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Hydrovolcanic eruptions occur when rising magma violently fragments while mixing with shallow surface water or groundwater. These eruptions, among the most violent on Earth, generate hundreds to thousands of explosions throughout the course of an eruptive event. Each of these explosions ejects a mixture of juvenile and accidental clasts, gas, and water droplets. The solid materials either fall to the ground from tephra jets or collapse to form pyroclastic density currents (PDCs). The deposits of these eruptions build up rings of bedded tuff around the vent, recording both a wide variety of pyroclastic depositional mechanisms, and important changes in the eruptive style with time. This field trip will explore the deposits of basaltic, hydrovolcanic eruptions in the Fort Rock–Christmas Valley basin, the location of an intermittent, fluctuating, widespread Pleistocene lake (Fossil Lake). Basaltic volcanoes erupted in the center of the lake basin are characterized by Surtseyan eruptions (standing water), along the lake basin margins by maar eruptions (groundwater), and beyond the lake margin by scoria cones. The focus of the trip will be Fort Rock (Surtseyan) and the Table Rock Complex (large Surtseyan-tuff cone, large maar, and seven minor craters). This trip offers the opportunity to examine (1) settings under which explosive hydrovolcanic eruptions occur, (2) depositional characteristics that infer eruptive conditions, (3) a wide variety of pyroclastic deposits (i.e., fallout through air or a body of water, eruption-fed subaqueous sediment gravity current deposits, PDC deposits), and (4) mega-dunes (200 m wavelength dunes associated with large scale, dilute, PDCs from an energetic, mafic hydrovolcanic eruption).

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