The Eastern Snake River Plain

A Volcanic Wonderland

4/3  The Eastern Snake River Plain:  A volcanic wonderland  –  Presented by Mike McCurry, Idaho State University, Department of Geosciences.

For over 10 million years the Eastern Snake River Plain (ESRP) has been the scene of some of earth’s most extreme forms of volcanism. These have varied from development of older, rhyolitic caldera-dominated supervolcanoes, to younger basaltic Hawiian-style shield volcanoes and linear rift systems, to steep-sided rhyolite lava domes.  Beginning 4-5 million years ago, as Idaho drifted relentlessly to the southwest off a putative mantle plume, supervolcano-type activity waned locally, shifting northeast into Wyoming (Yellowstone NP country).  Interestingly, however, instead of volcanism waning to inactivity as might be expected, the ESRP transitioned into a distinctive and sustained low-intensity form of volcanism that has produced many of the iconic features of southern Idaho such as the Three Buttes, Hell’s Half Acre, China Hat and Craters of the Moon National Monument.  This sustained low-intensity phase of volcanism is dominated by eruptions of hundreds of Hawaiian-like, geochemically primitive (i.e. mantle derived) basalt lava flows, such as are beautifully exemplified on Interstate 15 at the Hell’s Half Acre rest stop between Idaho Falls and Blackfoot.  These accumulated steadily, in concert with subsidence of the ESRP, to a thickness of up to 2 km in some areas, largely burying the older Yellowstone-like supervolcano systems.  Importantly, this phase of volcanic activity also includes eruptions of volcanic rocks that span an extreme range of composition, many types of which occur at Craters of the Moon.  These rocks vary from alkalic low silica basalt (‘basanite’) to evolved alkalic ferroan varieties of basalt, andesite, and dacite, and at their most extreme, some of the most geochemically evolved ferroan rhyolites on earth (e.g., Big Southern Butte).  This talk gives an overview of some of the wonders of ESRP volcanic geology.  It will summarize current ideas for the origin of the volcanic rocks, and for the evolving patterns in their distribution, geochemical characteristics, eruption rates and related volcanic processes.