DESCRIPTION:
Three-Fingered Jack Volcano, Oregon

The central High Cascade Range of Oregon is chiefly a Pleistocene volcanic platform of overlapping basalt and basaltic andesite lava flows whose aggregate thickness is generally unknown but probably exceeds 4,000 feet locally. This platform is elongate north-south and is 20-30 miles wide. A typical volcano of the platform is a broad shield of light-colored, vesicular basaltic andesite with a cinder cone core that has been invaded by plugs and radial dikes. ... Some of the basaltic andesite volcanoes developed large composite structures reaching 10,000 feet elevation on a shield base 10 miles wide. Examples include The Husband, North Sister, Mount Washington, and Three-Fingered Jack.

Three Fingered Jack (2,390 meters) is the most distinctive volcano in this part of the range -- (Central Oregon High Cascades south of Mount Jefferson to Santiam Pass). This deeply glaciated basaltic andesite shield volcano has around 800 meters of relief and is centered on a pyroclastic cone that underlies the summit of the mountain. The cone lacks a high-level conduit-filling plug, however, unlike other shield volcanoes such as nearby Mount Washington south of Santiam Pss. Three Fingered Jack is undated by radiometric methods, but its age probably lies between 0.50 and 0.25 million years ago, as inferred from its erosional state compared to other shield volcanoes in the High Cascades.

Taylor (1965) described Holocene eruptions between North Sister and Three-Fingered Jack, Oregon, that exemplify the activity of Cascade basaltic volcanoes, although the density of Holocene vents there is much greater than in other areas of the range. One type of activity was characterized by initial scoria and ash eruptions that formed cinder cones and by later extrusion of lava flows. The most recent such eruption in the Cascades occurred in 1851 A.D. at Cinder Cone, east of Lassen Peak. In many areas, this type of activity has formed fields of numerous scoria cones, which are typically arranged in a linear zone, and lava flows.

In the United States, the Cascade Range extends from northern California to northern Washington. In Oregon, south of Mount Hood, the Cascade Range is 50 to 120 kilometers wide, and is composed primarily of upper Eocene to Quaternary volcanic, volcaniclastic, sedimentary, and igneous intrusive rocks. The crest of the Cascade Range is generally at an altitude of 1,500 to 2,000 meters, with several of the high volcanoes exceeding 3,000 meters. The high stratovolcanoes and volcano remnants of the central Oregon Cascade Range include Mount Jefferson, Three Fingered Jack, Mount Washington, the three volcanoes of the Three Sisters, Broken Top, Mount Bachelor, Diamond Peak, Mount Thielsen, and Crater Lake Caldera (Mount Mazama). These are all Pleistocene stratovolcanoes of rhyolitic to basaltic composition and are formed mostly of interlayered thin lava flows and pyroclastic deposits overlying cinder cone cores. Of the large stratovolcanoes, there has been Holocene volcanic activity on the summit and flanks of South Sister and at Mount Bachelor as well as the caldera-forming eruptions of Mount Mazama. The general conical morphology is best preserved on the volcanoes of Middle Sister, South Sister, and Mount Bachelor; the rest have been deeply eroded by Pleistocene glaciation.

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