Mount
Rainier, Washington
At 14,410 feet, Mt. Rainier is the highest of the Cascade composite cones
(stratovolcanoes). While many of the classic volcanic peaks in the Cascades
are composite cones (volcanoes formed by alternating layers of pyroclastic
fragments and lava flows), the mountain range is built upon extensive
basalt flows and also includes andesites and rhyolites.
The slopes of Mt. Rainier reveal the layered sequence of lava flows and volcanic fragments that form the mountain. (Link to USGS Mt. Rainier info)
Note behind the climber the layers of rock and and cinders that have eroded into a stairstep pattern, as the lava flows are relatively resistant to erosion and the cinders are easily eroded away.
This aerial view to the north from Portland, Oregon shows Mt. St. Helens (foreground) and Mt. Rainier (covered with glaciers and snow), two of the major stratovolcanoes in Washington.
A closer view of Mt. St. Helens, which erupted explosively in May, 1980, blasting 3.7 billion cubic yards of rock out of the mountain and lowering the summit by 1,314 feet (to 8,363 feet). (Link to USGS Mt. St. Helens info)
A winter aerial view of Mt. St. Helens in 2005.
This brilliant blue lake occupies the caldera formed during the collapse of Mt. Mazama ~7700 years ago. Prior to that series of eruptions, the mountain would have been similar to modern Mount Rainier or Mt. St. Helens. The eruptions are estimated to be 42 times the power of the 1980 Mt. St. Helens eruption, removing 5,000 feet from the summit of the mountain. (Link to USGS Crater Lake info)
View
South from Crater Lake, Oregon
This view from the "Watchman" fire lookout along the caldera
rim shows a typical Cascade landscape of small volcanic peaks built upon
the basalt "backbone" of the mountain range. Mt. McLoughlin
(9,495 feet) is in the distance.
This view of the caldera shows the internal layering of volcanic deposits that comprised Mt. Mazama, similar to those at Mt. Rainier (above).
Note the steep talus slope leading to the lakeshore and the "Phantom Ship," a rock spire that rises 167 feet above the water. Some of the oldest lava flows (~400,000 years) are near the Phantom Ship.
Crater
Lake, Oregon
Wizard
Island is a cinder cone that formed after the caldera collapse and now
rises 760 feet above the lake.
The lake is approximately 1,932 feet deep and appears an intense blue due to the clarity and depth of the water.
This view from the north shows the composite cone of the summit ("Hotlum cone", 14,192 feet), which has erupted as recently as ~200 years ago, the older Misery Hill cone (center, 13,384 feet, active 40,000 - 10,000 years ago), and the large Shastina cone to the right (12,330 feet), rapidly built by eruptions ~9,500 years ago. (Link to USGS Mt. Shasta info)
Approximately 300,000 years ago, the northern flank of the volcano collapsed, creating a 50+ km debris avalanche into the Shasta Valley below. The hill in the foreground is one of the large blocks of debris carried over this long distance in a fluid of volcanic material and gases.
Note the large intact blocks of volcanic rock that were once part of the ancestral Mount Shasta.
View toward the terminus of the debris avalanche, with several large blocks of debris visible along the highway.
This view shows the Hotlum glacier on Mount Shasta (upper mountain) and distinct "levees" of andesite from lava flows (lower mountain).
This view of the southeastern slopes shows Avalanche gulch (left, snowy) and the remnants of the Sargents Ridge cone on the right. The Sargents Ridge cone built during an eruptive period 200,000 - 100,000 years ago and was subsequently eroded by two glaciations. The remnant volcanic layers can be seen at "The Thumb" (center of photo, beds dipping to the left) and Shastarama Point (far right, high point on the ridge with beds dipping to the right).
Note the glacial moraine in the foreground of this view.
Near the dominating composite cone of Mount Shasta are numerous other volcanic features, many built from more fluid (less viscous) basalt flows. Mt. Herd, just to the northwest of Mt. Shasta, has the classic shape of a shield volcano. A seismograph is mounted atop Mt. Herd to monitor activity at Mt. Shasta.
Many areas of basalt flows contain numerous lava tubes, extensive cave systems that developed by lava solidifying at the surface, thus insulating the still-molten flow of basalt below. At the end of an eruption, the molten rock would flow out of the lava tube, leaving a cave behind.
Also around the flanks of Mt. Shasta are many small volcanic cones built from pyroclastic eruptions, such as this small tephra cone. Internal layering and the central vent were exposed by excavation. Note the numerous blocks and bombs ejected from this vent--some measuring 2m in diameter were thrown 200m from the vent.
Here the deposits of two separate eruptions are visible. The different layers (an "angular unconformity") were separated in time by an erosive period.
One of the greatest natural hazards at Mt. Shasta are debris flows, mixtures of loose rock, mud, and water that can flow at high velocities. Debris flows are common on Mud Creek, where the deposits of previous flows are exposed along the banks.
Immediately downstream of the previous photo is a 1920s catch basin built to protect downstream areas from debris flows. Like many such structures, it filled almost immediately and provides no protection today.
View of debris flow sediments along Mud Creek.
This view illustrates the classic low, broad shape of a shield volcano, built primarily from successive flows of relatively fluid basalts. Note the numerous smaller volcanic cones around the shield volcano. (Link to USGS Medicine Lake Volcano info)
View
from Medicine Lake Volcano, California
This view from Little Mt. Hoffman, part of the Medicine Lake volcanic
complex, shows numerous smaller volcanic cones built near the periphery
of the Basin and Range.
Eruptions at Medicine Lake Volcano are a result of the subduction zone to the west and the tensional stresses common to the Basin and Range province to the east. This locale is at the extreme northwest of the Basin and Range, where normal faults reveal the crustal extension that has provided some pathways for magma to reach the surface.
This tensional fractures is the product of a rising dike of magma that forced the ground above to split (~1,000 years ago) but never produced lava at this location.
A similar view of the same tensional fracture.
In other sites along the flanks of Medicine Lake Volcano, lava did reach the surface to produce features such as these spatter cones, which formed approximately 10,000 years ago.
Note the scale of the spatter cones relative to the person.
Like many volcanic features, the spatter cones are aligned above a linear weakness in the rocks upon which they are built.
Much of the lava produced by these spatter cones is scoria, a basalt with numerous gas bubbles from steam and other gases that erupted with the lava (quarter for scale).
At other locations around Medicine Lake Volcano, rhyolite eruptions produced different rocks and landforms. Glass Mountain is largely rhyolite and is mantled with obsidian produced by the very rapid cooling of lava that prevents mineral grains from developing. This entire hill slope is covered with obsidians
Obsidian exhibits excellent conchoidal fracture. This site was used by Native Americans as a source of stone for manufacturing tools such as spearheads.
Rhyolite lavas are very viscous and thus flow slowly, producing distinct landforms. The steep faces of this rhyolite flow indicate slow-moving, viscous lava that erupted ~1,100 years ago. Mount Shasta is in the distance.
