Mount Thielsen

Mount Thielsen
Mount Thielsen

Mount Thielsen's eroded crater from Diamond Lake
Elevation 9,184 ft (2,799 m) NAVD 88[1]
Prominence 3,342 ft (1,019 m) [2]
Location Douglas / Klamath counties, Oregon, USA
Range Cascades
Coordinates 43°09′10.2″N 122°03′59.5″W / 43.152833°N 122.066528°W / 43.152833; -122.066528Coordinates: 43°09′10.2″N 122°03′59.5″W / 43.152833°N 122.066528°W / 43.152833; -122.066528[1]
Topo map USGS Mount Thielsen
Type Shield volcano
Age of rock About 290,000 years
Volcanic arc/belt Cascade Volcanic Arc
First ascent 1883 by E. E. Hayden[3]
Easiest route Scramble

Mount Thielsen, or Big Cowhorn, is an extinct shield volcano in the Oregon High Cascades, near Mount Bailey. Because Mount Thielsen stopped erupting 250,000 years ago, glaciers have heavily eroded the volcano's structure, creating precipitous slopes and its horn-like peak. The spire-like shape of Thielsen attracts lightning strikes and causes the formation of fulgurite, an unusual mineral. The prominent horn forms a centerpiece for the Mount Thielsen Wilderness, a reserve for recreational activities such as skiing and hiking.

Thielsen was produced by subduction of the Juan de Fuca Plate under the North American Plate.[4] Volcanism near the Cascades dates back to 55 million years ago (mya), and extends from British Columbia to California. Thielsen is part of the High Cascades, a branch of the main Cascades range that includes Oregonian volcanoes less than 3.5 million years old. It is a member of a group of extinct volcanoes distinguished by their sharp peaks.

The area surrounding the volcano was originally inhabited by Chinook Native Americans, and was later encountered by Polish settlers. One of the visitors was Jon Hurlburt, an early explorer of the area who named the volcano after the engineer Hans Thielsen. Later explorers discovered nearby Crater Lake. The volcano was not studied scientifically until 1884, when a team from the United States Geological Survey sampled its fulgurite.



Mount Thielsen, as seen from the air

The area was originally inhabited by Chinook Native Americans, who referred to the mountain as "Hischokwolas".[5] Jon Hurlburt, a Polish explorer, renamed the volcano after Hans Thielsen, a railroad engineer and builder[6] who played a major role in the construction of the California and Oregon Railroad.[7]

In 1884 a United States Geological Survey team headed by J. S. Diller began studying the mountains of the Cascade Range. Their intended destinations included Thielsen, which was climbed and sampled by one member who retrieved multiple samples of fulgurite. Thielsen's spire-like top is hit by lightning so frequently that some rocks on the summit have melted into a rare mineraloid known as lechatelierite, a variety of fulgurite. The mountain has earned the nickname "the lightning rod of the Cascades".[5][8]

Apart from study, Thielsen and the rest of the Crater Lake area features heavily into nineteenth and early twentieth century exploration. In 1853, miners from Yreka first described Crater Lake; one called it "the bluest water he had ever seen", another "Deep Blue Lake." The first published description was written by Chauncy Nye for the Jacksonville Sentinel in 1862. Nye recalled an expedition of gold prospectors where they passed a lake of deep blue color. Native Americans lived in the area and grew irritable towards new settlers in the area. In 1865, Fort Klamath was built as a protective sanctuary. A wagon road was built to connect the Rogue Valley to the building. In late 1865, two hunters ventured upon the lake and their sighting became pervasive. By then, the lake became famous for its distinctive blue color and crowds came to see it. The first non-Native American to stand on the shore of Crater Lake was Sergeant Orsen Stearns, who climbed down into the caldera. A friend, Captain F.B. Sprague, gave it the name "Lake Majesty." Tourism continued until May 22, 1902; on that day, Theodore Roosevelt awarded the lake and surrounding area national park status.[9]



The Juan de Fuca Plate is being subducted under the North American Plate, generating gradual, diverse volcanism.

The Cascade Range was produced by convergence of the North American Plate with the subducting Juan de Fuca Plate. Active volcanism has taken place for approximately 36 million years, and a nearby range features complexes as old as 55 mya. Most geologists believe that activity in the Cascades has been relatively intermittent, producing up to 3,000 volcanic calderas at a time. Holocene volcanism (within the last 10,000 years) has taken place frequently and stretches from Mount Garibaldi in British Columbia to north California's Lassen Peak complex. Remarkably different from state to state, the volcanism ranges from sparse but large volcanoes to extensive zones of smaller activity such as lava shields and cinder cones. It is divided into two large sectors, called the High Cascades and the Western Cascades. Thielsen is part of the High Cascades, which are east of the Western Cascades.[4]


This map of the Southern Oregon Cascade Range lists several of the major volcanoes around Thielsen.

