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Mount Tehama

Coordinates:40°26′44″N121°33′34″W / 40.44556°N 121.55944°W /40.44556; -121.55944
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From Wikipedia, the free encyclopedia
Eroded Andesitic stratovolcano in Northern California
Brokeoff Volcano
Broken northeast face of Mount Tehama
Highest point
Elevation9,239 ft (2,816 m) NAVD 88[1]
Prominence915 ft (279 m)[1]
ListingCalifornia county high points 17th
Coordinates40°26′44″N121°33′34″W / 40.44556°N 121.55944°W /40.44556; -121.55944[2]
Geography
Map
LocationShasta /Tehama counties,California, U.S.
Parent rangeCascade Range
Topo mapUSGS Lassen Peak
Geology
Mountain typeErodedstratovolcano
Volcanic arcCascade Volcanic Arc
Climbing
Easiest routeHike
Former extent of Brokeoff Volcano
Former extent of Brokeoff Volcano

Mount Tehama (also calledBrokeoff Volcano orBrokeoff Mountain) is an erodedandesiticstratovolcano in theCascade Volcanic Arc and theCascade Range inNorthern California. Part of theLassen volcanic area, its tallest remnant, Brokeoff Mountain, is itself the second highest peak inLassen Volcanic National Park and connects to the park's highest point,Lassen Peak. Located on the border ofTehama County andShasta County, Brokeoff's peak is the highest point in the former. The hikers that summit this mountain each year are treated to "exceptional" views of Lassen Peak,[3] theCentral Valley of California, and many of the park's other features. On clear days,Mount Shasta can also be seen in the distance.

Brokeoff Volcano was active from 590,000 to 387,000 years ago, during which period it erupted numerous flows with compositions ranging from basaltic andesite to dacite. At its peak, Brokeoff Volcano reached an estimated height of 3,350 metres (10,990 ft) and had a basal circumference of approximately 12 kilometres (7.5 mi). At approximately 313,000 years ago volcanism began shifting to the northeast of Brokeoff Volcano, with volcanism currently focused around the modernLassen Peak. Following the end of volcanism at Brokeoff Volcano, cooling magma beneath the surface continued to drive hot, highly reactive hydrothermal fluids circulating near the central vent of the volcano. These hydrothermal fluids chemically weathered the volcanic rocks near the central vent, making them particularly susceptible to physical erosion.Glaciers and streams were able to rapidly erode the hydrothermally altered rocks near the central vent, creating the deep Mill Canyon and exposing deep into the core of the ancient volcano.

Many of the lava flows along the flanks of Brokeoff Volcano were not as extensively chemically weathered, resulting in the flanks of much of the volcano being preserved. Remnants of the flanks of Brokeoff Volcano include Brokeoff Mountain, Mount Conard, Pilot Pinnacle, Mount Diller, and Diamond Peak.

History

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The area near Lassen Peak became a haven for new settlers throughout the 1800s.Wagon trains followed winding trails on theNobles Emigrant Trail which cut through the Lassen Peak vicinity nearSacramento Valley. One of the main landmarks along this trail was a volcano. CalledLassen Peak after Peter Lassen, a prominent blacksmith and guide who escorted California settlers, the volcano and the area around it were given merit for their gripping volcanic phenomena, which included lava beds and extinct volcanic cones. In May 1907, Lassen Peak was declared anational monument by PresidentTheodore Roosevelt to protect the area for "future generations to study and enjoy".[4] Roosevelt had been told that the area was actuallyextinct in terms of eruptive activity. Despite assurance from Native Americans in the area that the mountain was indeed active, settlers continued to think that Lassen Peak was extinct and "dead" and so continued to settle nearby. The natives continued to declare that "one day the mountain would blow itself to pieces".[4] In May 1914, almost exactly seven years later, the volcano began a large explosive eruption sequence. More than a hundred eruptions of varying size took place over the next seven years, attracting national interest and the designation ofnational park in 1916.[4]

Geography and geology

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The state of California is located on thePacific Ring of Fire, an area of highly active tectonic activity that includes seismicity and volcanism. Brokeoff and the other volcanoes near Lassen Peak were produced bysubduction of the oceanicJuan de Fuca Plate under theNorth American Plate.[5]

