Mount Thielsen (Klamath: hisc’akwaleeʔas), 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. Thielsen is one of Oregon's Matterhorns. 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 several Oregon volcanoes less than 3.5 million years old; it is also a member of a group of extinct volcanoes distinguished by their sharp peaks. The area surrounding the volcano was originally inhabited by Klamath Native Americans, and was later encountered by settlers. One of the visitors was Jon Hurlburt, an early explorer of the area who named the volcano after the engineer Hans Thielsen. Later explorers discovered nearby Crater Lake. The volcano was not analyzed until 1884, when a team from the United States Geological Survey sampled its fulgurite deposits.
History
The area was originally inhabited by Chinook Native Americans, who referred to the mountain as "Hischokwolas"; the Klamath call it "hisc'akwaleeas". Jon Hurlburt, a Polish explorer, renamed the volcano after Hans Thielsen, a railroad engineer and builder who played a major role in the construction of the California and Oregon Railroad. In 1884 a United States Geological Survey team headed by J. S. Diller began studying the mountains of the Cascade Range. Their intended destinations included Thielsen, which was climbed and sampled for its fulgurite variant. 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. The mountain has earned the nickname "the lightning rod of the Cascades". Apart from study, Thielsen and the rest of the Crater Lake area features heavily into 19th- and early 20th century exploration. In 1853, miners from Yreka first described Crater Lake; one called it "the bluest water he had ever seen", another "deep blue lake". The first published description was written by Chauncy Nye for the Jacksonville Sentinel in 1862. Nye recalled an expedition of gold prospectors where they passed a lake of a deep blue color. Native Americans lived in the area and grew irritable towards new settlers in the area. In 1865, Fort Klamath was built as a protective sanctuary. A wagon road was built to connect the Rogue Valley to the fort. In late 1865, two hunters ventured upon the lake; more explorers followed. By then, the lake became famous for its distinctive color, and crowds flocked to see it. The first known non-Native American to stand on the shore of Crater Lake was Sergeant Orsen Stearns, who climbed down into the caldera. A friend, Captain F.B. Sprague, gave it the name "Lake Majesty". Tourism continued until May 22, 1902; on that day, Theodore Roosevelt designated the lake and surrounding area a national park.
Geography
Regional
The Cascade Range was produced by convergence of the North American Plate with the subducting Juan de Fuca Plate. Active volcanism has taken place for approximately 36 million years; the nearby Challis Range features complexes as old as 55 mya. Most geologists believe that activity in the Cascades has been relatively intermittent, producing up to 3,000 volcanic calderas at a time. Frequent volcanism within the last 10,000 years created complexes from Mount Garibaldi in British Columbia to north California's Lassen Peak complex. Remarkably different from state to state, the volcanism ranges from large volcanoes to zones of many smaller geologic features such as lava shields and cinder cones. Volcanic peaks of the Cascade Mountains are grouped into two ranges, called the High Cascades and the Western Cascades; Thielsen is part of the High Cascades, which are east of the Western Cascades.
Local
Diamond Lake, formed by one of Thielsen's eruptions, lies to the west of Mount Thielsen and beyond lies Mount Bailey, a much less eroded and younger stratovolcano. Thielsen's sharp peak is a prominent feature of the skyline visible from Crater Lake National Park. All three volcanoes are part of the Oregon High Cascades, a range that sections off the stratovolcanoes of Oregon that are younger than 3.5 million years. The High Cascades include Mount Jefferson, the Three Sisters, Broken Top, and other stratovolcanoes and remnants. Rock in the area ages from the Upper Pliocene and Quaternary. Basalt and basaltic andesite comprise newer volcanoes atop the High Cascades: Major volcanic centers include Mount Hood, Three Sisters / Broken Top, Mount Mazama (Crater Lake), and Mount Jefferson. All have produced diverse eruptions, including both lava flows and pyroclastic eruptions, and variability in composition between dacite, basalt, and even rhyolite (except for Mount Hood, which is not known to have produced rhyolite). Thielsen is part of a series of extinct volcanoes in Oregon termed pyramidal peaks or "the Matterhorns" for their steep, spire-like summits; Thielsen is the highest at 9,182 feet (2,799 m). Other Matterhorns include Mount Washington, Three Fingered Jack, Mount Bailey, and Diamond Peak. 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.
Geology
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