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STRUCTURAL GEOLOGY OF NORTH AMERICA 



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Fig. 39.10. Distribution of volcanoes in Alaska. Taken from Smith, 1939. Dots are active or recently 

 active volcanoes. 



less plentiful; most of them are present either as small bodies of highly glassy 

 lava or as blankets of light-colored pumice. 



Relation of Volcanism to Structure 



In general, the volcanoes are superficial structures, built upon a basement 

 of Tertiary and older rocks that is exposed at intervals throughout the length 

 of the arc. The nature of the structures that have determined the position of 

 the eruptive centers can be determined in few places. Some writers have 

 thought that the line of volcanoes, because of its narrowness, represents the 

 trace of a great thrust plane or fault, movement on which is thought to be 

 responsible for the frequent earthquakes. In detail, the volcanic line does not 

 form a perfecdy simple arc, but consists of segments of different lengths; the 

 included angles between adjacent segments may be as little as 140°. Certain 

 volcanoes, like Bogoslof and Amak, lie some distance away from the main line, 

 on the concave side of the arc. In the Aniakchak region, Knappen mapped a 



tension fault with an east-west trend, along or close to which several volcanic 

 structures are alined; he considered that the site of the eruptive center was 

 determined by the existence of the fault. It is probable that similar relationships 

 exist elsewhere in the arc and that most of the volcanoes have had their sites 

 determined by minor tensional fractures striking at an angle to the major 

 overthrust zones. The distance of a volcano from the major active zone of 

 movement is probably dependent upon the depth at which such a tensional 

 fracture, originating in and limited to an overthrust block, taps eruptible 

 magma (Coates, 1950). 



Seismicity 



The distribution of earthquake foci (Gutenberg and Richter, 1941, 1945) is 

 such that the epicenters of shallow earthquakes tend to be south of the chain; 

 those of intermediate-depth earthquakes ( deeper than 60 kilometers ) are in the 

 islands of north of the chain. It seems probable that the general structural picture 



