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
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
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
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.