (Hopi Buttes volcanic field, Arizona, USA), exposed similar to 150 m below the paleosurface, present an outstanding record of this transition. They record pulsatory magma rise and eruption, fissure extension, vent stepping, and conduit wall-rock failure, providing a critical link to activity witnessed and geophysically monitored during historic eruptions of well-observed and -monitored large-volume basaltic volcanoes. Castle Butte comprises four linked and closely spaced spatter-dikes and maar-diatremes. The spatter-dikes consist of bedded, variably welded deposits plus wall-rock debris in multiple sequences demarcated by truncation surfaces that indicate progressive southwest-northeast emplacement. They reveal a shallow plumbing cycle of pulsating, weak, hot spatter fragmentation, concurrent wall-rock failure and periodic slips that truncated and downdropped bedded deposits. In addition to vertical pulsation, we infer from the northeastward-younging spatter succession that repeated magma diversions facilitated episodes of magma withdrawal that caused the slips during northeastward extension of the complex toward the large maar-diatreme.