27 Nature of the Lavas

The Geology of Crater Lake National Park, OregonWith a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams

The Main Andesite Cone of Mount Mazama


Nature of the Lavas

By far the greater part of Mount Mazama is composed of thin flows of hypersthene-rich augite andesite, similar to the lavas of Mounts Rainier and Baker and the bulk of Mount Shasta. Andesites rich in hornblende with or without biotite are almost completely absent. Invariably the lavas are characterized by abundant and large phenocrysts of. feldspar. Texturally, they show wide variation, even within a single flow, from holocrystalline to glassy. Rarely, however, are they as rich in glass as the dacites, nor do they show the hair-fine banding characteristic of the latter. Indeed, except on a broad scale, it is difficult to detect fluxion. In color, the andesites vary from black to pale gray, according to the content of glass, and many flows are pale brown, pink, or dark brick red where they have been affected by escaping gases. Nevertheless, flows which, for these reasons, seem in the field to be of widely different types are singularly alike under the microscope.

One of the most distinctive features of the Mazama andesites is the widespread occurrence of basic inclusions. Few lavas are entirely devoid of them, and in some they make up no less than a quarter of the volume. Generally they are more plentiful among the later flows, where many exceed a foot in length. Most of these inclusions are ovoid and range from I inch to 3 or 4 inches across. Compared with the enclosing lavas, they are much more porous and crystalline, and most of them stand out conspicuously in the darker matrix by reason of their pale-gray or pink color. They consist of crisscross prisms of hornblende, pyroxene, and feldspar separated by a small amount of interstitial glass. Similar inclusions are common among the lavas of other Cascade volcanoes; they represent fragments torn from the walls of the conduits and the roofs of the magma chambers as the lavas rose to the surface.

In the great cliff sections along the south, east, and west walls of the caldera, the Mazama andesites usually range between 10 and 80 feet in thickness, most of them measuring between 20 and 30 feet thick. They are considerably thinner than the andesites of Mount Rainier, for which Coombs1 estimates an average thickness of about 80 feet. But a cursory examination shows that the Mazama flows pinch and swell with great rapidity and that the massive lavas commonly merge into red, slaggy breccias, like those plainly visible on the Castle Crags, east of the Park Headquarters.