Mount Thielsen
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| Mount Thielsen | |
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 Mount Thielsen from the air | |
| Elevation: | 9,184 feet (2,799 metres) |
| Coordinates: | Template:Coor dms |
| Location: | Oregon, USA |
| Topo map: | USGS Mount Thielsen |
| Range: | Cascades |
| Type: | Stratovolcano |
| Age of rock: | ~ 290 Kyr |
| First ascent: | 1883 by E. E. Hayden |
| Easiest route: | scramble |
Mount Thielsen is a stratovolcano in southern Oregon that 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. The reason for this is two-fold; Thielsen is a relatively old Cascade volcano and it stopped cone-building eruptions relatively early. It therefore was not able to repair damage caused during the last two or three ice ages.
Thielsen's spire-like top is hit by lightning so often that some of its summit rocks have melted into a rare mineraloid called lechatelierite or fulgurite, and the mountain itself has earned the nickname "the lightning rod of the Cascades."
West of Thielsen is Diamond Lake and west of the lake is Mount Bailey (a much less eroded and younger stratovolcano). Physically, Thielsen is made of a shield volcano with a composite cone on top.
See also: Map of the Southern Oregon Cascade Range
Geology
The rock formations on Thielsen have normal magnetic polarity, meaning they all likely erupted within the last 700,000 years (it is possible, yet unlikely, that some of the earliest lavas flowed during a previous period of normal polarity).
Thielsen's initial cone was made of pyroclastic material and probably reached a height of 1000 to 3000 feet (~300-900 meters). Around this cone there were peripheral eruptions of basaltic-andesite, which deposited scoria, cinders, and thin flows of lava. Thielsen's construction was later completed when a half mile wide plug of basaltic andesite was emplaced in the volcano's main vent.
Since then numerous glaciers have eroded most of the mountain's volume.
Reference
- Fire Mountains of the West: The Cascade and Mono Lake Volcanoes, Stephen L. Harris, (Mountain Press Publishing Company, Missoula; 1988) ISBN 0-87842-220-X