The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams
The Foundations of Mount Mazama
The High Cascades Between Mounts Shasta and Mazama
We have seen that in the Western Cascades few scattered intrusions remain to suggest the position of former vents. In the High Cascades, on the contrary, even the oldest volcanoes preserve much of their original form, and the younger cones have scarcely been modified by erosion.
Viewed as a whole, the High Cascades form a plateau surmounted by steep-sided shield volcanoes of olivine basalt and olivine-bearing basic andesites, and by younger composite cones, chiefly composed of hypersthene andesite, that lie along or near the crest. The latter so far dwarf all other volcanoes in height that one is likely to gain the false impression that the High Cascades are predominantly composed of andesite. On Shasta, McLoughlin, Mazama, Hood, Adams, Rainier, and Baker, hypersthene andesite is either the dominant or the only type of lava. But large as these volcanoes are, they form only a small part of the total volume of lava in the High Cascades, and they are chiefly Pleistocene and Recent accumulations on a far more voluminous basement of Pliocene basalts and olivine-bearing basic andesites. The first recognition of this important fact we owe to Thayer.18
The earlier volcanoes of the High Cascades were formed almost exclusively by quiet effusions of basic lava. Explosive eruptions did pile small cinder cones on the summits of some of the basaltic shields, but these were merely weak, culminating explosions. In the younger andesitic volcanoes, on the other hand, explosive activity alternated with quiet effusive activity throughout growth, and in general explosions became more frequent and violent in the later stages. Moreover, in the closing phases the andesitic magma of several of the younger volcanoes was strongly differentiated so that dacites and basalts were erupted, the former as pumice and viscous flows and the latter chiefly as scoria. No such differentiation took place among the older lavas in the southern part of the High Cascades.
For emphasis, then, it may be repeated that the High Cascades in the region that concerns us here were constructed primarily by copious outpourings of basic, olivine-bearing lavas of striking uniformity, some of which escaped from central vents to form steep-sided shield volcanoes of Icelandic type, while others escaped from fissures near the western border of the belt and poured down deep canyons cut through the older lavas of the Western Cascades. While these basaltic volcanoes were active in the High Cascades, others, even more numerous, were erupting wide floods of similar basalt on the plateau to the east.
Toward the close of the Pliocene period, or early in the Pleistocene, at many points along and near the crest of the Cascades a series of imposing andesitic cones began to develop. These include the major volcanoes with which the name Cascades is usually associated. Among them must be counted Mount Mazama, the ancestral cone in the summit of which Crater Lake lies.
The eruption of olivine basalt and olivine-bearing andesite did not cease with the rise of the great hypersthene andesite cones. On the contrary, it went on at the same time from neighboring vents, and has only ceased within the past few centuries. Accordingly, we see in the High Cascades, side by side with the composite andesite cones, well preserved basaltic shield volcanoes, like the Goose Nest, 20 miles north of Mount Shasta, and Brown Mountain.
The salient features of the High Cascades having been enumerated, it seems proper to refer to the results of a reconnaissance of that part of the range which lies adjacent to Crater Lake and extends thence southward to Mount Shasta. For convenience, the description may be separated into two main parts, dealing with (I) the volcanic cones, and (2) the intracanyon flows along the western margin of the High Cascades.