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The western Snake River Plain (SRP) is a southeast-northwest–trending complex graben bounded on the SW by the Owyhee Front. This graben, which merges with the central SRP at its southeast end near Bruneau Canyon, is a subsidiary tectonic feature that resulted from southwest-northeast extension as the main SRP–Yellowstone hotspot trend evolved. Silicic volcanism during the late Miocene along the Owyhee Front and in the central SRP resulted in large welded-tuff and rhyolite lava flows being erupted; these are well exposed southwest of the western Snake River Plain (WSRP) and in the western Mount Bennett Hills, northeast of where the western and central SRP merge. As the WSRP graben developed, it held a large lake, into which some of the rhyolite units flowed. At various stops described in this field guide, the characteristics of rhyolite lavas versus rheomorphically deformed welded tuffs, and of subaerially deposited versus subaqueously deposited rhyolite units, are displayed. During Pliocene and Pleistocene time, basaltic volcanism partially filled the WSRP graben and developed a basalt plateau across much of the central SRP. At various stops described in this field guide, the characteristics of subaerial versus subaqueous basalt flows, and of phreatomagmatic vent complexes, are displayed. Altogether, the stops described provide a guide to the wide variety of rhyolitic and basaltic volcanism phenomena in the western SRP, Owhyee Front, western Mount Bennett Hills, and Bruneau Canyon areas.

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