Union Peak is a heavily eroded shield volcano

Shield volcano

A shield volcano is a type of volcano usually built almost entirely of fluid lava flows. They are named for their large size and low profile, resembling a warrior's shield. This is caused by the highly fluid lava they erupt, which travels farther than lava erupted from more explosive volcanoes...

 in the Cascade Range

Cascade Range

The Cascade Range is a major mountain range of western North America, extending from southern British Columbia through Washington and Oregon to Northern California. It includes both non-volcanic mountains, such as the North Cascades, and the notable volcanoes known as the High Cascades...

 of central Oregon

Oregon

Oregon is a state in the Pacific Northwest region of the United States. It is located on the Pacific coast, with Washington to the north, California to the south, Nevada on the southeast and Idaho to the east. The Columbia and Snake rivers delineate much of Oregon's northern and eastern...

, located along the Cascade Crest less than 7 miles (11.3 km) southwest of the rim of Crater Lake

Crater Lake

Crater Lake is a caldera lake located in the south-central region of the U.S. state of Oregon. It is the main feature of Crater Lake National Park and famous for its deep blue color and water clarity. The lake partly fills a nearly deep caldera that was formed around 7,700 years agoby the...

. Ice Age

Ice age

An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...

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

Cirque (landform)

thumb|250 px|Two cirques with semi-permanent snowpatches in [[Abisko National Park]], [[Sweden]].A cirque or corrie is an amphitheatre-like valley head, formed at the head of a valley glacier by erosion...

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

Three Fingered Jack

Three Fingered Jack, named for its distinctive shape, is a Pleistocene volcano in the Cascade Range of Oregon. It is a deeply glaciated shield volcano and consists mainly of basaltic andesite lava...

, Mount Washington

Mount Washington (Oregon)

Mount Washington is a deeply eroded shield volcano in the Cascade Range of Oregon. The mountain dates to the Late Pleistocene. However, it does have a line of basaltic andesite spatter cones on its northeast flank which are approximately 1,330 years old according to carbon dating...

, Howlock Mountain

Howlock Mountain

Howlock Mountain is a heavily eroded shield volcano in the Cascade Range of central Oregon, located along the Cascade Crest just north of Mount Thielsen. Ice Age glaciers eroded away most of the flanks of the volcano, leaving numerous deep cirques surrounding a central ridge capped by several horns...

, and Mount Thielsen

Mount Thielsen

Mount Thielsen, or Big Cowhorn, is an extinct shield volcano in the Oregon High Cascades, near Mount Bailey. Because Mount Thielsen stopped erupting 250,000 years ago, glaciers have heavily eroded the volcano's structure, creating precipitous slopes and its horn-like peak...

 from other shield volcanoes farther north in the Oregon Cascades.

Union Peak is located within Crater Lake National Park

Crater Lake National Park

Crater Lake National Park is a United States National Park located in southern Oregon. Established in 1902, Crater Lake National Park is the sixth oldest national park in the United States and the only one in the state of Oregon...

, and the summit was used as a post for fire lookout

Fire lookout

A fire lookout 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 a good view of the surrounding terrain, to spot smoke caused by a wildfire.Once a possible fire...

s from the 1930s to the 1950s. A 5.5 miles (8.9 km) trail

Trail

A trail is a path with a rough beaten or dirt/stone surface used for travel. Trails may be for use only by walkers and in some places are the main access route to remote settlements...

 leads to the summit from the Pacific Crest Trail

Pacific Crest Trail

The Pacific Crest Trail is a long-distance mountain hiking and equestrian trail on the Western Seaboard of the United States. The southern terminus is at the California border with Mexico...

 off Oregon Route 62.