_Origin of the Mountain._—Visitors to Crater Lake find they must ascend
extensive slopes of volcanic ash or pumice to view the lake resting in a
crater approximately 5 miles in diameter, with walls from 500 to 2,000
feet high. Geologists tell us this rim is the remnant of an ancient
mountain which stood more than 14,000 feet high.
In 1896 the Mazama Club, a mountain-climbing group of Portland, Oreg.,
visited the lake and with fitting ceremonies gave to the ancient
mountain, never viewed by man, the name Mount Mazama.
In comparatively recent geologic time enormous flows of molten rock
poured out over an area of more than 100,000 square miles, extending
into Oregon, Washington, Montana, Idaho, Nevada, and California. These
masses of lava came to the surface largely through great cracks or
fissures in the earth’s surface. A typical example of such extensive
flows may be seen in the lava beds forming the Columbia River Gorge.
Numerous volcanoes were formed in this lava region during the relatively
late outpourings of molten rock through small openings. The mass of
these volcanoes represents only an extremely small volume in proportion
to the total mass of lava. Mount Mazama at Crater Lake is one of these
volcanoes, likewise are the volcanic cones of the Cascade Range. The
more noted of these are Mount Rainier (14,408 feet), Mount Shasta
(14,161 feet), Mount Adams (12,326 feet), Mount Hood (11,225 feet),
Mount Baker (10,750 feet), Mount Lassen (10,453 feet), and Mount St.
Helena (9,697 feet).
The mountain in which Crater Lake rests was built principally by lava
flows, poured out layer upon layer, and to a lesser degree by the piling
up of volcanic ash, soil, and the deposits of streams and glaciers as
they flowed down the mountain. At Dutton Cliff we see an example of
successive layers of lava and volcanic ash. Near Discovery Point, in
addition to layers of lava and volcanic ash, one may also see examples
of glacial deposits and glacial striae or scratches.
One can understand Crater Lake in its relation to the volcano only when
the mountain is considered as the result of a building process extending
over long periods in which many changes took place. The following is an
explanation of some of the processes.
_Lava outpourings through splitting of the 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.
[Illustration: _Grant photo_
NATURALIST GUIDING A PARTY OF VISITORS OVER THE RIM TRAIL]