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 prominent horn forms a centerpiece for the Mount Thielsen Wilderness, a reserve for recreational activities such as skiing and hiking.

[5] Volcanism near the Cascades dates back to 55 million years ago (mya), and extends from British Columbia to California.

One of the visitors was Jon Hurlburt, an early explorer of the area who named the volcano after the engineer Hans Thielsen.

The area was originally inhabited by Chinook Native Americans, who referred to the mountain as "Hischokwolas";[6] the Klamath call it "hisc'akwaleeas".

[9] In 1884 a United States Geological Survey team headed by J. S. Diller began studying the mountains of the Cascade Range.

Thielsen's spire-like top is hit by lightning so frequently that some rocks on the summit have melted into a rare mineraloid known as lechatelierite, a variety of fulgurite.

Active volcanism has taken place for approximately 36 million years; the nearby Challis Range features complexes as old as 55 mya.

Unlike other mountains in the High Cascades, all these volcanoes became extinct 250,000–100,000 years ago, and their summits endured the last few ice ages, accounting for their distinct shapes.

[6] On the sides of the mountain are bands of palagonite, a clay formed from iron-rich tephra making up the body of the volcano.

The volcanic cone of Mount Thielsen sits atop prior shield volcanoes, and has a volume of 2 cubic miles (8.3 km3).

The cone was built from basaltic andesite, a common component of other shield volcanoes in the Oregon Cascades,[14] breccia, and tuff, and it is intruded by dikes.

[18] Whilst the glaciation was previously extensive, volcanic ash from eruptive activity at Mount Mazama has almost certainly masked contents.

Lightning strikes the summit regularly, creating patches of "brownish black to olive-black glass"[10] that resemble "greasy splotches of enamel paint".

Inspection of the fulgurite reveals a homogenous glass over a layer of basalt; in between, a stratum made of materials such as feldspar, pyroxene, and olivine exists.

[10] A grove of enormous incense cedars exists near Diamond Lake, and there is a forest of ponderosa pine at the nearby Emile Big Tree Trail.

The forest is inhabited by avian species such as mallards, American bald eagles, Canada geese, and whistling swans.

[26] It also contains 26 miles (42 km) of the Pacific Crest National Scenic Trail, accessible from a trailhead along Oregon Highway 138.