DESCRIPTION:
Mount Thielsen Volcano, Oregon

Thielsen87_aerial_thielsen_09-87.jpg
Aerial view Mount Thielsen, Oregon.
USGS Photograph taken in September 1987, by W.E. Scott.
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Mount Thielsen (2,800 meters; 9,187 feet) is a normally polarized shield volcano comprising approximately 8 cubic kilometers of basaltic andesite built atop a broad pedestal (24 cubic kilometers) of older lava. Thielsen is remarkable even at a distance for its colorfully interbedded pyroclastic rocks that dip away from the jagged spire of the central plug, often called the "lightning rod of the Cascades". The most spectacular views are on the north and east sides (accessible only by foot or horseback) where now-vanished glaciers have carved precipitous cirque walls that reveal the construction. Thielsen's age is approximately 290,000 years (whole-rock K-Ar), and its geomorphology is a reference point for assigning Cascade Range volcanoes to the age division 0-0.25 million years (younger than Thielsen) or 0.25-0.73 million years (older than Thielsen). Very little of Thielsen's underpinnings are exposed because Holocene Mazama ash, which erupted from vents at Crater Lake National Park (20 kilometers south), forms a shroud 4-20 meters thick in the Thielsen area.

Mount Thielsen is similar to many of the basaltic andesite shields that form the bulk of the High Cascades in Oregon. It consists of a central pyroclastic cone built of scoriaceous to pumiceous cindery tuff and coarse breccia. Variations in grain size define the bedding, which is made more spectacular by the alteration of glassy tephra to colorful palagonite. The beds mainly dip 10-40 degrees away from the central conduit-filling plug, although locally these beds have been steepened and even overturned during the plug's intrusion, a feature unreported from other shields in the Cascade Range. Dikes and sills lace the cone.

The lava of Mount Thielsen forms stacks of gently dipping flows and breccia as much as 100 meters thick. Single lava flows are as thin as a few centimeters near their vents but thicken to more than 10 meters downslope. Fountaining from dike-fed eruptions around the edge of the cone generated coalescing spatter, which formed many of the flows. In the eastern cirque wall, lava is preserved draining back toward the dikes. Lava moving downslope from these satellitic vents probably rafted away parts of the cone. ...

Thielsen's summit spire is a thick sheet of two-pyroxene basaltic andesite 500 meters across at the lowest exposures but dividing upward. The conduit-filling magma congealed without multiple intrusion.

Thielsen has been glaciated many times since its eruptions around 300,000 years ago. A talus rampart in the north cirque still protects a very small permanent snowfield ("Oregon's southernmost glacier") shown as Lathrop Glacier on the new 7.5-minute quadrangle map.

Mount Thielsen is located near Diamond Lake and north of Crater Lake National Park, in the Umpqua National Forest. ... Trails from Oregon Highway 138 lead up the south and west sides of Thielsen.

The Trailhead:

Go 1.5 miles north of the Highway 230/Highway 138 road junction near Diamond Lake. The trailhead parking lot is located along the east side of Highway 138. A USFS Northwest Forest Pass is required to park at this trailhead. Umpqua National Forest Recreation Map reference number is L-6. Located at Township 28 south, Range 5 1/2 east, Section 16.

Attractions and Considerations:

The trail leads through lodgepole pine for the first mile. Timberline is at approximately 7,200 feet elevation. The mountain hemlock-true fir type is predominant. Inviting glimpses of Mount Thielsen are evident as one progresses. A breath taking view of Mount Thielsen presents itself just beyond the Spruce Ridge Trail junction 1.6 miles. As you reach the Pacific Crest National Scenic Trail 2.9 miles, the glacial features of this volcanic peak loom before you. From the Pacific Crest National Scenic Trail, a scramble trail works its way to the spire pointed peak at 9,182 feet elevation. The trail is a steep climb, particularly above timberline beyond which there are no markers. The last 200 feet is a difficult hand-over-hand climb. Elevation gain is 3,782 feet. Hiking boots and caution are highly recommended for climbing on the loose rocky slopes. Those brave and energetic enough to make it to the top, should add their names to the climbing register found there. The view of the east and west sides of the Cascades, from the Sisters to Mount Shasta, is incredible. This trail is used in the winter time by Nordic skiers up to the Pacific Crest National Scenic Trail.

Much of the Mount Thielsen Wilderness is made up of high alpine forests and open meadows. Elevations range from 5,000 feet to the 9,182 summit of Mount Thielsen, the "lightening rod of the Cascades." The Pacific Crest National Scenic Trail winds through the Mount Thielsen Wilderness for 26 miles along the summit of the Cascade Range. A trailhead on Highway 138, one mile east of the north entrance to Crater Lake National Park, is the southern entrance point.

Mount Thielsen, (Diamond Lake Ranger District; Umpqua National Forest) Also known as Big Cowhorn. This mountain was known as Hischokwolas to Indians of the area. This rugged horn-like mountain is unique and very distinguishable.

About 1872 it was named Mount Thielsen by John A. Hurlburt of Portland, in honor of Hans Thielsen, prominent pioneer railroad engineer and builder.

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