The Face of Hotlum Cone, Mount Shasta volcano, California

Hotlum cone, Mount Shasta

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Mount Shasta's youngest cone
Photo by R. Christiansen in May 1981	The Hotlum cone, named for the glacier that mantles its northeast slope, is the youngest part of Mount Shasta. It started to grow during the activity of Shastina and we find lava flows from the two cones interlayered on the north side of the mountain. However, much of the cone seems to have grown since the disappearance of glaciers even larger than those now partly covering it–glaciers that probably existed until about 6,000 years ago. Thus, it seems likely that the latest cone-building eruptions occurred within the past few thousand years.
This view of Mount Shasta is toward the southwest from U.S. Highway 97. Erosion has done little to alter the smooth, nearly symmetrical Hotlum cone.

Mount Shasta's youngest cone

Photo of the northeast flank of Mount Shasta volcano, California

Photo by R. Christiansen in May 1981

The Hotlum cone, named for the glacier that mantles its northeast slope, is the youngest part of Mount Shasta. It started to grow during the activity of Shastina and we find lava flows from the two cones interlayered on the north side of the mountain. However, much of the cone seems to have grown since the disappearance of glaciers even larger than those now partly covering it–glaciers that probably existed until about 6,000 years ago. Thus, it seems likely that the latest cone-building eruptions occurred within the past few thousand years.

This view of Mount Shasta is toward the southwest from U.S. Highway 97. Erosion has done little to alter the smooth, nearly symmetrical Hotlum cone.

Lava flows spread from crater of Hotlum cone
Photo by R. Christiansen on 24 July 1975	Just like the other cones of Mount Shasta, lava erupted from a crater at the summit of Hotlum cone and moved downslope. Each flow moved slowly down the precipitous slopes of the volcano like a gigantic tractor tread. As its outer surfaces cool and harden, the lava breaks into angular blocks. These blocks are carried along on the surface of a moving flow, sometimes piling up as levees at its margins (note flow levees in photo at left). Ultimately some of the blocks are dumped over the steep flow margins and overridden by the continually moving flow so that the molten lava core becomes completely encased in these jagged broken blocks. The inexorable march of such a lava flow is generally slow enough so that it seldom presents a life-threatening hazard, but any stationary objects in its path are destroyed. This aerial view toward the southwest shows the Military Pass lava flow.

Lava flows spread from crater of Hotlum cone

Photo by R. Christiansen

on 24 July 1975

Just like the other cones of Mount Shasta, lava erupted from a crater at the summit of Hotlum cone and moved downslope. Each flow moved slowly down the precipitous slopes of the volcano like a gigantic tractor tread. As its outer surfaces cool and harden, the lava breaks into angular blocks. These blocks are carried along on the surface of a moving flow, sometimes piling up as levees at its margins (note flow levees in photo at left).

Ultimately some of the blocks are dumped over the steep flow margins and overridden by the continually moving flow so that the molten lava core becomes completely encased in these jagged broken blocks. The inexorable march of such a lava flow is generally slow enough so that it seldom presents a life-threatening hazard, but any stationary objects in its path are destroyed.

This aerial view toward the southwest shows the Military Pass lava flow.

Lava dome forms Summit of Mount Shasta
Photo by R. Christiansen on 24 July 1975	The summit crater of the Hotlum cone, like those of the older cones, is capped with a dome of solidified lava. Partial collapse of the dome during its extrusion spread pyroclastic flows onto the flanks of Mount Shasta below. The dome is creased by an elongate fissure where the largest snowbanks in this view are seen. Summit activity was sufficiently recent that its residual heat produces steam vents, sulfur deposits, and a small hot spring just below the summit. An eruption witnessed by a sailing expedition at sea in 1786 probably exploded from this summit fissure and left deposits of volcanic ash that charred some of the vegetation it fell on.

Lava dome forms Summit of Mount Shasta

Aerial view of the summit dome Hotlum cone, Mount Shasta, California

Photo by R. Christiansen on 24 July 1975

The summit crater of the Hotlum cone, like those of the older cones, is capped with a dome of solidified lava. Partial collapse of the dome during its extrusion spread pyroclastic flows onto the flanks of Mount Shasta below. The dome is creased by an elongate fissure where the largest snowbanks in this view are seen.

Summit activity was sufficiently recent that its residual heat produces steam vents, sulfur deposits, and a small hot spring just below the summit. An eruption witnessed by a sailing expedition at sea in 1786 probably exploded from this summit fissure and left deposits of volcanic ash that charred some of the vegetation it fell on.

Volcanic "flows" spread from base of Mount Shasta

Photo by C.D. Miller in August 1976
Some of the best records of past volcanism are found in the broad aprons of debris that spread from the bases of most volcanoes. Here, we typically find layers of deposits formed by falling ash and pumice, fast-moving pyroclastic flows, and lahars (volcanic debris flows). Such aprons of debris are sometimes the favorite sites on which to build communities.

