556 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



upper part is composed of two Recent lava-scoaria cones of olivine basalt, the 

 younger of which has a well-preserved crater that may have been active 

 during the present millenium. 



The largest Pleistocene andesite-dacite volcano was undoubtedly Mount 

 Mazama, the ancestral mountain in the collapsed summit of which lies Crater 

 Lake. This volcano, and its parasitic cone, Mount Scott, grew to full height by 

 eruption of pyroxene andesites; then, in late Pleistocene time, more siliceous 

 andesites and dacites were discharged from vents on a semicircular fissure on 

 the northern slopes of the volcano, while a cluster of dacite domes rose near 

 its eastern base and many basaltic cinder cones were formed elsewhere on 

 the mountainsides. 



During Pleistocene time, long flows of massive, pale-gray olivine basalt 

 poured down the ancestral canyons of several of the principal rivers that now 

 traverse the Western Cascades, such as the North Santiam, North Umpqua, 

 and Rogue rivers, and the North Fork of the Willamette River. These flows 

 did not issue from the central vents of the High Cascade volcanoes, but from 

 fissures near the feet of these volcanoes and others farther west. They accumu- 

 lated to a thickness of 1,600 feet in the ancestral canyon of the North Santiam, 

 to about 1,000 feet in the North Umpqua, and to lesser thicknesses in other 

 canyons. No doubt their eruption took place intermittently over a long span of 

 time. 



The principal eruptions of Pliocene and early Pleistocene time were 

 from vents close to the crest of the range, but later eruptions are numer- 

 ous on the eastern flank and on the adjacent plateau farther east. One 

 of the most impressive recent lava fields is around and north of Relknap 

 and Little Relknap crater. A line of cinder cones in the northern part of 

 this field betrays rise of magma along a fissure. Another recent field 

 stretches from Rachelor Rutte through Sheridan Mountain to Lookout 

 Mountain. More than 15 cinder cones and lava-scoria cones lie along a 

 fissure system here. 



A third large recent volcanic field is that around Newberry Crater 

 (N, Fig. 36.1) which is 40 miles east of the crest of the High Cascades. 

 According to Williams (1957) again: 



The Newbury volcano is an approximately circular shield volcano about 20 

 miles in basal diameter which rises 4,000 feet above the surrounding plateau 

 (Williams, 1935). On top there is a caldera, 5 miles long and 4 miles wide. 

 The oldest visible lavas of the volcano are rhyolites exposed on the walls of 

 the caldera. The rhyolites are overlain by basaltic flows and fragmental ejecta 

 and by subordinate flows of andesite, and these in turn are capped by rhyolite 



flows that aggregate 1,000 feet in thickness, forming Paulina Peak. Presumably 

 the volcano grew to its full height during the Pleistocene epoch; then its summi 

 collapsed along ring fractures, probably in consequence of drainage of tha 

 underlying reservoir either by subterranean migration of magma or, more likely 

 by copious eruptions of basalt from flank fissures. Thereafter eruptions tool- 

 place within and outside the caldera. No basaltic flows and only a few basaltic 

 cinder cones occur within the caldera, where most of the eruptions involved 

 discharge of rhyolite. Outside the caldera on the flanks of the Newberry shield 

 no less than 150 basaltic cinder cones were built and innumerable basaltic flow; 

 issued from them. 



The row of stratovolcanoes of the High Cascades is continued north-, 

 ward by Glacier Peak ( G. P. ) and Mt. Raker ( R ) which are cones built 

 on the Nevadan batholithic complex and isolated from the main volcaniq 

 complex of the Cascades. Even farther north in Rritish Columbia 40 to 123, 

 miles north of the city of Vancouver other Pleistocene volcanic cones!i 

 occur. Mount Garibaldi (G on map, Fig. 37.1) has recently been described] 

 by Mathews ( 1958 ) . There about 6 cubic miles of lava and pyroclastics 

 have been erupted in good part during the Wisconsin stage of the Pleisto- 

 cene. The extrusives are basalt and dacite; the dacite is most voluminous,, 

 Andesite in minor amounts is noted. Proceeding still farther north otheiy 

 volcanic mountains occur which are Mt. Clayley, Meager Mountain, and 

 an unnamed one at 51°00'N. Lat. 



These cones give the stratovolcanic row a length from Mount Shasta 

 on the south to Meager Mountain on the north of 750 miles. The nexj 

 known volcanic cone northward is Mt. Hoodoo, 400 miles from Meagei| 

 Mountain, but it is possible that other volcanic cones occur between; 

 which have not yet been discovered. The rows of stratovolcanoes of tha 

 South American Andes range from 650 to 900 miles long, and henceftl 

 are of the same order of magnitude as the Cascades volcanic row. 



The volcanic rocks of the older Western Cascades are classed as 

 tholeiitic by Waters ( 1955 ) , and he describes them as pyroxene andesites 

 and basaltic andesite constituting about 75 percent of the total and tho- 

 leiitic basalt, hypersthene basalt, and dacite pumice accounting for most 

 of the rest. Some olivine basalt and rhyolte are also present. 



Many of the lava flows and pyroclastic rocks contain abundant 



