Hillside Of Pinnacles

The Pinnacles are erosional remnants of a pyroclastic flow. The pyroclastic flow is called the Wineglass Welded Tuff by geologists. The color change from the base of the flow to the top is dramatic evidence for chemical zonation in the magma chamber of Mount Mazama. The lower light-colored layer is rhyodacite pumice. It was erupted early and tapped the upper part of the magma chamber. As the eruption continued it tapped progressively deeper layers in the magma chamber. These deeper layers were more mafic in composition which gives them their dark color. Since they were erupted later they were deposited on top. The dark layer is a basaltic andesite scoria. Photograph by Steve Mattox, August 1987.

Mafic: An igneous composed chiefly of one or more dark-colored minerals.