Volcanoes 



207 



Two other factors may assist this pro- 

 cess: The shrinking of the earth, which 

 allows the molten mass to rise through 

 fissures and generate steam more rap- 

 idly, and the change of load at the 

 surface, caused by deposition or deg- 

 radation, which thus disturbs the law 

 of equilibrium. 



Of eruptions in general Professor 

 Russell says : 



' ' The cause of the rise of the molten 

 rock in a volcano is still a matter of 

 discussion. Certain geologists contend 

 that steam is the sole motive power, 

 ■while others consider that the lava is 

 forced to the surface owing to pressure 

 ■on the reservoir from which it comes. 

 The view perhaps most favorably enter- 

 tained at present in reference to the gen- 

 eral nature of volcanic eruptions is that 

 the rigid outer portion of the earth be- 

 comes fractured, owing principally to 

 movements resulting from the shrink- 

 ing of the cooling inner mass, and that 

 the intensely hot material reached by 

 the fissures, previously solid owing to 

 pressure, becomes liquid when pressure 

 is relieved, and is forced to the surface. 

 As the molten material rises, it invades 

 the water-charged rocks near the sur- 

 face and acquires steam or the gases re- 

 sulting from the decomposition of water, 

 and a new force is added, which pro- 

 duces the most conspicuous and at times 

 the most terrible phenomena accompa- 

 nying eruptions. 



' ' The recent volcanic outbreaks on 

 Martinique and St. Vincent were erup- 

 tions of the explosive type The volca- 

 noes have been dormant for years, and 

 the lava in the summit portion of their 

 conduits was cold and hard. Move- 

 ments in the earth's crust caused a fresh 

 ascent of lava from deep below the sur- 

 face, the molten material came in con- 

 tact with water in the rocks it invaded, 

 and steam explosions resulted. 



' ' These explosions were similar to 

 what would happen if water should be 

 poured on a mass of molten slag such 

 as comes from an iron furnace." 



SOME ERUPTIONS OF THE PAST 



Prof. William M. Davis and Mr. 

 William H. Snyder, in their excellent 

 ' ' Physical Geography ' ' ( Ginn & Co. ) , 

 give a number of interesting instances 

 of volcanic eruptions. I quote the fol- 

 lowing : 



"Monte Nuovo (New Mountain) is 

 a small volcano that was formed on the 

 north side of the Gulf of Naples, in 

 Italy, in 153S. Earthquakes occurred 

 thereabouts for two years before the 

 eruption, when in a week's time a cone 

 was built up 440 feet high, half a mile 

 in diameter at the base, and with a 

 crater over 400 feet deep. Masses of 

 lava ' as large as an ox ' were shot 

 into the air by the bursting of great 

 bubbles of gas or steam that ascended 

 through the lava in the vent. Finer 

 ashes fell over the country for several 

 miles around. The people of the neigh- 

 boring villages fled in terror from their 

 homes. 



' 'A great eruption took place in Mex- 

 ico in 1750, when the volcano Jorullo 

 (pronounced Ho-rul-yo) was built on 

 the central plateau, burying fertile fields 

 of sugar cane and indigo. The outburst 

 was preceded by earthquakes ; the erup- 

 tion continued half a year, building six 

 cones and pouring out extensive lava 

 flows. The highest cone, Jorullo, rose 

 700 feet above the plateau. The flows 

 retained a perceptible heat for over 20 

 years. 



" Many examples might be given of 

 marine eruptions. In 1867 a shoal was 

 discovered among the Tonga Islands of 

 the Pacific (latitude 20 20' south, lon- 

 gitude 175 20' west), the surrounding 

 sea floor being about 1 ,000 fathomsdeep. 

 In 1877 smoke was seen ascending from 

 the sea surface over the shoals. In Oc- 

 tober, 1885, an island had been formed 

 two miles long and 200 feet high. At 

 this time a terrific eruption was in 

 progress, enormous clouds of constantly 

 changing form rising over the island. 

 The shocks of the explosion were felt 