This view shows 10 distinct volcanic layers from Mount Shasta along the bank of Squaw Creek within about 1.5 km of McCloud. Distance to the volcano's summit is about 16 km. The deposits were laid down during formation of the Misery Hill and Hotlum cones. This streambank consists of 5 lahar deposits formed by material sweeping down from Hotlum cone. The bottom layer is a pyroclastic flow (geologist is standing at the base of this unit), probably from the Misery Hill cone.

Volcanic "flows" spread from base of Mount Shasta
Photo by C.D. Miller in August 1976	Some of the best records of past volcanism are found in the broad aprons of debris that spread from the bases of most volcanoes. Here, we typically find layers of deposits formed by falling ash and pumice, fast-moving pyroclastic flows, and lahars (volcanic debris flows). Such aprons of debris are sometimes the favorite sites on which to build communities.
This view shows 10 distinct volcanic layers from Mount Shasta along the bank of Squaw Creek within about 1.5 km of McCloud. Distance to the volcano's summit is about 16 km. The deposits were laid down during formation of the Misery Hill and Hotlum cones. This streambank consists of 5 lahar deposits formed by material sweeping down from Hotlum cone. The bottom layer is a pyroclastic flow (geologist is standing at the base of this unit), probably from the Misery Hill cone.

Debris flows occur with or without volcanic activity
Mudflows or other debris flows and muddy floods pose a potential danger to people and property around any high, snow-covered volcano in the Cascades. The steep slopes of Mount Shasta are seasonally covered by heavy snows that generally melt off through the summer months. Eruptions, especially explosive ejections hot debris onto the volcano, can trigger rapid melting of such a snowpack. Deposits from the past several thousand years on the lower slopes around Mount Shasta attest to numerous such debris flows carrying hot volcanic materials. Even without eruption, accelerated summer melting on the steep snow-covered slopes and persistent glacial ice of the volcano can produce major debris flows. For example, in August of 1924, debris flows coursed down Mud Creek, the major drainage on the south side of Mount Shasta. The flows inundated large areas near the town of McCloud and sent mud into the McCloud River, through the Sacramento River, and all the way to San Francisco Bay. Nowadays, the large reservoir of Shasta Lake would impound such a flow, but even larger events could endanger the reservoir if it were nearly full.
Photo by C.D. Miller in September 1976	The 1924 debris flows spilled over the banks of Mud Creek, encasing trees with mud, gravel, and boulders. Subsequent erosion along the banks of Mud Creek exposed the 1924 deposits and tree trunks (left of geologist in photo) rooted below the deposits. In July of 1977 several small debris flows swept down Ash Creek on the east side of the volcano, leaving behind boulderly deposits (bottom left photo) and burying parts of several roads. Thus, even without eruptions, debris flows can endanger areas below Mount Shasta on all sides, and an eruption during the winter or spring months could produce very large debris flows and muddy floods.
Photo by R. Christiansen on 26 July 1977

Debris flows occur with or without volcanic activity

Mudflows or other debris flows and muddy floods pose a potential danger to people and property around any high, snow-covered volcano in the Cascades. The steep slopes of Mount Shasta are seasonally covered by heavy snows that generally melt off through the summer months. Eruptions, especially explosive ejections hot debris onto the volcano, can trigger rapid melting of such a snowpack.

Deposits from the past several thousand years on the lower slopes around Mount Shasta attest to numerous such debris flows carrying hot volcanic materials. Even without eruption, accelerated summer melting on the steep snow-covered slopes and persistent glacial ice of the volcano can produce major debris flows.

For example, in August of 1924, debris flows coursed down Mud Creek, the major drainage on the south side of Mount Shasta. The flows inundated large areas near the town of McCloud and sent mud into the McCloud River, through the Sacramento River, and all the way to San Francisco Bay. Nowadays, the large reservoir of Shasta Lake would impound such a flow, but even larger events could endanger the reservoir if it were nearly full.

Debris flow deposits exposed along bank of Mud Creek, Mount Shasta, California

Photo by C.D. Miller in September 1976

The 1924 debris flows spilled over the banks of Mud Creek, encasing trees with mud, gravel, and boulders. Subsequent erosion along the banks of Mud Creek exposed the 1924 deposits and tree trunks (left of geologist in photo) rooted below the deposits.

In July of 1977 several small debris flows swept down Ash Creek on the east side of the volcano, leaving behind boulderly deposits (bottom left photo) and burying parts of several roads. Thus, even without eruptions, debris flows can endanger areas below Mount Shasta on all sides, and an eruption during the winter or spring months could produce very large debris flows and muddy floods.

Recent debris flow deposit in Ash Creek, Mount Shasta, California

Photo by R. Christiansen on 26 July 1977

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References

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U.S. Department of the Interior, U.S. Geological Survey, Menlo Park, California, USA
URL http://volcanoes.usgs.gov/Hazards/Where/ShastaDanger/hotlum.html
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Last modification: Tuesday, 23-Apr-2002 23:35:09 EDT (SRB)