Rheomorphism in outflow sheets of McDermitt Tuff, Nevada and Oregon

Folds in outflow sheets of McDermitt Tuff, from the eruption that formed McDermitt Caldera in Nevada and Oregon about 16.4 million years ago.  The folding is called “rheomorphism” and occurred as the hot ash deposit flowed under its own weight shortly after it was deposited over preexisting topographic highs and lows.  The texture resembles that of rhyolite lava flows, making it very difficult to distinguish lava and ash deposits in this area.  This example is from the Double H Mountains south of the caldera.  Photo by Chris Henry, Nevada Bureau of Mines and Geology.