Mount Thielsen

Mount Thielsen from the air

Mount Thielsen is a shield volcano in southern Oregon, USA. The original crater was destroyed by strong glacial erosion and formed a pyramid- like summit. The fact that the last of the cone- forming eruptions took place long before the last ice age, has been preserved in the form.

The peak of Mount Thielsen is frequently struck by lightning, which rocks melt near the summit and this creates Lechatelierite or Fulgurit. Therefore, the volcano also bears the nickname The lightning rod in the Cascade Range.


From the natives of the area of ​​the mountain Hischokwolas was called. He is also known under the name Big Cowhorn (big cow horn ). The volcano was loud Oregon Geographic Names by Lewis A. McArthur, published in 1982 by Oregon Historical Society Press, about the year 1872 named by John A. Hurlburt of Portland to Hans Thielsen, a well-known railway engineer and pioneer.


In the vicinity of the 2799 m high stratovolcano are similar resulting Howlock Mountain and the younger Mount Bailey. The latter rises to the west of Thielsen on the other side of Diamond Lake. Only about nine kilometers south begins the Crater Lake National Park. The Mount Thielsen lies on the border between the counties in the West Douglas and Klamath in the east. Likewise, the national forests share ( English singing. National Forest ) Umpqua and Winema the mountain.


The mountain is within the Cascade Range of the oldest volcanoes. The period of the volcanic cone forming eruptions of Mount Thielsen was relatively short. Glaciers eroded the summit region such that the volcano no longer has a crater eruption today. The present form was created during the last two or three ice ages.


The volcanic cone of Mount Thielsen was built on older shield volcanoes and has a volume of 8.3 km ³. It consists of breccia, tuff and basaltic andesite, a frequently occurring in the Cascade Range volcanic rocks. The summit area of the volcano is characterized by glacial erosion during the ice ages. The ice deformed the cone, which parts of the volcano inside were exposed. Potassium - argon dating showed the mount Thielsen an age of lies of at least 290 000 years. The last eruptions occurred years ago 100000-250000, which means that the period of activity of the volcano was quite short. The outbreaks took place in three different time periods: a period of cone- forming lava flows, another with frequent eruptions and a final period in which won the bowling especially on diameter and arose extensive deposits.

The solidified lava flows on the slopes of Mount Thielsen are between 30 cm and 10 m thick. The volcano has columnar formations of breccia and Palagonitspuren. In the basalt of Mount Thielsen pyroxene, hypersthene and feldspar were detected.


Glaciers of the Pleistocene eroded caldera. The volcano was glaciated until after the end of the Little Ice Age. Even in the early 20th century, there were on the Mount Thielsen large ice masses. The small Lathrop Glacier in northern Kar is the only glacier today.


Exclusively in the summit area, a maximum of 3 m below the highest point, Fulgurit was found. The material is produced by lightning in rocks. The Fulguritformen on Mount Thielsen have diameters of a few centimeters, and can reach lengths of up to 30 cm. Lightning strikes on a regular basis in the summit area of the volcano and provide for the origin of the Fulgurits that has very dark brown to olive green colors.