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California Volcanoes and Volcanics

Cascade Range

From: Wood and Kienle, 1990, Volcanoes of North America: United States and Canada: Cambridge University Press, 354p., p.149, Contribution by Charles A. Wood.
Holocene volcanism in the Cascades extends from the Garibaldi Volcanic Belt in southern British Columbia to the Lassen volcanic complex in northern California. Pronounced differences in the nature of volcanism occur along the arc. In Washington there are five, generally large, widely spaced stratovolcanoes, with only one ( Mount Adams) having significant nearly basaltic volcanics. In marked contrast, Oregon has six generally smaller stratovolcanoes, but the entire state is traversed by a 40-50-kilometer-wide band of basaltic to andesitic lava shields, cinder cones, and smaller stratovolcanoes that the "Cascade" cones rise above. South of Crater Lake, the Cascade arc bends perceptibly toward the southeast, and continues along this trend to Lassen Peak. Both Lassen and Shasta are associated with eastward halos of mafic shields and lava fields which, near Shasta, culminate in the huge shield volcano of Medicine Lake.

From: Swanson,, 1989, Cenozoic Volcanism in the Cascade Range and Columbia Plateau, Southern Washington and Northernmost Oregon: AGU Field Trip Guidebook T106.
The Cascade Range has been an active arc for about 36 million years as a result of plate convergence. Volcanic rocks between 55 and 42 million years ago occur in the Cascades, but are probably related to a rather diffuse volcanic episode that created the Challis arc extending southeastward from northern to northwest Wyoming. Convergence between the North American and Juan de Fuca plates continues at about 4 centimeters per year in the direction of North-50-degrees-East, a slowing of 2-3 centimeters per year since 7 million years ago. According to most interpretations, volcanism in the Cascades has been discontinuous in time and space, with the most recent episode of activity beginning about 5 million years ago and resulting in more than 3000 vents.

In Oregon, the young terrane is commonly called the High Cascades and the old terrane the Western Cascades, terms that reflect present physiography and geography. The terms are not useful in Washington, where young vents are scattered across the dominantly middle Miocene and older terrane. ...

In Washington and Oregon, a striking contrast has existed for the past 5 million years in the style of volcanism in the Cascades relative to geography. North of Mount Rainier, young volcanism is concentrated in only a few isolated andesitic and dacitic composite cones (notably Glacier Peak, Mount Baker, and the volcanoes of the Garibaldi belt in British Columbia), whereas south of Mount Hood moderate-sized andesitic and dacitic composite cones are relatively unimportant features of a landscape dominated by small andesite and basalt vents. The area between Mounts Rainier and Hood is transitional; large andesite and dacite composite cones ( Rainier, Adams, St. Helens, Hood, and the extinct Goat Rocks volcano) occur together with fields and scattered vents of olivine basalt ( Indian Heaven, Simcoe Mountains, and the King Mountain fissure zone south of Mount Adams. ...

The southern Washington Cascades are seismically active. Most earthquakes occur along the 100-kilometer-long, north-northwest trending St. Helens seismic zone, where most focal mechanisms show dextral slip parallel to the trend of the zone and consistent with the direction of plate convergence. Other crustal earthquakes concentrate just west of Mount Rainier and in the Portland (Oregon) area. Few earthquakes occur north of Mount Rainier or south of Mount Hood.

From tomography, Rasmussen and Humphreys (1988) interpret the subducted Juan de Fuca plate as a quasi-planar feature dipping about 65 degrees to about 300 kilometers under the southern Washington Cascades. The plate is poorly defined seismically, however, owing to a lack of earthquakes within it. Guffanti and Weaver (1988) show that the present volcanic front of the Washington Cascades, defined by the westernmost young vents, parallels the curved trend of the subducting plate reflected by the 60 kilometer-depth contour. The front trends northwest in northern Washington -- where Glacier Peak, Mount Baker, and the volcanoes of southern British Columbia occur along a virtually straight line -- and northeast in southern Washington. A 90-kilometer gap free of young volcanoes between Mount Rainier and Glacier Peak is landward of that part of the subducting plate with the least average dip to a depth of 60 kilometers. South of Portland, the volcanic front is offset 50 kilometers eastward and extends southward into California, probably still parallel to the trend of the convergent margin.

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From: "Catalog of Active Volcanoes of the World" (CAVW), Smithsonian Institution, Global Volcanism Program Website, 2002
From: Wood and Kienle, 1990, Volcanoes of North America: United States and Canada: Cambridge University Press, variety of contributors
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09/20/02, Lyn Topinka