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. 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.
Type: Summit Activity: Snowshoeing,Climbing,Hiking Prominence: 3342 ft/1019 m Peak elevation: 8,717 feet (2,656.94 m)