VOLCANOES AND IGNEOUS INTRUSIONS 



337 



It assumes that there exists, at a depth estimated at 40 kilometers (about 23 miles), 

 a basaltic (basic) substratum which underlies an almost universal shell of acid rock 

 (granite, etc.). Because of the pressure of the overlying rocks, this substratum of 

 actually or potentially fused rock is so rigid as to act as a solid. It holds that all igneous 

 action is the result of the mechanical intrusion of the substratum basalt into the over- 

 lying crust. The intrusion of the magma is accomplished largely by " stoping"; that 

 is, cracks in the overlying rocks are entered by the molten mass, blocks are wedged off 

 and are ultimately absorbed in the magma. At the contact, solution takes place to 

 some extent, but this is believed to be of secondary importance to stoping. The vent 

 of the volcano or fissure for the last few hundred or thousand feet may have been 

 opened by explosions or by Assuring. Once the movement of the molten magma is 

 started, the original heat of the intrusion is maintained by chemical and exothermic 

 reactions (the heat liberated in the formation of chemical compounds). 



The explosiveness of volcanoes, according to this theory, is the result of the original 

 gases of the molten rock, as well as of the water which the magma absorbs from the 

 intruded rocks in its ascent. 

 The cause of the extinction 

 of volcanoes is shown in 

 the diagram (Fig. 332). 

 The active vent is situated 

 at the highest point of the 

 injected lava, and it is in 

 this place that the gases of 

 the lava accumulate. The 

 temperature of the lava in 

 such situations is believed 

 to be not only that of its 



primal heat but also to be 



: j 1 1 • 1 ^ IG - 33 2 - — Ideal section illustrating the abyssal m- 



increased by chemical re- jection theory The middle yent {s actiye because k 



actions and by other means or i g inates at the highest point in the injected body. The 

 connected with the pres- other vents are extinct because of the advantage of the 

 ence of the gas. Vents middle vent. The arrows show the movement of the gas; 

 become extinct when, be- the solid black, the crystallized portion of the injection, 

 cause of the higher position 



of the lava in other locations, the gases which cause the fusion accumulate elsewhere. 

 According to this theory, the composition of the lava ejected from a volcano depends 

 upon whether it is composed entirely of the basaltic lava of the substratum, or is a 

 mixture produced by the solution of the rock through which the basaltic lava has 

 passed. 



Resume of Present Knowledge of Volcanism 



There is no agreement as to the origin of lava ; (1) some investiga- 

 tors hold that a portion is derived from deeply buried sedimentary- 

 rocks which have a high temperature as a result of the rise of heat 

 from the earth's interior, so that when the pressure of the overlying 

 rocks is relieved, the more fusible strata liquefy; (2) some hold that 

 it is formed as the result of heat produced by friction between great 



