ALASKA AND THE YUKON 



621 



and andesite, with rhyolite die most abundant, and basalt practically 

 absent. They are so widespread that they must have been erupted 

 from several craters or from fissures. Mid-Tertiary intrusive granite 

 -ocks are probably later. In the younger group the acidic varieties occur, 

 but basalt is common. Some of the more basic members have inclusions 

 of ultrabasic composition. All the younger units are post-Miocene and 

 in part Quaternary. 



Farther down the Yukon in the Chandaler Valley and in the Koyukuk 

 Valley volcanic rocks believed to correlate with the older unit of the 

 Tanana region occur. These rocks are partly in the Laramide belt. 



In the lower Yukon Valley volcanic rocks undoubtedly of several ages 

 occur with the older presumably more acidic than the younger. 



Volcanics of the Laramide Oogenic Belt 



i Other than the Tertiary volcanic rock occurrences in the Yukon Valley 

 which are partly in the Laramide belt, the main eruptions have been in 

 the Seward peninsula. Three large fields are shown on the tectonic map 

 bf D. J. Miller (1959). One has an area considerably more than 1000 

 square miles. Old flows occur but the bulk of the volcanic rock is typically 

 ;Recent. The ropy surface is preserved, Quaternary gravels are covered, 

 and stream drainages blocked. The sources are not evident, and perhaps 

 ithe flows issued from fissures. The composition is basaltic (Smith, 1939). 



Aleutian volcanic belt 



i 



Kinds of Volcanoes 



A great arc of volcanoes extends from Mount Spurr on Cook Inlet 

 along the whole Alaska peninsula and the Aleutian Archipelago. See map, 

 Fig. 39.10. This arc is 1500 miles long. Unfortunately, most of the 

 Volcanoes are situated in regions of sparse population little visited by 

 outsiders, and therefore their grandeur is seldom seen. The highest stand 

 3000 to 11,000 feet above the sea and excel in beauty many of the vene- 

 rated volcanoes of better-known lands. The Wrangell volcanic field and 

 Mt. Edgecumbe extend the belt of active or recently active volcanoes 

 another 1000 miles to the east and southeast. 



Southward from the Mount Spurr group at the extreme northeastern 

 limit of southwestern Alaska, the sites of Tertiarv to Recent vok.misin 

 become increasingly evident until, at Mt. Veniaminof they include prac- 

 tically all the features of the bedrock. The lofty modern volcanoes that 

 overshadow all the other topographic features are dominant in almost 

 every landscape. 



According to Coats (1950) there are at least 76 major volcanoes, 

 active and extinct in the arc from Mt. Spurr to Buldir Island. Of these, 

 36 have been active since 1760. Seventeen calderas have been recognized. 

 These are volcanic depressions, more or less circular, and over 1 

 mile in diameter. Of the 17 calderas the three largest are Fisher on 

 Unimak Island which measures 10x11 miles, Aniakchak, 9.7x8.4 

 miles, and Veniaminof, 8.4 miles. 



A number of volcanic domes have also been recognized. As defined, 

 these are steep sided, viscous protrusions of lava forming a more or 

 less dome-shaped mass around the vents. 



The older volcanoes of the arc seem to include both shield volcanoes, 

 characterized by many relatively thin flows, with a small proportion of 

 fragmental material, accumulated on slopes of low declivity, and stratovolcanoes 

 or composite cones, made up both of flows and fragmental material, the 

 slopes of which approach the angle of repose of the fragmental material. The 

 major active volcanoes of the arc are without exception composite cones 

 (Coates, 1950). 



Petrographic Character 



Smith (1939) summarizes the general petrography as follows: 



The composition of the lavas has in the main been fairly comparable with 

 that of normal andesites, but more basic phases analogous to basalt and more 

 acidic phases approaching rhyolite are by no means unknown. 



Coates (1950) depicts them as follows: 



The volcanic rocks of the Aleutian arc range from olivine basalt to rhyolite. 

 They include basalts characterized by olivine and andesites without olivine, in 

 both of which hornblende and hypersthene occur separately or together. 

 Relatively high percentages of conspicuous calcic plagioclase crystals and 

 usually less conspicuous green augite characterize most of the rocks. Those 

 that are comparatively rich in silica, such as dacites and rhyolites, are much 



