Abstract

Large (0.3–4 km3) andesite and dacite lava flows at Mount Rainier, Washington, sit atop or are perched along the sides of high ridges separating deep valleys. Early researchers proposed that these ridge-forming lavas flowed into paleovalleys and displaced rivers to their margins; entrenchment of the rivers then left the lavas atop ridges. On the basis of exceptional flow thickness, ice-contact features, and eruption age measurements, we propose that the lavas flowed beside and between valley glaciers that filled the adjacent valleys in the Pleistocene. When the glaciers retreated, the flows were left high on the adjacent ridges. These lavas were never situated at valley floors and do not represent products of reversed topography. Instead, ridge-forming and perched lava flows at Mount Rainier and at many other high stratovolcanoes illustrate the ability of ice to dam, deflect, and confine flowing lava.

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