106 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



time before complete extinction. Short-lived vents, opened above 

 satellitic and therefore relatively small injections, will, of course, have 

 no such tendency to great systematic change in explosiveness. 



Lava Outfloiv at Central Vents. 



A noteworthy feature of all central eruptions is the relatively insig- 

 nificant size of their individual lava flows. Thoroddsen has estimated 

 the volume of the celebrated fissure eruption of Skaptar Jokull in 

 Iceland at 12,3-2() millions of cubic meters. He gives the volume of 

 one prehistoric flow as 43,160 millions of cubic meters; of a second, 

 500,000 millions of cubic meters. In striking contrast are the follow- 

 ing examples of the larger recorded flows at volcanic cones. 



Geological investigation shows that the flows from the central vents 

 of Paleozoic and later periods have been of the same order of magni- 

 tude as the flows of the human period. With very few exceptions or 

 none at all, these larger flows have issued from lateral fissures in the 

 cones, and a large part of the volume of each flow is readily explained 

 as the lava drained out, hydrostatically, from the upper part of each 

 conduit. Without recorded exception all overflows at the main cra- 

 ters are incomparably smaller than those noted in the foregoing table. 

 Therefore, the ascensive force in central conduits is either slight, or, if 

 powerful, is applied for short periods. 



The smallness of individual overflows clearly suggests that the 

 magma chambers >vhich continue to feed central vents are very sel- 

 dom deformed by important movements of the earth's crust. If the 

 magma in the chamber were diastrophically pinched, we should expect, 

 at times, relatively enormous lava-floods from central vents. Some 

 authors hold, on the contrary, that the growth of a great cone sometimes 

 occasions subsidence, so that crustal movement may be a consequence 

 rather than a direct cause of lava overflow at central vents. 



