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 which formed Llao Rock filled an ancient glacial notch.
Forming of the Caldera.
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 United States 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 pumice was 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. The total volume of the ejected lava was about 5 cubic miles. It is believed that an additional 1.5 cubic miles of old rock were carried away at the same time.
Accompanying these eruptions, which occurred within the past 7,000 years, 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 probably opened parallel to the axis of the Cascade Range. Thus was formed the great pit that was to become Crater Lake.
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 caldera 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.
