Union Peak is a heavily eroded

shield volcano A shield volcano is a type of volcano named for its low profile, resembling a shield lying on the ground. It is formed by the eruption of highly fluid (low viscosity) lava, which travels farther and forms thinner flows than the more viscous lava ...

in the

Cascade Range The Cascade Range or Cascades is a major mountain range of western North America, extending from southern British Columbia through Washington (state), Washington and Oregon to Northern California. It includes both non-volcanic mountains, such as m ...

of central

Oregon Oregon ( , ) is a U.S. state, state in the Pacific Northwest region of the United States. It is a part of the Western U.S., with the Columbia River delineating much of Oregon's northern boundary with Washington (state), Washington, while t ...

, located along the Cascade Crest less than southwest of the rim of

Crater Lake Crater Lake ( Klamath: ) is a volcanic crater lake in south-central Oregon in the Western United States. It is the main feature of Crater Lake National Park and is a tourist attraction for its deep blue color and water clarity. T ...

Ice age An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages, and g ...

glaciers A glacier (; or ) is a persistent body of dense ice, a form of rock, that is constantly moving downhill under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires ...

eroded away most of the flanks of the volcano, leaving numerous deep

cirques A (; from the Latin word ) is an amphitheatre-like valley formed by glacial erosion. Alternative names for this landform are corrie (from , meaning a pot or cauldron) and ; ). A cirque may also be a similarly shaped landform arising from fluvi ...

surrounding a central glacial horn. This is the same process that formed the summit horns of

Three Fingered Jack Three Fingered Jack is a summit of a shield volcano of the Cascade Range in the U.S. state of Oregon. Formed during the Pleistocene epoch, the mountain consists mainly of basaltic andesite lava and was heavily glaciated in the past. While ot ...

Mount Washington Mount Washington is an ultra-prominent mountain in the state of New Hampshire. It is the highest peak in the Northeastern United States at and the most topographically prominent mountain east of the Mississippi River. The mountain is notorio ...

, Howlock Mountain, and Mount Thielsen from other shield volcanoes farther north in the Oregon Cascades. Union Peak is located within

Crater Lake National Park Crater Lake National Park is a national park of the United States located in southern Oregon. Established in 1902, Crater Lake is the fifth-oldest national park in the United States and the only national park in Oregon. The park encompasses t ...

, and the summit was used as a post for

fire lookout A fire lookout (sometimes also called a fire watcher) is a person assigned the duty to look for fire from atop a building known as a fire lookout tower. These towers are used in remote areas, normally on mountain tops with high elevation and ...

s from the 1930s to the 1950s. A

trail A trail, also known as a path or track, is an unpaved lane or a small paved road (though it can also be a route along a navigable waterways) generally not intended for usage by motorized vehicles, usually passing through a natural area. Ho ...

leads to the summit from the

Pacific Crest Trail The Pacific Crest Trail (PCT), officially designated as the Pacific Crest National Scenic Trail, is a long-distance hiking and equestrian trail closely aligned with the highest portion of the Cascade and Sierra Nevada mountain ranges, which lie ...

off Oregon Route 62.

References

* * * * {{Crater Lake Crater Lake National Park Shield volcanoes of the United States Subduction volcanoes Cascade Volcanoes Volcanoes of Oregon Mountains of Oregon Landmarks in Oregon Cascade Range Mountains of Klamath County, Oregon Volcanoes of Klamath County, Oregon Pyramidal peaks