EARTHQUAKES; VOLCANOES 231 



Others began where there were fissures in rocks that were 

 rising or were already uplifted. In one case a single erup- 

 tion began and ended the life of a volcano two hundred 

 feet high, south of Sicily. The cone has since been 

 swept away by the sea and nothing but a ledge of rock 

 remains. 



An eruption is probably caused, in most cases, by the 

 great pressure of the steam formed in regions where water 

 has percolated to heated rock far below the surface. Since 

 unconfined steam occupies about 1,700 times as much space 

 as the water from which it is made, its pressure when con- 

 fined is tremendous. As it pushes its way out, it often forces 

 out solid rock above it, as well as the liquid rock which is 

 near it. Some of the lava rises into the air in a fine spray, 

 as water does from a hose, but it solidifies in the air and falls 

 as a fine dust, called volcanic ash. 



Before an eruption, the earth is often violently jarred and 

 shaken by the expanding steam, and the resulting earth- 

 quake is thus a warning of an eruption. The whole side 

 of the volcano is sometimes blown away by an especially 

 explosive eruption, as in Krakatoa, in the Straits of Sunda, 

 near Java, in 1883. 



Volcanoes, like earthquakes, are more numerous in young 

 mountain regions than in old ones, because while the folding 

 of the rocks is going on, the fractures provide openings for 

 the release of the melted rock, which is always under pres- 

 sure below. 



251. Lava Sheets and Dikes. Volcanic action does not 

 always produce a typical, cone-shaped volcano. Sheets of 

 lava may flow over a great extent of country, through a 

 fracture one hundred or more miles in length. Such old 

 lava flows are found in states west of the Rocky Moun- 

 tains, where they cover an area of 150,000 square miles in 

 Idaho, Washington, and Oregon. 



When the liquid rock cools in the fissures, it closes the 