Diamond Lake (formed by one of Thielsen's eruptions)[10] lies to the west of Mount Thielsen and beyond lies Mount Bailey, a much less eroded and younger stratovolcano. Thielsen's sharp peak is a prominent feature of the skyline visible from Crater Lake National Park. Both of the volcanoes are part of the Oregon High Cascades, a range that sections off the stratovolcanoes of Oregon that are younger than 3.5 million years. The High Cascades include Mount Jefferson, the Three Sisters, Broken Top, and other stratovolcanoes and remnants.[11]

Rock in the area is primarily of Upper Pliocene and Quaternary age. Basalt and basaltic andesite volcanoes exist on top of the older rocks of the High Cascades: major volcanic centers include Mount Hood, Three Sisters-Broken Top, Mount Mazama (Crater Lake), and Mount Jefferson. All have produced eruptions with a degree of diversity, including both lava flows and pyroclastic eruptions, and variability in composition between dacite, basalt, and even rhyolite (except for Mount Hood, which is not known to have produced rhyolite).[4] Thielsen is part of a series of extinct volcanoes in Oregon termed the Matterhorns for their steep, spire-like summits. Thielsen is the highest Matterhorn at 9,182 feet (2,799 m). Other Matterhorns include Mount Washington, Three Fingered Jack, Mount Bailey, and Diamond Peak. Unlike other mountains in the High Cascades, all these volcanoes became extinct 100,000 to 250,000 years ago. Their summits were subjected to the last few ice ages, accounting for the difference between the Matterhorns and other nearby volcanoes.[12]


Aerial view of Mount Thielsen.

Thielsen has been so deeply eroded by glaciers that there is no summit crater and the upper part of the mountain is more or less a horn. Thielsen is a relatively old Cascade volcano and cone-building eruptions stopped relatively early. Erosion caused during the last two or three ice ages remains visible.[12] Subsidence of the last material in Thielsen's crater moved its youngest lava more than 1,000 feet (305 m) above the-then active crater.[13]

On the mountain past lava flows are diverse, some being as thick as 33 feet (10 m) at one sector and as thin as 1 foot (30 cm) at others. Stack-like figures composed of breccia and past flow deposits are as thick as 328 feet (100 m). The placement of these flows suggest that they were generated by splatter emitted by fountains in the cone.[5] On the sides of the mountain are bands of palagonite, a clay formed from iron-rich tephra making up the body of the volcano.[14] Basalt taken from the volcano contained pyroxene, hypersthene material, and feldspars.[15]

Other notable formations in the vicinity include Hemlock Mountain, Windigo Butte, and Tolo Mountain. Other than Crater Lake, little water flows on the surface. In canyons excavated by glaciers, small streams have formed.


The volcanic cone of Mount Thielsen lies on prior shield volcanoes, and is 2 cubic miles (8.3 km3) in volume. The cone was built from basaltic andesite, a common component of other shield volcanoes in the Oregon Cascades,[12] breccia, and tuff, and is intruded by dikes.[14] A coalesced volcanic cone, it formed as pyroclastics erupted and fountains spewed lava.[12] Glaciers cut and deformed the cone, eroding its upper sector. This erosion opened the interior of Thielsen for observation. Within the cone, lava flows, pyroclastic flow deposits, and strata of tephra, and volcanic ash, are easily visible.[12] Potassium-argon dating of deposits in the cone suggests that Thielsen is at least 290,000 years old. Since its eruption stopped about 100,000- 250,000 years ago, the period of eruptive activity was short in time. The eruptions of the cone came in three phases: a period where lava flows built up its cone, one where more explosive pyroclastic eruptions took place, and the final period, in which pyroclastic and material of lava-based origin were erupted together forming a weak cone encircled by long deposits.[12]


Glaciers were present on the volcano until the conclusion of the Little Ice Age, at the beginning of the 20th century.[11] Pleistocene glaciers have largely eroded Thielsen's caldera—leading to exposure of its contents.[12] The small Lathrop Glacier in the northern cirque of the volcano is the only extant glacier on Mount Thielsen. While the glaciation was extensive, volcanic ash from eruptive activity at Mazama Volcano has almost certainly masked contents.[5]


Fulgurites (substances that form when ligntning melts rock)[12] on the volcano are restricted to the very pinnacle of the mountain, and are only found between the top 5 feet (2 m) and 10 feet (3 m) of its summit. Lightning strikes the summit regularly, creating patches of "brownish black to olive-black glass"[8] that resemble "greasy splotches of enamel paint".[12] These range from a few centimeters in diameter to long, narrow lines up to 30 centimeters (12 in) long. Their appearance also varies: while some patches are rough and spongy, others are flat. Inspection of the fulgurite reveals a homogenous glass topping a layer of basalt. In between, a stratum made of materials such as feldspar, pyroxene, and olivine exists.[8]


Mule deer are known to inhabit the forest around Thielsen.

