Mount Thielsen: Wikis


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Mount Thielsen

Mount Thielsen from Diamond Lake with its eroded crater.
Elevation 9,184 ft (2,799.3 m) NAVD 88[1]
Prominence 3,342 ft (1,018.6 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 43122B1
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
Map of the Southern Oregon Cascade Range

Mount Thielsen, or Big Cowhorn, is an extinct shield volcano in the Oregon High Cascades, near Mount Bailey. Mount Thielsen stopped erupting 250,000 years ago, and subsequent glacial erosion has formed a horn-like shape. 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, an area with many recreational activities.


History and geography

The area was previously inhabited by Native Americans, who referred to the mountain as "Hischokwolas".[4] The volcano's current name was coined by Jon Hurlburt, a Polish explorer. The man dedicated the name to Hans Thielsen, a railroad engineer and builder known in the area.[5]

Diamond Lake lies to the west of Mount Thielsen and beyond lies Mount Bailey, a much less eroded and younger stratovolcano. Its 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 stratovolcanos and remnants.[6]


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.

The volcano is composed of strata of past shield volcanoes. Its main cone is built from basaltic andesite, a common component of the Oregon Cascades shield volcanoes, and is about two cubic miles wide. Much erosion from glacial movement has led to significant deformity at the cone—its upper sector is almost completely eroded. However, this offers its contents to observation. Within the cone lava flows, pyroclastic flow deposits, and strata of tephra, or volcanic ash, are easily visible.[7]

Aerial view of Mount Thielsen.

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, its eruptive activity was somewhat limited. It can at least be sectioned into three main divisions: 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.[7]

On the mountain past lava flows are diverse, some being as thick as 10 metres (33 ft) 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.[4]



Glaciers were present on the volcano until the conclusion of the Little Ice Age, at the beginning of the 20th century.[6] Pleistocene glaciers have largely eroded Thielsen's caldera—leading to exposure of its contents.[7]


Thielsen's spire-like top is hit by lightning so often that some rocks on the summit have melted into a rare mineraloid called lechatelierite, a variety of fulgurite. The mountain itself has earned the nickname "the lightning rod of the Cascades."[8][9]


The lower slopes of Mount Thielsen are heavily forested with low diversity of plant species.[10] A forest of mountain hemlock and fir grows up to the timberline at about 7,200 feet (2,200 m).[11]


Mount Thielsen lies within the Mount Thielsen Wilderness which is part of the Fremont–Winema National Forest. 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.[12] The forest sector contains 26 miles (42 km) of the Pacific Crest National Scenic Trail, accessible from a trailhead along Oregon Highway 138.[13] In 2009 the trail was selected as Oregon's best hike.[14] 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 2008-11-17. 
  2. ^ "Mount Thielsen, Oregon". Retrieved 2008-04-02. 
  3. ^ a b Amar Andalkar (January 15, 2003). "Mount Thielsen". Retrieved March 29, 2009. 
  4. ^ a b "Description: Mount Thielsen Volcano, Oregon". United States Geological Survey. May 28, 2002. Retrieved March 28, 2009. 
  5. ^ Lewis, MacArthur (1982). Oregon Geographic Names: Western Imprints. Press of the Oregon Historical Society. 
  6. ^ 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. 
  7. ^ a b c Harris, pages 157-165.
  8. ^ Purdom, William B. (1966). "Fulgurites from Mount Thielsen, Oregon" (PDF). The Ore-Bin (Oregon Department of Geology and Mineral Industries) 28 (9). Retrieved 2008-04-01. 
  9. ^ "Cascade Range Volcanoes Summaries". USGS Cascades Volcao Observatory. Retrieved 2008-04-01. 
  10. ^ Wuerthner, George (2003). Oregon's Wilderness Areas. Big Earth Publishing. p. 133. ISBN 9781565794344. 
  11. ^ "Mount Thielsen Wilderness". Retrieved 2009-04-02. 
  12. ^ Grubbs, page 187
  13. ^ "Fremont–Winema National Forests: Recreational Activities". US Forest Service. April 27, 2005. Retrieved March 27, 2009. 
  14. ^ "Readers' Choice Awards 2009", Backpacker 3: 53–59, January 



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