Mount Thielsen, Extinct shield volcano in southern Oregon, United States

Mount Thielsen is an extinct shield volcano in the High Cascades of Oregon, reaching 9,182 feet (2,799 m). Glaciers eroded it after eruptions ceased 250,000 years ago, leaving a horn-like summit of pale, fractured rock with no crater. Broad slopes of loose material with vertical gullies fan out below, and dense forest covers the base.

Jon Hurlburt, an early explorer of the area, named the volcano after Hans Thielsen, a railroad engineer who helped build the California and Oregon Railroad. In 1884 a United States Geological Survey team led by J. S. Diller climbed the peak and sampled its fulgurite deposits. Eruptive activity had ceased about 250,000 years ago, and glaciers later carved the summit into a horn.

The Klamath people called this peak hisc'akwaleeʔas before settlers renamed it after railroad engineer Hans Thielsen. Hikers today follow trails through the wilderness area that carries the mountain's current name.

Hiking trails wind through 55,100 acres of wilderness around the peak and connect with the Pacific Crest Trail. Most routes require good fitness, especially for the steep summit climb over loose rock.

Lightning strikes hit the summit so often that they create fulgurite, a glass-like mineral formed from melted rock. These lightning tubes sometimes lie scattered across the upper slopes, showing the electrical activity of the area.