Three Fingered Jack, named for its distinctive shape, is a Pleistocene

Pleistocene

The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....

 volcano 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 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...

. It is a deeply glaciated

Glacier

A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...

 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...

 and consists mainly of basaltic andesite

Basaltic andesite

Basaltic andesite is a black volcanic rock containing about 55% silica. Minerals in basaltic andesite include olivine, augite and plagioclase. Basaltic andesite can be found in volcanoes around the world, including in Central America and the Andes of South America. Basaltic andesite is common in...

 lava

Lava

Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...

. Compared with other heavily glaciated volcanic peaks such as 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...

 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...

, which display eroded volcanic necks as their cores, Three Fingered Jack's present summit is a narrow ridge of loose tephra

Tephra

200px|thumb|right|Tephra horizons in south-central [[Iceland]]. The thick and light coloured layer at center of the photo is [[rhyolitic]] tephra from [[Hekla]]....

 supported by a dike

Dike (geology)

A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...

 only 10 feet (3 m) thick on a generally north-south axis. Radiating dikes and plugs from a series of overlapping cinder and composite cones that support this summit have been exposed by glaciation.

The sole glacier

Glacier

A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...

 is the unofficially-named Jack glacier, located in a shaded cirque

Cirque

Cirque may refer to:* Cirque, a geological formation* Makhtesh, an erosional landform found in the Negev desert of Israel and Sinai of Egypt*Cirque , an album by Biosphere* Cirque Corporation, a company that makes touchpads...

 on the northeast side at an unusually low altitude for the Oregon Cascades. The last cone-building eruptions were more explosive in nature than earlier shield-building eruptions, producing mostly unconsolidated strata. Cone-building eruptions ceased before the Pleistocene glacial period, allowing the expanded glacial cover to remove most of the cone.

Sources disagree about who first reached the summit, It was either six boys from Bend, Oregon

Bend, Oregon

Bend is a city in and the county seat of Deschutes County, Oregon, United States, and the principal city of the Bend, Oregon Metropolitan Statistical Area. Bend is Central Oregon's largest city, and, despite its modest size, is the de facto metropolis of the region, owing to the low population...

 or members of The Mazamas

The Mazamas

The Mazamas is a mountaineering club based in Portland, Oregon, United States, founded in 1894.-Name:The name Mazamas means mountain goat, from Nahuatl mazatl, deer. for etymology...

. Sources agree that the ascent occurred in 1923.

Three Fingered Jack is located in the Mount Jefferson Wilderness

Mount Jefferson Wilderness

The Mount Jefferson Wilderness is a wilderness area located on and around Mount Jefferson in the central Cascade Range of Oregon in the United States. It is situated where the Willamette, Deschutes, and Mount Hood National Forests meet...

 and is not accessible by paved road, but can be approached by 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...

. It is regarded as extinct and has not been assessed for volcanic hazards.