The rhyohte plateau in Yellowstone Park is made up of flows and welded tuff with subsidiary rhyolite domes, basalt, and rhyolite-basalt mix-lava. The two most important units are the Yellowstone tuff, exposed over 600 square miles within the Park, and a group of younger flows, the Plateau flows, which cover 1000 square miles in the Madison, Central, and Pitchstone plateaus.

The Plateau flows occupy a tectonic basin rimmed by Yellowstone tuff and older rocks in central and southwestern Yellowstone Park. Individual flows range up to 1000 feet in thickness and cover areas up to at least 100 square miles. Surficial features, including vent domes, are locally well preserved. Exposed portions of these flows are, for the most part, banded obsidian and perlite and breccias formed by incorporation of fragments of pumiceous crust in the moving flows.

The Yellowstone tuff is welded to obsidian at its base and grades upward through indurated, lithoidal rhyolite to loose ash at the top of an uneroded section. Probably all the Yellowstone tuff was erupted in a single rapid series of eruptions from dispersed vents.

Textural and structural evidence suggests that the Yellowstone tuff was emplaced as pyroclastic flows. A thermodynamic analysis of this type of eruption is given. Energy changes within the conduit and during emplacement are evaluated. The minimum temperature at which rhyolite glass will weld, determined experimentally, is approximately 600°C. The data of Tuttle and Bowen (1958) are used to fix the range of initial magma temperature. Welding in the Yellowstone tuff can be explained if the tuff was emplaced as pyroclastic flows and if the magma contained less than about 4 per cent H2O at the start of the eruption.

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