Mount McLoughlin is a stratovolcano in the southern Oregon part of the Cascade Range. The mountain is north of Mount Shasta, south of Crater Lake, and west of Klamath Lake. The volcano was originally called Mount Pitt but was renamed after John McLoughlin, an official of the Hudson's Bay Company.

Except for a short monograph written by Arthur B. Emmons in 1886, little was known about McLoughlin's geology until the 1970s, Much of what is known about the volcano's geology comes from LeRoy Maynard of the University of Oregon's Center for Volcanology.

Geology

McLoughlin went through three or four stages of development and stopped growing some time before the current glaciation started 25,000 years ago.

The first known stage of the volcano's growth was explosive and built a large cinder cone from a single vent that may have reached the unusual height of 3000 feet (~900 meters) and may fill one third of McLoughlin's present interior (a steep-sided pre-existing shield volcano may exist within the cinder cone, thus explaining its extreme height). Some lava is associated with this stage but is confined to the lower parts of the cinder cone (this is common for cinder cones). McLoughlin's second cone-building stage was characterized by large andesite lava flows that poured out of a summit crater and in time buried the cinder cone. The volcano's third stage of development consisted of floods of blocky andesite lava erupting from below the summit and more fluid lavas erupting from basal fissures mostly confined to the south slope. North and South Squaw Tips on McLoughlin's west flank now mark the site of the two major blocky flow vents (there were two smaller vents). The entire third stage is thought to have happened in the Holocene (after the last ice age) due to a general lack of weathering and fresh appearance of the solidified lava.

During the last ice age a large ice cap buried most of the High Cascades and McLoughlin marked the southern terminus of this structure. The ice cap was 200 to 300 feet (60 to 90 meters) thick where it touched McLoughlin. A large ice age glacier on the northeast side of the volcano severely eroded that part of the mountain, making an asymmetrical cone (a cirque glacier occupied part of the depression until early in the 20th century). The volcano's crest is located south of the central vent and its crater has long since been eroded away (along with ~500 feet, ~150 meters of its former height).

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