RECONSTRUCTING THE SUBAQUEOUS PHREATOMAGMATIC ERUPTION AT THE 71 GULCH VOLCANO, WESTERN SNAKE RIVER PLAIN, IDAHO

Phreatomagmatic eruptions result from the interaction of magma with water and/or wet sediment. The resulting deposits provide information regarding the environment at the time of the eruption and the accessibility of water to the magmatic plumbing system. Data collected from the field helps determine the conditions of the eruption such as, the composition of the magma and host sediment, the depth of the lake bottom, and the contact surface between the lake and eruptive sediments. 71 Gulch is a shallow (<15 m below the surface) basaltic fissure structure proximal to the western Snake River Plain (WSRP) basin in southwest Idaho. The entire field stretches ~2 km west-to-east and contains evidence of multiple pyroclastic vents (≥8). Erosion has made 71 Gulch an ideal field site to investigate, because it exposes the intrusive to eruptive interface. Dikes intruding host and pyroclastic sediment (953 – 863 m asl) constrain the eruptive surface. The magma composition is mostly basaltic and host sediment is mostly quartz sandstone and phreatomagmatic lapilli tuffs. Field mapping marked the distribution, location, and elevation of in situ pillow lavas, which constrain the subaqueous regions of the field. The pillow lavas range 953 – 889 m and the pyroclastic vents range 969 – 900 m in elevation. In addition, mapped contacts between the undisturbed lake sediment and pyroclastic deposits (952 – 841 m) define the paleo-lake bottom as well as measure the depth of the water at the time of the eruption. There was a gradual 0.05° slope at the lake bottom, and by calculating the difference in pillow elevation, the minimum water height was 64 m. Extensive pillows suggest a wholly wet eruption, however both lava drips on the inside of a dike feeding the western vent and spatter deposits in the same vent suggest locally dry conditions at one time during the eruption. Reconstructing the 71 Gulch eruption by studying the exposed shallow plumbing, the explosive and effusive facies, and the relative water depths will provide insight into how subaqueous phreatomagmatic eruptions evolve.