VOLCANOES AND IGNEOUS INTRUSIONS 



339 



Crater 



found in the fluxing (Fig. 333) by intensely hot gas * from deep in 

 the earth. When the plug has been formed, heat is developed be- 

 neath it by the compression of the gas, by chemical reaction, and by 

 gas solution. After the plug 

 is shortened by the melting 

 away of its lower end in this 

 intense heat, the gas pressure 

 may become great enough to 

 blow out the remaining part. 

 After the explosion, the lava 

 in the throat of the volcano 

 again cools and a period of 

 inactivity ensues. The cause 

 of extinction is discussed on 



Fig. 333. — Section of a dormant volcano, 

 showing how the lava plug may be weakened 

 by gas fluxing. The broken line shows the 

 original depth of the solid plug and the prog- 

 ress made by the gas. (Modified after Daly.) 



P- 337- 



In individual cases, as for 

 example in that of Stromboli, 

 eruptions occur when gas has 

 accumulated under the scum of lava in the crater in sufficient volume 

 to cause an eruption, after which quiet ensues, the surface of the 

 lava hardens, and the gases again begin to accumulate. 



Influences of the Atmosphere, etc. — Volcanic eruptions seem to be 

 somewhat more prevalent when (1) the atmospheric pressure is high 

 than when it is low, (2) after heavy rains rather than before, and 

 (3) when tidal strains are unusually severe. None of these causes 

 could produce an eruption, but it is probable that the increased weight 

 of the atmosphere over a large area would aid in forcing out the lava, 

 as would also the weight of the water after heavy rains. Tidal 

 strains would have a similar effect. None of these agencies could be 

 effective unless the eruption was imminent, only a slight additional 

 force being necessary to start it. 



It has long been noticed that the volcano Stromboli (p. 299) dis- 

 charges a greater quantity of steam and bombs in stormy than in 

 fine weather, and the fishermen make use of it as a " weatherglass " : 

 the increase of activity indicating a falling barometer and conse- 

 quently stormy weather; and a diminution in activity promising 

 fair weather 



1 Such gases, called primeval gases, are believed to come directly from great depths and reach 

 the surface for the first time. They are distinguished from resurgent gases which have a second- 

 ary origin, that is, those which are absorbed from the intruded rock. 