Extent of Brokeoff Volcano shown by outline

Brokeoff Volcano represents the Pleistocene component of the Lassen volcanic complex, an area of active volcanism for more than three million years. It formed during a period of eruptive activity approximately 600,000 years ago on a series offaults;[6] the largest of these eruptions was about 50 times as powerful as the1980 eruption of Mount St. Helens. During thePleistocene, constantly renewedlava ofandesite flowed from the central vent of the volcano. The eruption of overlapping andesitic lava flows accumulated to build the volcano. By the time it reached its peak size, the volcano was immense, measuring at its base between 11 mi (18 km) and 15 mi (24 km) wide and more than 11,000 ft (3,353 m) tall.[7] Activity at Brokeoff lasted for approximately 200,000 years. Eruptions varied from ejection ofpyroclastic material toandesitic lava flows built primarily offeldspar,pyroxene, andolivine.[6]

Soon after the formation of Brokeoff, other, smaller volcanoes began to form.Shield volcanoes like Raker Peak, Red Mountain, Prospect Peak, and Mount Harkness appeared. At this point, thePleistocene was coming to an end. One additional and final vent formed on the northeastern flank of the volcano, eruptingglassy dacitic flows.[7] Over time, the volcano became dilapidated from extensive erosion and hydrothermal alteration,[6] and was eroded by creeks and glaciers. Later activity built more than thirty other cones known as theLassen Domes. Today the most active volcano in the area isLassen Peak. Other smaller craters younger than 50,000 years are also active.[8]

Brokeoff's remains are exposed to the southwestMount Lassen inShasta County, California.Sulphur Works, an area known for its sharp, putrid scent, is thought to be the center of the ancient volcano. The central vent of Brokeoff Volcano was located near modern Diamond Peak.

Brokeoff Mountain in winter.

After volcanism at Brokeoff ended, magmatism became moresilicic and began migrating toward Lassen Peak's northern flank. The last 400,000 years have seen at least three known flows, parts of which are still on Raker Peak and Mount Conard. A subsequent explosive eruption ejected 12 cu mi (50 km3) of material and created a large crater, which was soon covered by lava. Twelvelava domes were built throughout the area and the next period of eruptive activity began. Spanning 50,000 years it ejected 3.5 cu mi (15 km3) to 6 cu mi (25 km3) of material and was followed by a third period. At least twelve periods of activity have taken place since Brokeoff formed, consisting of activity including pyroclastic flows, pasty lavas, and additional lava domes.[6] Among the vents built by this activity is Lassen Peak.[9]

Flora and fauna

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Near the trailhead at lower elevations,alder thickets are present, as well asincense cedar,red firs, andwestern white pine. At higher elevations, groups ofmountain hemlock andlupines prevail.[3]Eagles andhawks can be frequently noted on high crags, andsquirrels andpika live about the mountain.[10]

Recreation

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Hiking is especially popular on the mountain during summer. The mountain trail lasts between four and six hours, spans 7.4 mi (12 km), and offers "exceptional" views of Mount Diller, Lassen Peak, Chaos Crags, and Mount Conard.[3]

Due to the characteristically intermittent eruptions of active volcanoes such as Lassen Peak, there is some threat from the volcanoes of the LVNP.Daciticexplosive eruptions have taken place within the last 50,000 years at Lassen Peak,Chaos Crags, andSunflower Flat, andeffusive eruptions of basalt have occurred at Tumble Buttes, Hat Mountain, and Prospect Peak.Pyroclastic flows andlahars could easily occur near glaciated areas and in river valleys like Hat Creek Valley.[8] In addition to thevolcanic hazards that could possibly occur, one volcano did erupt in the 20th century in LVNP: (Lassen Peak). Lassen's eruptions (1914–21, though most activity occurred between 1914 and 1917) were very small compared to the 1980 eruption of Mount St. Helens.

See also

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Notes

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  1. ^ab"Brokeoff Mountain, California".Peakbagger.com. Retrieved2010-02-03.
  2. ^"Brokeoff Mountain".Geographic Names Information System.United States Geological Survey,United States Department of the Interior. Retrieved2010-02-03.
  3. ^abcHeid, p. 227.
  4. ^abcHarris, Tuttle, and Tuttle, p. 542.
  5. ^Sinnotte, Barbara.
  6. ^abcdLopes and Lopes, p. 113.
  7. ^abHarris, Tuttle, and Tuttle, p. 549.
  8. ^abDiggles, Michael (May 24, 2005)."Volcano Hazards of the Lassen Volcanic National Park Area, California".United States Geological Survey.Archived from the original on November 2, 2009. RetrievedMay 15, 2010.
  9. ^Lopes and Lopes, p. 114.
  10. ^Lopes and Lopes, p. 119.

References

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External links

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