Subaqueous pyroclastic flows form almost one-half of the 10,000-foot Ohanapecosh Formation (Eocene and Oligocene?) in the eastern part of Mount Rainier National Park, Washington. Most of these flows probably originated by the sloughing of debris from the flanks of active underwater volcanoes during and after pyroclastic eruptions. Some of the pyroclastic flows caused directly by underwater eruptions may not have been completely quenched and could have traveled as steam-inflated slurries of pyroclastic debris and water. Most of them, however, were probably cold or only slightly warm as they flowed into deeper water.

The Ohanapecosh subaqueous pyroclastic flows are extensive, nonwelded deposits of lapilli-tuff or fine tuff-breccia ranging in thickness from 10 to more than 200 feet. They are interbedded with thinner and generally finer turbidity-current and ash-fall deposits formed by smaller and more water-rich slumps of pyroclastic debris from the underwater volcanoes and by ash falls that rained into the water.

The three main types of flows are thought to be related to three different kinds of volcanic activity. The most common flows—those containing a variety of lithic fragments and variable amounts of pumice—were probably produced by underwater phreatic eruptions. The flows rich in pumice and glass shards were probably caused by underwater eruptions of rapidly vesiculating magma which, on land, would have produced hot ash flows or ash falls. The least common flows—those containing only one or two kinds of lithic fragments—were probably derived from fairly homogeneous bodies, such as domes, spines, and lava flows that were erupted into water and fragmented by steam-blast explosions.

Remnants of the underwater Ohanapecosh volcanoes consist chiefly of coarse tuff-breccia piled around filled volcanic vents. Unbrecciated lava flows are subordinate and were probably deposited on land when the volcanoes succeeded in growing into islands.

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