Mount Thielsen, or Big Cowhorn, is an extinct shield volcano in the Oregon High Cascades, near Mount Bailey. Because eruptive activity ceased 250,000 years ago, glaciers have heavily eroded the volcano's structure, creating precipitous slopes and a horn-like peak. The spire-like shape of Thielsen attracts lightning strikes and creates 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. Thielson is one of Oregon's Matterhorns.
Mount Thielsen's eroded edifice from the Pacific Crest Trail
Mount Thielsen seen from the south in the 1930s
The Juan de Fuca Plate is being subducted under the North American Plate, generating gradual, diverse volcanism.
Aerial view of Mount Thielsen
A pyramidal peak, sometimes called a glacial horn in extreme cases, is an angular, sharply pointed mountain peak which results from the cirque erosion due to multiple glaciers diverging from a central point. Pyramidal peaks are often examples of nunataks.
The Matterhorn, a classic example of a pyramidal peak.
Kinnerly Peak in Glacier National Park in the U.S. state of Montana
Alpamayo in Huascarán National Park, Peru
Artesonraju in Huascarán National Park, Peru