VOLCANOES AND IGNEOUS INTRUSIONS 



297 



ash applied to this fine material is misleading, since dust and ash are 

 not the result of combustion, as the name seems to imply, but of the 

 shattering of the rock or lava by explosions, the pulverization of 

 lava by sudden cooling after it is hurled into the air, and the col- 

 lisions between stones as they are hurled from the crater or as they 

 fall back to the ground. No part of the work of volcanoes has a 

 greater geological importance than the production of dust. Some 

 of it is so fine that no watchcase is so closely fitted as to prevent its 

 entrance. Near the vents it is sometimes scores of feet thick, and 

 in regions several hundred miles away it is 

 sometimes deposited to a depth of several 

 inches. For example, in 1783 the dust from an 

 Icelandic volcano was carried to Scotland, a 

 distance of 600 miles, in sufficient quantity to 

 destroy the crops. 



The larger particles are termed cinders and 

 often- constitute the conspicuous deposits of the 

 volcanic cone, the fine dust having been carried 

 away by the wind. 



When a mass of molten lava is thrown into 

 the air, it takes a more or less globular form 

 and is called a bomb. Two kinds of bombs are 

 common : one spindle or almond-shaped (Fig. 

 290), with an exterior only slightly cracked ; 

 the other with a surface cracked and broken, 

 like that of the crust of a loaf of bread. The 

 cause of the difference is to be found in the 

 degree of liquidity of the lava. The spindle- 

 shaped bombs were formed from very liquid 

 lava, and their shape was produced by their gyratory motion in the 

 air, while the " bread-crust " bomb was formed from viscous lava 

 which was little affected by the rotation, and^nmich cracked in 

 cooling, forming a glassy surface and a porous interior. Bombs 

 vary in size from a few inches to several feet in diameter. 



When the ejected lava is blown full of holes by the expansion of 

 the gas which it contains, it becomes so cellular that it is practically 

 rock froth, the air^avities being sometimes eight or nine times 

 greater than the hoping glass, so that it is light enough to float upon 

 the water. Wh^Kt is in this condition, it is called pumice. After 

 the eruptions ofWertain volcanoes situated on shores, great quanti- 



Fig. 290. — Volcanic 

 bomb, Aukland. The 

 elliptical form is acquired 

 as fragments of rapidly 

 rotating liquid lava are 

 cooled as they pass 

 through the air. (After 

 Marshall.) 



