mountain.—In addition to spilling out as broad flows of melted rock, it is common for the tremendous mass of molten lava in a volcano to break through the mountain side. The lava filling of such a crack or fissure is known as a dike. After it cools the material filling these fissures is often harder than the surrounding rock. Subsequent wash of water may cut away the softer bordering material, leaving the hard filling of the fissure as a sharp ridge. Devils Backbone, on the west side of Crater Lake, is an illustration of such a lava dike.

Action of streams and glaciers on the mountain in the course of its building.—In the section of layers forming the rim of the mountain we find evidences of wash by water. In some places this is shown by the cutting of valleys; at others, by the accumulation of water-carried ash, gravel, and boulders.

Glacier ice carrying sand, pebbles, and boulders scratches or polishes the rock surface as it moves slowly over it. Glacial polish and thick beds of material carried by glaciers are common around the mountain. They are present on the surface rock and seem also to appear between earlier layers, showing that glaciers were present at various stages in the history of the mountain.

Broad U-shaped valleys cut at various points around the crater are also characteristic of glacial action. Kerr Notch is such an evidence of glacial erosion. It was through a similar ancient glacial notch that the lava forming Llao Rock flowed.

Forming of the crater.—The broken edges of rock layers seen on the crater slopes indicate widening of the crater in all directions. The edges of these rock layers inside the crater wall are clearly exposed because they have been sharply broken around the entire inner rim region. This fracturing occurred in the course of widening the crater. Increase in size of the opening at the summit of the mountain, which eventually formed the present crater, may have been caused by tremendous explosions, or by collapse of the peak, or by a combination of such activities.

If the activity of a volcano diminishes slowly, growth of the mountain may end in forming a symmetrical cone. If activity continues by spasmodic outbursts, explosions may blow away a considerable part of the peak. Other conditions may bring about undermining of the walls in such manner as to produce a wide cauldronlike crater, but without tremendous explosions.

Recent investigations by Howel Williams, under a grant from the National Academy of Sciences, have led to the conclusion that the crater owes its origin principally to collapse or engulfment of the mountain peak.

Formation of the crater by collapse was first proposed by J. S. Diller of the United States Geological Survey. Diller’s explanation differs from that of Williams principally in the method by which the void beneath the crest of Mount Mazama was formed. Diller thought that great quantities of molten rock were drained away through subterranean passages, thus weakening the support of the mountain peak and causing ultimate collapse.

In a report to be published by Williams, he describes great quantities of volcanic ash or pumice extending for a distance of more than 80 miles northeast of Mount Mazama. This ash is equivalent to more than 20 cubic miles of material and is thought to have been blown from the mountain in a catastrophic event, and carried northeastward by the prevailing winds. Analysis of this ash shows that it is new material derived from the magma within the volcano and not finely divided fragments of the original mountain walls.

Following this explosion the mountain is thought to have literally boiled over, pouring out great quantities of frothy magma which flowed down the sides of the mountain and overflowed the lowlands below. The greater quantity extended to the south and southwest for distances up to 35 miles. This material poured out from the crater as a series of avalanches which must have flowed at a terrific speed for those on the south and west sides of the mountain did not begin to deposit their load until they reached a distance of 4 or 5 miles from the crater.

Accompanying these explosions and the outpouring of this lava material, cracks developed in the flanks of the mountain and eventually the top collapsed and was engulfed within the void produced by the outpourings of ash and molten rock, thus forming the crater as we see it today.

By projecting the present slopes of the crater rim upward and making adjustments to conform to the slopes of similar volcanoes, it has been estimated that approximately 17 cubic miles of old lava has been removed, which formed the upper part of ancient Mount Mazama.

Toward the close of the activity forming the crater, minor eruptions produced Wizard Island and possibly other cones.—If the lake were removed, the crater would be seen as a relatively flat-floored cavity extending as a maximum about 2,000 feet below the present lake surface. In this great depression Wizard Island would appear as one of perhaps several volcanic cones produced by pouring out of lava and cinders in the last period of volcanic activity. Forming of the present floor probably involved many stages, during some of which the cauldronlike crater may have been occupied by wide stretches of molten lava, as in the “lake of fire,” at Kilauea, in Hawaii.

Origin of the lake.—The water of Crater Lake is derived from rainfall and snowfall over this crater region, together with snow blown into the depression. The lake is not known to have an outlet except by seepage. The conditions of evaporation, seepage, and precipitation are in a state of balance which makes possible this accumulation of water and maintenance of approximately this water level. If the region were at a different altitude, or in a different location, the lake might not have been formed.

It is conceivable that in the course of late stages in its history, and under climatic conditions different from those of the present, the crater may at times have been filled in part with ice.

The existence of Crater Lake was made possible by the building of a mountain, in the elevated summit of which there could be formed a wide and deep cavity having no outlet, except by seepage, and no inlet. The conditions required for the accumulation of a body of water with the peculiar beauty of this lake are furnished in a crater produced by a combination of those tremendous forces found in the power and heat of a volcano.

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 blue of the deeper water is brought out in contrast with the brilliant green of shallow areas along the margin.

The deep blue of the lake is believed to be caused chiefly by the scattering of light in water of exceptional depth and clearness. The color is thought to be due to the same cause that produces the blue of the sky where light passes through deep atmosphere.

The extraordinary beauty of the lake arises in part from its great depth, the clearness of the water, and of the atmosphere above it, and from favorable conditions presented in viewing it from the high crater rim.

Grant photo
NATURALIST GUIDING A PARTY OF VISITORS OVER THE RIM TRAIL

PLACES OF INTEREST

SINNOTT MEMORIAL

In recognition of great service to Crater