Voluminous flows of olivine basalt also poured down the canyon of Butte Creek from fissures at the western base of the Rustler Peak, Blue Rock, and Mount McLoughlin volcanoes. On lithological grounds it is impossible to say where the intracanyon flows begin and the lavas of the cones end, though the boundary is indicated approximately by a distinct topographic break at an elevation of about 3600 feet.
Finally, an area of olivine basalt, 60 square miles in extent, forms the eastern half of Shasta Valley, separating the older volcanics of the Western Cascades from the cones of the High Cascades. The source of this basalt must be in concealed fissures at the northwest base of Mount Shasta, and when the lava was erupted it must have been distinctly fluid, for it poured for a distance of almost 20 miles down a gentle gradient. Here and there large tubes traverse the lava, one of which, Pluto’s Cave, is traceable for more than a mile and has a maximum bore of 70 by 80 feet. The preservation of ropy surfaces and many “Schollendome,” and the general absence of weathering suggest that the flow is more likely Recent than Pleistocene in age.
To summarize: After the folding and faulting of the Western Cascade series and the intrusion of dioritic stocks into them at the close of the Miocene, quiet eruptions of olivine basalt and basic, olivine-bearing andesite built a broad north-south belt of shield volcanoes of Icelandic type. This process probably occupied most of Pliocene time. Throughout this long interval the lavas of the High Cascades underwent little or no differentiation. Toward the close of the Pliocene, or early in the Pleistocene period, a few vents in the High Cascade chain began to erupt hypersthene andesite. These continued to be active throughout the Pleistocene and some have ceased to erupt only within the past few thousand years. They now form the crowning peaks of the range, Mounts Shasta, Mazama, Hood, Adams, Rainier, and Baker. At some of these centers, particularly among those at the southern end of the Cascades, differentiation became more and more pronounced as activity proceeded, until eruptions of hypersthene andesite finally gave way to eruptions of dacite and basalt. On the other hand, at the northern end of the chain, on Mounts Rainier and Baker, hypersthene andesite remained the only type of lava even to the end.
While these andesitic cones grew, the eruption of olivine basalt from other centers did not cease. On the contrary, large basaltic shields were formed at the same time, and extensive floods of olivine basalt poured westward down canyons cut through the older lavas of the High Cascades and the underlying volcanics of the Western Cascades.
We are now in a position to appreciate more fully the details of the geology of Crater Lake National Park, the main theme of this report. Our concern in the pages that follow is with the growth and destruction of one of the andesitic volcanoes, Mount Mazama, and with the irregular basement left by the outpourings of older basaltic cones on which it grew.
***previous*** — ***next***
