THE GEOLOGIC STORY OF CRATER LAKE

Origin of the Mountain.—Geologists say that the slope, which visitors ascend to view the lake, and the crater wall rising 500 to 2,000 feet above the water are the remnant of a mountain, which stood more than 12,000 feet high. This ancient peak, now destroyed, is known as Mount Mazama.

In comparatively recent geologic time, numerous volcanic peaks were formed near the western edge of a vast lava plateau covering portions of Oregon, Washington, Idaho, Montana, Nevada, and California, These make up the Cascade Range, of which Mount Mazama was one of the commanding peaks. It was built by successive lava flows with some accumulation of volcanic ash. The cone thus formed was modified by streams and glaciers which carved valleys in its sides and deposited rock debris on its flanks, The layered character and different formations of the mountain are now clearly exposed in numerous places within the crater wall.

Forming of Dikes.—In addition to broad surface flows, it is common for molten lava to be squeezed into cracks, or fissures, that develop in a volcano. Such filling results in dikes, or walls, frequently harder than the enclosing rock. At Crater Lake the destruction of the mountain and subsequent erosion have exposed numerous dikes in the wall, of which the Devil's Backbone on the west side of the crater is the outstanding example.

Action of Streams and Glaciers on the Mountain in the Course of Its Building.—In the layers forming the crater wall there is evidence of the action of water, In some places this is shown by the cutting of valleys; in others, by the accumulation of water-carried gravel and boulders.

Glacial ice, carrying sand, pebbles, and boulders, scratches and polishes rock surfaces over which it moves. Glacial polish and thick beds of glacial debris are common around the mountain. They occur on the surface rock and between earlier layers, showing that glaciers existed at various stages in the history of the mountain.

U-shaped valleys, such as Kerr Notch, Sun Notch, and Munson Valley on the southeast slope of Mount Mazama, are evidence of glaciation. The lava flow forming Llao Rock filled an ancient glacial notch.

Forming of the Crater.—Many geologists have concluded that the basin occupied by the lake resulted from the collapse and subsidence of the volcanic cone of Mount Mazama. This explanation was first proposed by J. S. Diller, of the U. S. Geological Survey, who considered that the support of the summit was weakened by drainage of great quantities of molten rock through subterranean cracks. The pit thus formed grew progressively larger in all directions, as is indicated by the broken edges exposed around its rim today. Extensive study by Prof. Howel Williams of the University of California, led him to practically the same conclusion.

In his delightful, popular, and scientifically accurate book, "Crater Lake, The Story of Its Origin," Williams describes great quantities of pumice extending more than 80 miles northeast of Mount Mazama. This amounts to more than 10 cubic miles of material, thought to have been blown from the mountain in a catastrophic event and carried northeastward by the prevailing winds. Analysis shows that this is material derived from the heart of the volcano and not finely divided fragments of the original mountain walls.

Following this eruption, the crater is believed literally to have boiled over, pouring out great quantities of frothy material as a series of glowing avalanches. These avalanches must have traveled at a terrific speed down the valleys, for those to the south and west did not begin to deposit their load until they had reached a distance of 4 to 5 miles, The greater quantity flowed down the mountain to the south and southwest for distances up to 35 miles from the source.

Accompanying these eruptions, cracks developed in the flanks of the mountain so that the top collapsed, being engulfed in the void produced by the ejection of the pumice and lava and the withdrawal of 10 cubic miles of molten rock into swarms of cracks that opened parallel to the axis of the Cascade Range. Thus was formed the great pit as we see it today.

By projecting the slopes of the mountain remnant upward, conforming to the slopes of similar volcanoes, it has been estimated that approximately 17 cubic miles of the upper part of ancient Mount Mazama was destroyed by the collapse.

The Growth of Wizard Island. After the destruction of the peak, volcanic activity within the crater produced Wizard Island and perhaps other cones. These cones rise above a relatively flat floor, the lowest part of which is almost 2,000 feet below the surface of the present lake. Its flatness indicates that the crater may at times have been occupied by wide stretches of molten lava.

Origin of the Lake.—The water of Crater Lake is derived from rainfall and snowfall, also from the snow blown into the depression. The lake has no inlet and no outlet, except by seepage. Evaporation, seepage, and precipitation are in a state of balance which maintain an approximately constant water level. If the basin lake were at a different altitude, or in a different location, the lake might not have been formed.

Color an Outstanding Character of Crater Lake.—The color of Crater Lake is generally recognized as the most attractive feature of this region. Among spectacular lakes of the world there are none in which the depth of color and brilliance of blue are more striking.

The deep blue of the lake is believed to be caused chiefly by the scattering of sunlight in water of exceptional depth and clearness, the blue rays of sunlight being reflected from the water, rays of other colors being absorbed.

The extraordinary beauty of the lake arises in part from its great depth, the clearness of the water and of the atmosphere above it, the multicolored towering walls, and from the favorable view points on the rim.