A grove of enormous incense cedars has established itself near Diamond Lake, and there is a forest of ponderosa pine at the nearby Emile Big Tree Trail. Umpqua National Forest features swordferns and Douglas firs.[10] In Fremont-Winema National Forest Rocky Mountain elks, pronghorn antelopes, and mule deer, bobcats, black bears, and mountain lions are found. The forest's rivers support populations of trout and the lakes contain fish such as the large-mouth bass. The forest is inhabited by avian species such as mallards, American Bald Eagles, Canada Geese, Whistling Swans. Peregrine Falcons and Warner Suckers are also known to infrequently enter its boundaries.[16]

The lower slopes of Mount Thielsen are heavily forested, with a low diversity of plant species.[17] A forest of mountain hemlock and fir grows up to the timberline at about 7,200 feet (2,200 m).[18] Near the peak of the volcano, Whitebark Pine alone predominates.[19]


Mount Thielsen lies within the Mount Thielsen Wilderness, part of the Fremont–Winema National Forest. This land is part of the Oregon Cascades Recreation Area, a 157,000-square-mile (406,628 km2) area set aside by Congress in 1984.[20] The wilderness and forest offer several activities related to the mountain, such as hiking and skiing. The wilderness covers 55,100 acres (86 sq mi) around the volcano, featuring lakes and alpine parks.[21] The forest sector contains 26 miles (42 km) of the Pacific Crest National Scenic Trail, accessible from a trailhead along Oregon Highway 138.[22] In 2009 the trail was selected as Oregon's best hike.[23] Three skiing trails exist on the mountain, all of black diamond rating. They follow several trails through the wilderness from the bowl of the mountain.[3]


  1. ^ a b "Mt Thielsen". NGS data sheet. U.S. National Geodetic Survey. Retrieved November 17, 2008. 
  2. ^ "Mount Thielsen, Oregon". Retrieved April 2, 2008. 
  3. ^ a b Andalkar, Amar (January 15, 2003). "Mount Thielsen". Retrieved March 29, 2009. 
  4. ^ a b c Topinka, Lyn (December 15, 2004). "Description: Cascade Range Volcanoes and Volcanics". United States Geological Survey. Retrieved April 23, 2010. 
  5. ^ a b c d Topinka, Lyn (May 28, 2002). "Description: Mount Thielsen Volcano, Oregon". United States Geological Survey. Retrieved March 28, 2009. 
  6. ^ McArthur, Lewis A.; McArthur, Lewis L. (1982) [First published 1928]. Oregon Geographic Names (5 ed.). Portland, Oregon: Oregon Historical Society Press. ISBN 9780875951140. OCLC 8846716. 
  7. ^ Heer and Heer, p. 47.
  8. ^ a b c Purdom, William B. (December 1966). "Fulgurites from Mount Thielsen, Oregon" (PDF). The Ore-Bin (Oregon Department of Geology and Mineral Industries) 28 (9). Retrieved April 1, 2008. 
  9. ^ Topinka, Lyn (April 18, 2008). "Description: Mount Mazama Volcano and Crater Lake Caldera, Oregon". United States Geological Survey. Retrieved January 4, 2011. 
  10. ^ a b Molhlenbrock.
  11. ^ a b O'Connor; Hardison III, and Costa (2001). Debris Flows from Failures of Neoglacial-Age Moraine Dams in the Three Sisters and Mount Jefferson Wilderness Areas, Oregon: USGS Professional Paper 1606. United States Geological Survey. 
  12. ^ a b c d e f g h i Harris, pp. 157-165.
  13. ^ Diller and Patton, p. 21.
  14. ^ a b Bishop and Allen, p. 121.
  15. ^ Clarke, p. 166.
  16. ^ "Fremont-Winema National Forests: Ecology, Fish & Wildlife". United States Forest Service. July 19, 2006. Retrieved January 7, 2011. 
  17. ^ Wuerthner, George (2003). Oregon's Wilderness Areas. Big Earth Publishing. p. 133. ISBN 9781565794344. 
  18. ^ "Mount Thielsen Wilderness". Retrieved 2009-04-02. 
  19. ^ Murray, Michael (2005). Our Threatened Timberlines: The Plight of Whitebark Pine Ecosystem. 12. Whitebark Pine Ecosystem Foundation. 
  20. ^ "Wilderness Areas on the Umpqua". US Forest Service. Retrieved January 1, 2011. 
  21. ^ Grubbs, page 187
  22. ^ "Fremont–Winema National Forests: Recreational Activities". US Forest Service. April 27, 2005. Retrieved March 27, 2009. 
  23. ^ "Readers' Choice Awards 2009". Backpacker 3: 53–59. January. 


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