The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams
The Parasitic Scoria Cones of Mount Mazama
The Younger Cones
This familiar landmark on the south slope of Mount Mazama rises 750 feet above the undulating crest of Vidae Ridge, a short distance from Tututni Pass. It rests upon and is surrounded by glacial drift. The cone itself has not been modified by glaciation and is as well preserved as any within the park. Most of it consists of lapilli and bombs of red, brown, and black porphyritic basaltic andesite like that which makes up the cone of Wizard Island. Few fragments exceed even 3 inches across, though occasional bombs about a foot in diameter are also present. Some of the ejecta are angular and have smooth, plane surfaces. Probably these were blown out when solid. Others are sub-rounded and scoriaceous and must have been erupted in a viscous state. With these are many lumps of old andesite and dacite torn from the underlying basement. Much brick-red ash was also blown from Crater Peak and may be seen as far as 2 miles from the cone. The activity at Crater Peak was not wholly explosive, for several dark, scoriaceous flows of lava issued from the flanks of the cone and spread over the surrounding flats. Possibly the dark, scoriaceous lavas exposed about 2 miles southeast of the cone are also related to this vent. The summit crater is a shallow, saucer-shaped basin, approximately 150 feet deep, and is breached on its eastern side.