Reconstructing the 71 Gulch eruption: Implications for the Evolution of Phreatomagmatic Eruptions and their Products

Abstract

71 Gulch is a basaltic fissure structure located in the western Snake River Plain, southwestern, Idaho that erupted roughly 4 Ma. The entire volcanic field stretches 2 km west to-east and produced 9 phreatomagmatic vents and subaqueous deposits including: billowed dikes, blocky and fluidal peperites, pillow lavas, glassy pyroclastic bombs, and mingled clasts. Data collected from the field helps determine the paleoenvironment at the time of the eruption: the composition of the magma and the host sediment, the local and regional water levels, and the depth of intrusive features, such as dikes and peperites. Sediment-magma mingling is prominent both on the large- (meter) and small- (micrometer) scale and is related to billowed and peperitic textures, which is important when determining the explosive behavior of a phreatomagmatic eruption. Variables such as lithostatic and hydrostatic pressure may contribute to the way sediment and magma interact and are ultimately preserved, which is important in showing whether an eruption was explosive or nonexplosive. Extensive subaqueous deposits suggest a wholly wet eruption, however local subaerial deposits imply locally dry conditions occurred before the end of the eruption. Reconstructing the 71 Gulch eruption by understanding the exposed shallow plumbing, the explosive to effusive deposits, and the relative local water levels provide insight into how subaqueous phreatomagmatic fissure eruptions evolve.