Bimodal basalt-rhyolite magmatism in the central and western Snake River Plain, Idaho and Oregon

The purpose of this trip is to examine Miocene to Pleistocene basalt and rhyolite flows, ignimbrites and hypabyssal intrusions in a transect from the western Snake River Plain graben across the older part of the Snake River Plain "hot-spot-track." The earlier, dominantly explosive rhyolitic phase of volcanism will be examined primarily in the Cassia Mountains, near Twin Falls, Idaho. The second day of the field trip will focus on the Graveyard Point intrusion, a strongly differentiated diabase sill in easternmost Oregon. This late Tertiary sill is well exposed from floor to roof in sections up to 150 m thick, and is an example of the type of solidified shallow magma chamber that may be present beneath some Snake River Plain basalt volcanoes. The field trip will conclude with an examination of the diverse styles of effusive and explosive basaltic volcanism in the central and western Snake River Plain.