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STRUCTURAL GEOLOGY OF NORTH AMERICA 



above its normal value by conduction with each fresh overturn of the con- 

 vection cell. This should correlate with intermittent magma generation, 

 and possibly the development of the 7.5-kilometer-per-second seismic 

 velocity layer (Chapter 31). 



Heat generated by crustal deformation has been held very significant 

 by some, and the epigram "Diastrophism is the mother of volcanism" is 

 commonly recited; yet widespread, and in places voluminous, magmatic 

 activity has occurred in regions of crustal stability. As concluded in the 

 chapters on the Rocky Mountains, igneous activity may be the cause of 

 the diastrophism. There must be a real tie between the batholiths of the 

 Nevadan belt and crustal deformation, and also between the later 

 andesitic-basalt eruptions and diastrophism. In reference to the great 

 mantle fault, shown in Fig. 38.3. Benioff suggests that considerable 

 heat is generated in the aftershocks which are a manifestation of creep 

 strain in the rocks, and that this heat may be sufficient for the apparently 

 related volcanic activity. He says, 



A rough idea of the magnitude of energy released, say per year, by the 

 aftershock sequences in a region on one side of the fault can be obtained by 

 taking one fourth of the energy released in the same time by seismic waves 

 in the principal earthquakes. Thus, in the case of South America, the shallow 

 and intermediate earthquake sequences each liberate approximately 4 x 10 21 

 ergs per year. Thus roughly 10 J4 ergs per year is being released in the fault 

 blocks. The writer has no knowledge of the amount of energy per year 

 required to maintain the South American system of volcanoes, and consequently 

 it is not possible to say whether or not the energy requirements are met on 

 this hypothesis. Moreover there must be a large time lag between the liberation 

 of heat in the depths and its appearance in the form of volcano output. Thus 

 the present rate of volcanic energy release should be equated to a phase of 

 seismic-heat generation which occurred long ago, rather than to the present 

 rate. 



The problem of the origin of magma is not one of quantity of energy 

 according to Turner and Verhoogen (1951): 



. . . radiogenic heat in the earth seems to be ample to account for all geologic 

 (including igneous) phenomena, but what must still be sought is some process 

 which will concentrate this energy locally, and raise the temperature sufficiendy, 

 at points of concentration. Igneous activity itself testifies to the operation of 

 some such process. But its precise nature remains an unsolved problem. 



Primary Magmas 



Definition. Primary magma, by definition, originates by partial or 

 complete fusion in great volume of pre-existing rock. It is conceivable 

 that some igneous bodies have come from a primitive liquid still existing 

 from an early stage in the earth's history but no satisfactory evidence for 

 such has been recognized (Turner and Verhoogen, 1951). The modifica- 

 tion of a primary magma results in derivative magmas. 



Criteria by which a primary magma may be recognized as such are somewhat 

 vague. Probably the most satisfactory is a pronounced tendency for the 

 magma to appear repeatedly throughout geologic time, in great quantities and 

 in extensive individual bodies (lava floods, batholiths, lopolithic sheets, etc.), 

 over large sectors of the earth's crust. A further criterion is predominance of 

 corresponding rocks within one or more rock associations, the other members of 

 which could have been derived from the primary magma by accepted modifying 

 processes — differentiation, assimilative processes, etc. 



Conversely there is a tendency to regard magmas as belonging to the 

 derivative class when they occur habitually in small quantities, when they 

 are constantly found in association with a magma conventionally considered as 

 primary, and when derivation from the latter can be explained in terms of 

 accepted modifying processes (Turner and Verhoogen, 1951). 



Classification. There is general agreement that two broad primary 

 magma families exist, namely, granitic and basaltic. By granitic is meant 

 the common associates, granodiorite, quartz monzonite, and granite, and 

 perhaps tonalite, diorite, and others closely akin which in places occur in 

 great volume. Extrusive andesite is regarded by Waters (1955) as a pri- 

 mary magma, but this is questionable. Its relation to the granitic group 

 will be discussed later. 



The basalt family is made up of two main varieties, namely, olivine and 

 tholeiitic. Gradational varieties are common. 



Magmas of the Alkalic Igneous Province 



Prevalence of Olivine Basalt as Primary Magma. Under a previous 

 heading in connection with Fig. 36.6 it was concluded that the exposed 

 igneous rocks of the alkalic igneous province of the western United States 

 were derived from a primary olivine basalt magma. It was also postulated 

 that the surficial intrusions and extrusions come from megasills in the sur- 



