Geology Reference
In-Depth Information
Three Fingered Jack, Mount Washington, and the Three Sisters of the central Cascades looking toward
the north (photo courtesy Dave Rohr)
andacite and dacite stratocones and shield volcanoes
waters reach a depth of 1,996 feet, which is the deepest
from numerous flows. These early eruptions spread
fresh water body in the United States. A balance
thin layers of lava 20 to 30 feet thick over the land-
between precipitation, seepage, and evaporation has
scape in all directions. Following repeated eruptions
kept the lake level near its present position throughout
and dormant periods, a large magma chamber devel-
historic time.
oped under the Mt. Mazama stratovolcano during the
In the late 1980s commercial concerns applied
final eruptive stages. At the same time small vents and
for
permission to drill for geothermal power in the
cones on the flanks of the dome extruded viscous
vicinity of Crater Lake. They were opposed by environ-
silica-rich rhyodacitic lavas as late as 7,000 years ago.
mental groups which feared that drilling, even a
Shortly after this, a sudden violent explosion
distance away from the lake shore, might interfere with
from a single vent in the main caldera sent out tower-
yet unknown plumbing systems connected to the
ing clouds of ash and fiery fragments with such force
springs said to exist on the lake floor. Research con-
that layers of debris 6 inches deep covered the area of
ducted with a remote-controlled submersible and later
present day Bend 70 miles away. Following the incan-
with a manned submersible revealed red and brown
descent ash cloud, hot glowing avalanches of pumice
pigments in lake floor sediments as well as what appear
sped down the sides in immense quantities obliterating
to be algal mattes, both of which strongly point to the
all topography. Approximately six cubic miles of
presence of hydrothermal vents. Additional evidence of
magma drained from the chamber before the roof
thermal springs on the bottom of the lake is indicated
collapsed into the caldera. The entire event took place
by the richness of dissolved solids, high amounts of
so rapidly that the youngest flow had yet to cool before
radon and helium gas, and slightly raised temperatures,
the caldera had been formed.
These discoveries, while not conclusive, have retarded
Before the eruption and collapse, Mt. Mazama
the issuing of permits for hydrothermal drilling.
is estimated to have been 10,000 to 12,000 feet high.
Of the 40 or more caves of significant size
Today the caldera averages 8,000 feet high, so that
within Crater Lake National Park, 30 are located near
approximately 2,000 feet of the cone collapsed into the
the lake level in the caldera. Most of these caves are
caldera with the explosion. Several small cones, includ-
the result of erosion of the lavas or layers of mudflows
ing Wizard Island, were built up from lava oozing from
between the lavas. Caves within the rim are shallow
the floor after the caldera developed. The cold lake
and may be seen along the water level, whereas those
waters that occupy the caldera are remarkably clear
caverns outside the caldera have more depth and
because of the lack of algal activity. The bright blue
topography. All of the caves are hazardous, and climb-
