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Memoir 47—Field excursions to volcanic terranes in the western United States, Volume II:
Cascades and Intermountain West

Edited by C. E. Chapin and J. Zidek, 1989, ix + 285 pp., 18 tables, 234 figs.

This publication is produced for the International Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI) General Assembly held in Santa Fe, New Mexico, on 25 June-1 July 1989. Contains a combination of scientific papers and field guides designed to familiarize the reader with the geology of a particular volcanic field and then lead him through it. Memoir 47 contains seven excursions to volcanic fields in Washington, Oregon, Montana, Wyoming, Idaho, Nevada, and California. The individual excursions were compiled from contributions of three to 11 authors each; a total of 105 authors contributed to the 16 excursions of Memoir 46 and Memoir 47. The technical papers form integral parts of the excursions, contain much new data, and are expected to serve as important references for many years. This set includes Bulletin 131 and Memoirs 46 and 47. Bulletin 134 is a companion to this set.

Excursions in this memoir include:

Cenozoic volcanism in the Cascade Range and Columbia Plateau, southern Washington and northernmost Oregon by D. A. Swanson, et. al. This text is for a six-day excursion between Issaquah, Washington, and Portland, Oregon, that emphasizes Tertiary and Quaternary volcanic geology of the western Columbia Plateau and the Cascade Range of southern Washington and northern Oregon. It summarizes the geology of selected areas along the route and provides a brief introduction to the general volcanic history of the Columbia River Basalt Group and the southern Washington Cascades. An extensive but not exhaustive list of references is included. The road logs are designed to be self-guiding; as such, they are more complete than necessary for guided bus excursions.

Recent volcaniclastic deposits and processes at Mount St. Helens volcano by R. B. Waitt, et. al. Mount St. Helens is a young composite volcano situated in southwestern Washington State, USA. Participants of IAVCEI field excursion 2A will investigate deposits that range from prehistoric to modern. Most attention will be given to deposits of the 18 May 1980 eruption. In the course of six days, most flanks of the volcano will be visited.

Eocene through Miocene volcanism in the Great Basin of the western United States by M. G. Best, et. al. Cenozoic magmatic rocks in the western US define a complex space-time-composition pattern that for many years has posed numerous interpretive challenges. One of the more intriguing tectonomagmatic provinces is the northwestern segment of the Basin and Range province in Nevada, western Utah, and minor adjacent parts of Oregon, Idaho, and California. This segment, characterized for the most part by internal drainage, is called the Great Basin. This paper summarizes characteristics of volcanic rocks in the Great Basin, primarily ash-flow tuff and subordinate lava, which were deposited from Eocene through Miocene time. Although widespread younger volcanic deposits exist, their volume is much less and they are not considered here. Only a few general and preliminary interpretations of the writers' observations are presented.

Silicic volcanic rocks in the Snake River Plain-Yellowstone Plateau province by B. Bonnichsen, et. al. The Snake River Plain volcanic province extends from southwest to northeast across southern Idaho and adjoining parts of Oregon, Nevada, and Wyoming. During the past 14 m. y. this elongate zone has been the site of extensive, bimodal, basalt-rhyolite volcanism. Although many exceptions exist, the general location of active volcanism has progressed from southwest to northeast along this zone and, at any particular place or time, the type of volcanism has changed from silicic ignimbrites to rhyolite lava flows to basalt lava flows. The major time-transgressive component of the volcanism was the silicic activity that swept from west to east. The silicic volcanism appears to have started in the region near the junction of Idaho, Oregon, and Nevada, and perhaps at the other sites to the north in the vicinity of the Idaho–Oregon state line. After this activity ceased, widespread basaltic eruptions occurred and, even later in the history of the evolving Snake River Plain, sporadic, small-volume silicic volcanic eruptions and additional basaltic eruptions took place.

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South Cascades arc volcanism, California and southern Oregon by L. J. P. Muffler, et. al. The Cascade Range is a late Tertiary and Quaternary volcanic arc that extends north from northeastern California through Oregon and Washington into British Columbia. The volcanic arc lies above an easterly dipping active subduction zone along which the Juan de Fuca, Gorda, and Explorer plates are thrust beneath the North American plate. In addition to the major composite volcanoes that have erupted andesites, dacites, and even rhyolites, there are many smaller, commonly monogenetic vents that erupted primarily calc-alkaline basalt and basaltic andesite throughout the history of the arc. The volcanic arc is active, with certain historic eruptions at Lassen Peak and Mount St. Helens, and possible historic eruptions at Mt. Shasta, Mt. Baker, Mt. Hood, Mt. Rainier, and Cinder cone. This field guide is designed as a six-day introduction to the volcanic geology of the Cascade Range in northern California and southern Oregon. Emphasis is placed on four major Quaternary volcanic centers of the High Cascades.

Long Valley caldera and Mono-Inyo Craters volcanic chain, eastern California by R. A. Bailey, et. al. Long Valley caldera is located at the western edge of the Basin and Range province straddling the eastern frontal fault escarpment of the Sierra Nevada, in which it forms a reentrant or offset commonly referred to as the "Mammoth embayment." The floor of the caldera ranges in elevation from 2,000 m ((656 ft) in its eastern half, where it is dominated by Lake Crowley and sage and grass-covered Long Valley, to 2,600 m ((8,530 ft) in its western half which is hillier and heavily forested. The caldera walls rise steeply to elevations of 3,000-3,500 m (9,842-11,483 ft) on all sides except the east and southeast where the floor rises only 150 m (492 ft) before merging with the Volcanic Tableland at 2,300 m elevation. The Mono-Inyo Craters volcanic chain extends from the western part of Long Valley caldera northward from Mammoth Mountain to Mono Lake, a distance of 50 km (31 mi). Although commonly described as subparallel to the Sierran front, the chain trends nearly due north at a noticeable angle to the northwest-trending Sierran faults. The prevolcanic basement in the area is mainly Mesozoic granitic rocks of the Sierra Nevada batholith plus Paleozoic metasedimentary rocks and Mesozoic metavolcanic rocks of the Mt. Morrison and Ritter Range roof pendants. The late Tertiary terrain upon which Long Valley volcanism was initiated was a maturely eroded upland drained by westward-flowing streams.

Cordilleran volcanism, plutonism, and magma generation at various crustal levels: Montana and Idaho compiled by D. W. Hyndman. This six-day field trip examines emplacement of Late Cretaceous to Eocene felsic magmas at various crustal levels, from deep continental crust to the Earth's surface. The trip is in three parts, each approximately two days long: 1) Idaho batholith: deep plutonic, with no preserved volcanic rocks that are known to be related. 2) Boulder batholith-Elkhorn Mountains Volcanics: shallow plutons with some preserved volcanic rocks erupted from them. 3) Challis Volcanics: broad volcanic field erupted from calderas, with some exposure of magma-chamber plutons.

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