USGS/Cascades Volcano Observatory, Vancouver, Washington
Three-Fingered Jack Volcano, Oregon
Three-Fingered Jack Information
Central High Cascade Roadside Geology: Bend, Sisters, McKenzie Pass, and Santiam
IN: Guides to Some Volcanic Terranes in Washington, Idaho, Oregon, and Northern
California: USGS Circular 838.
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
of light-colored, vesicular basaltic andesite with a
core that has been invaded by plugs and radial dikes. ...
Some of the
basaltic andesite volcanoes developed large
reaching 10,000 feet elevation on a
10 miles wide. Examples include
Mount Washington, and
Wood and Kienle, 1990, Volcanoes of North America: United States and Canada:
Cambridge University Press, 354p., p.179,
Contribution by David R. Sherrod
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
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.
Hoblitt, et.al., 1987,
Volcanic Hazards with Regard to Siting Nuclear-Power Plants in the Pacific Northwest: USGS Open-File Report 87-297
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
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.
From: O'Connor, Hardison III, and Costa, 2001, Debris Flows from Failures of Neoglacial-Age Moraine Dams in the Three Sisters and Mount Jefferson Wilderness Areas, Oregon: USGS Professional Paper 1606, 93p.
In the United States, the
Cascade Range extends from northern
California to northern
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
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09/07/04, Lyn Topinka