The Geology of Crater Lake National Park, Oregon

With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams

Introduction

CRATER LAKE NATIONAL PARK lies on the crest of the Cascade Range in southern Oregon, in the heart of the great volcanic province of the Pacific Northwest. No matter from which direction the traveler approaches, at least the last sixty miles of his journey are over volcanic rocks. Within the park, the road climbs steadily, mile after mile up the flanks of an old volcano until it brings him with breath-taking suddenness to the very brink of the precipitous walls that encircle Crater Lake. The scene is one of overwhelming beauty. Twenty miles of cliffs, delicately tinted in shades of gray, pink, purple, orange, and brown, enclose a circular body of water unexcelled in the rich depth of its blueness. The thoughtful visitor cannot remain long on the edge of this vast depression without asking himself how it came to be. He cannot doubt that the lake lies within an extinct volcano, for on all sides the lavas and fragmental deposits dip outward; nor can he doubt that the summit of the original cone rose far above its present ruins. How else can he explain those great U-shaped glacial valleys beheaded by the caldera walls? Surely glaciers powerful enough to carve such valleys must have had their source on a vanished peak. This much is clear from a cursory study. Somehow this ancestral, ice-clad peak disappeared, leaving in its place a giant depression more than 5 miles wide and 4000 feet deep. Shortly thereafter new eruptions began on the floor of the caldera, and rain and melting snows covered it with water. The scene of desolation left by the catastrophe slowly took on its present grandeur.

In order to appreciate most clearly the relation of Crater Lake to the neighboring volcanoes of the Cascade Range, the visitor should climb to the top of the Watchman, a peak on the western rim, or, better, to the summit of Mount Scott, which rises a short distance to the east of the rim. From either vantage point, he is sure to be impressed by the fact that Crater Lake does indeed occupy a basin in the summit of a decapitated volcano, and, giving fancy sway, he may – picture the remnant slopes sweeping upward to a peak a mile above the crater rim. To this imaginary peak others have given the name Mount Mazama. In a rough way, the panorama from the summit of the Watchman or Mount Scott may be separated into four parts, distinct both topographically and geologically. Recognition of these units is fundamental to an understanding of the history of the region.

There is, first, a north-south belt, generally between 20 and 25 miles wide, that passes through Crater Lake. This belt, known as the High Cascades, consists of Pliocene and younger volcanic cones. Here the typical slopes are constructional. To this belt belongs the chain of majestic cones that extends from Lassen Peak in the south to Mount Baker in the north, 500 miles removed. To it belong Mounts Shasta, McLoughlin, and Thielsen, the Three Sisters, Mounts Hood, St. Helens, Adams, Rainier, and Baker, and a host of lesser peaks.

West of the High Cascades lies a second belt, parallel and slightly wider, composed of older volcanic rocks ranging in age born Eocene to Upper Miocene. It is customary to speak of this belt as the Western Cascades. Here the conical peaks characteristic of the High Cascades are conspicuous by their absence; here there are no original volcanic slopes. Westward-flowing rivers with headwaters among the High Cascades, such as the Rogue and the Umpqua, have cut deep canyons across the gently folded rocks of the Western Cascades, and subsequent tributaries have carved broad north-south valleys along the strike. In contrast with the youthful topography of the High Cascades, the topography here is mature or submature. It is a region of narrow-crested, winding ridges. By reason of these contrasts, the accustomed eye has no difficulty in following from afar the boundary between the two belts.

Beyond the Western Cascades, on the distant sky line, rise the rugged peaks of a third region, the Klamath-Siskiyou Mountains, composed for the most part of Jurassic and older schists intruded by large bodies of igneous rock ranging in character from, serpentine to granite. This metamorphic and igneous series makes up the so-called “bedrock complex” beneath the Western and High Cascades. Undoubtedly such rocks pass beneath Crater Lake, for they reappear from below the Tertiary volcanic cover in eastern Oregon. Since Cretaceous time, all but the fringe of the Klamath-Siskiyou Mountains have remained above the sea, and though suffering erosion throughout that period, the mountains have continued to rise.

The topographic contrasts between the three provinces just described, striking as they are, seem small in comparison with that which distinguishes the fourth, for when the observer turns eastward, away from the “sea of peaks and ridges,” he looks on an immense plateau of basalt, relieved here and there by steepscarped, tilted fault-block mountains. This is the Interior Platform, which stretches northward into Washington and eastward, beyond the range of vision, into Idaho. Much of its surface is composed of Pliocene basalt, but where the plateau is entrenched by deep box canyons the older, gently folded Tertiary volcanic rocks are exposed. These pass beneath the High Cascade lavas and interfinger with the volcanic deposits of the Western Cascades.

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