Reference 1318

Abstract

Owyhee County in southwestern Idaho is the site of voluminousrhyolites that have lavalike textures and structures. Unlike typicalrhyolite lavas, however, the rhyolites of southwestern Idaho formtabular sheets that are distributed over many tens of squarekilometers, and in some places many hundreds of square kilometers.This report describes the petrography and chemistry of these unusualrocks and presents various lines of evidence that indicate that most,if not all, were erupted as ash flows at temperatures of 1,100 degreesC or higher, and that the ash flows coalesced to viscous liquidsbefore final emplacement and cooling. The tuffs were erupted fromdepths that were too great to allow calderas to form.

45 samples from this reference