LAND FORMS, THEIR DEFORMATION AND FORMATION 



59 



Figure 4.21 Subterranean and surface features associated with vulcanism. 



they range from masses a few inches thick and cover- 

 ing less than an acre to those many thousands of feet 

 thick and covering large areas. 



Laccoliths, a kind of dome mountain, are formed in 

 much the same v^ay as sills. In both cases, magma 

 comes up through a zone of weakness and is trapped 

 between rock layers; however, in a laccolith the 

 magma collects in a lens-shaped pool. 



Stocks and batholiths form from the deepest and most 

 extensive accumulations of magma; their top and bot- 

 tom margins normally are not parallel. Segregation 

 of these two land forms is rather arbitrary. Stocks 

 have an area of less than 40 square miles; batholiths 

 cover a greater area, some of them being over 100 

 miles wide and 1000 miles long. One of the largest 

 provides the substance of California's Sierra Nevada 

 Mountains. Both batholiths and sills, prior to solidi- 

 fication, may give rise to any of the overlying kinds 

 of vulcanism land forms. Therefore, the deepest in- 



trusive igneous activity is often a reservoir for more 

 superficial intrusive and extrusive flows. 



EXTRUSIVE VULCANISM 



In contrast to the rock and land form features of 

 intrusive flows, extrusive rocks either have crystals so 

 small that few, if any, can be seen, or they are non- 

 crystalline. This difference is due to the fact that 

 extrusive rocks cool much faster than intrusive ones, 

 and the faster a rock cools and hardens, the smaller 

 are its crystals. The associated land forms are always 

 surface features, namely volcanoes and related struc- 

 tures (Figure 4.22). 



The types of volcanoes and allied land forms are 

 related to the nature of the igneous activity. When 

 activity is explosive, volcanoes are in the form of cinder 

 cones, and fragmental materials are hurled from the 

 volcano. This fragmental debris consists of ovoid 



