Maar-diatreme phreatomagmatism at Hopi Buttes,Navajo Nation (Arizona), USA

Remnants of an extensive maar-diatreme volcanic field are magnificently exposed at various depths of erosion in the Hopi Buttes volcanic field of northeastern Arizona. Field and petrographic studies of both the maar and diatreme elements of a selection of volcanoes within the field show that: (1) lower sections of the maar rim sequences are typically rich in sandy mudrock derived from the pre-eruptive Mio-Pliocene Bidahochi Formation, and the muddy Bidahochi sediment was soft and wet at the time of maar eruptions; (2) beds higher within the rims contain generally increased proportions of sandstone clasts from the Triassic Wingate Formation. In the diatremes, late-emplaced breccia has deeper-seated lithics than more marginal breccia emplaced earlier; and (3) many vents are topped by megacryst-enriched scoria and spatter, and deep-seated xenoliths are known only from upper diatreme and craterfilling tephra. These observations show that: (1) eruptions at Hopi Buttes involved interaction of magma with unconsolidated mudrock at shallow levels, and the phreatomagmatic processes that provided the bulk of the energy involved in the violent eruptions were driven by the interaction of magma and wet sediment; (2) the locus of explosive activity migrated downward as eruptions progressed; and (3) the closing stage of many eruptions was characterized by rapid magma rise and relative depletion of water.