DESCRIPTION:
Mount Washington Volcano, Oregon

MtWash84_mount_washington_oregon_10-01-84.jpg
Mount Washington, Oregon, as seen from the east.
USGS Photograph taken October 1, 1984, by Lyn Topinka.
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Mount Washington

Location: Oregon

Latitude: 44.332 N

Longitude: 121.837 W

Height: 2,376 Meters

Type: Shield Volcano

Remarks: Late Pleistocene shield volcano surmounted by composite summit cone of cinders, lavas, and intrusives²

Although no isotopic ages are available, all of the Mount Washington lavas and the underlying basalt appear to be of normal paleomagnetic polarity; the age of Mount Washington is probably no more than a few 100,000 years, similar to that of other central High Cascade stratovolcanoes. During the late Pleistocene, cirques were excavated into the flanks of the summit cone by valley glaciers which extended more than 12 kilometers east and west. The is no evidence of recent reactivation of Mount Washington volcanism, but a series of aligned small basaltic andesite spatter cones erupted on the northeast flank approximately 1,330 years ago (carbon-14). Access to the Mount Washington Wilderness is restricted to foot trails. The west and southwest sides of the mountain are crossed by the Skyline Trail, 5 kilometers from a trailhead at Big Lake, near U.S. 20. The best long-distance viewpoints on main highways are from U.S. 20 near Blue Lake and from Oregon 242 at Windy Point.

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.

The High Cascade Range adjacent to Mount Washington is covered by lavas of high-alumina diktyaxitic basalt. Part of this lava field was produced from scattered cinder cones, now represented by glacially scoured, inconspicuous buttes along the Cascade crest. Most of the basalt emerged from vents now buried beneath Mount Washington and poured down the east and west Cascade slopes. To the east, these basaltic lavas are exposed along the perimeter of Mount Washington in the canyons of Cache Creek and Dry Creek, south to Dugout Mountain. Westward, basalt occurs as glaciated outcrops from Patjens lakes to the McKenzie River, where they form intracanyon benches, 16 kilometers from Mount Washington.

Subsequent eruptions of relatively uniform basaltic andesite lavas produced a shield volcano, 5 kilometers in diameter, surmounted by a summit cone that probably reached an elevation of 2,600 meters, around 1,200 meters above the pre-existing basalt field. Mafic ash accumulated on the flanks of the shield and has been preserved as thick sections of palagonitic tuff on the southwest and northeast sides of the summit cone. The volcano was intruded by a micronorite plug which now forms the central pinnacle, 0.4 kilometers in diameter. Dikes are exposed in many parts of the summit cone; their predominant orientation is north-south. A swarm of narrow dikes extends northward form the summit plug.

Although no isotopic ages are available, all of the Mount Washington lavas and the underlying basalt appear to be of normal paleomagnetic polarity; the age of Mount Washington is probably no more than a few 100,000 years, similar to that of other central High Cascade stratovolcanoes. During the late Pleistocene, cirques were excavated into the flanks of the summit cone by valley glaciers which extended more than 12 kilometers east and west. The is no evidence of recent reactivation of Mount Washington volcanism, but a series of aligned small basaltic andesite spatter cones erupted on the northeast flank approximately 1,330 years ago (carbon-14). Access to the Mount Washington Wilderness is restricted to foot trails. The west and southwest sides of the mountain are crossed by the Skyline Trail, 5 kilometers from a trailhead at Big Lake, near U.S. 20. The best long-distance viewpoints on main highways are from U.S. 20 near Blue Lake and from Oregon 242 at Windy Point.

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