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. Compared with other heavily glaciated volcanic peaks such as Mount Washington and Mount Thielsen, which display eroded volcanic necks as their cores, Three Fingered Jack's present summit is a narrow ridge of loose tephra supported by a dike only 10 feet (3.0 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.
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