VOLCANO 



6111 



VOLCANO 



Causes of Eruption. "Of all the natural 

 catastrophes," says an eminent geographer, 

 "the explosive eruption of a great volcano is 

 the most terrible." We cannot imagine a force 

 which will rend asunder a great mountain, hurl 

 pieces weighing tons thousands of feet in the 

 air, and scatter the fragments over hundreds of 

 square miles of territory and destroy thousands 

 of lives. Yet disasters of this kind accompany 

 almost every great volcanic eruption. In view 

 of these facts, we do not wonder that scientists 

 have studied for centuries to discover thr 

 causes of volcanic eruptions. While they are 

 not able to give reasons that are entirely con- 

 clusive in all cases, they are generally agreed 

 upon the following chief causes of volcanic 

 eruptions : 



(1) Pressure. The rocks are constantly chang- 

 ing, and these movements cause great pressure 

 over certain areas. Geologists claim that this 

 pressure and the friction caused by the move- 

 ment of the rocks upon each other are suf- 

 ficient to create a temperature that will melt 

 any kind of rock. In this way pools of molten 

 rock are formed beneath the surface. The 

 molten matter is forced into the adjoining 

 crevices and occasionally finds its way to the 

 surface and flows out as lava. 



(2) Steam. Steam is by far the most power- 

 ful cause of volcanic eruptions. We now and 

 then read of the destruction caused by the 

 explosion of a steam boiler. But if we multiply 

 the force exerted by the boiler by a million we 

 will not have a force as great as that operating 

 in the explosion of a great volcano. We know 

 that the pressure of steam increases with the 

 temperature. Water coming in contact with 

 molten rock is instantly changed to steam 

 having a higher temperature than it is possible 

 to reach in a steam boiler. As the steam con- 



s to form, the pressure increases until it 

 is sufficient to burst the walls of rock by winch 

 it is confined, and the explosion occurs with a 

 force that grinds most of the rock to powder, 

 which forms the so-called volcanic ash that 

 often falls over wide areas. Oxygen, hydrogen 

 and carbon dioxide, all of which are found in 

 considerable quantities, probably aid in the 

 explosion. 



A few volcanoes are in a constant state of 

 eruption. The most noted examples of these 

 are Mauna Loa in Hawaii, and Stromboli, 

 D called "the sailors' lighthouse/' m 

 iterranean. Mount Etna and Mount Ve- 

 suvius are types of volcanoes whose periods of 

 eruption arc followed by periods of quiet. 



Usually the longer the period between two suc- 

 cessive eruptions, the more violent the de- 

 layed eruption. Mount Pelee, on the island of 

 Martinique, whose eruption in 1902 destroyed 

 the city of Saint-Pierre and caused the loss of 

 nearly 30,000 lives, had been quiet for more 

 than fifty years and free from any violent 

 eruption for more than 150 years. An eruption 

 is usually preceded by an increase in the tem- 

 perature of the earth, the drying up of springs 

 and wells, earth tremors and occasionally s* 

 earthquake shocks, but the inhabitants of the 

 affected region seldom heed these warnings. 



Products of Eruption. Misunderstanding of 

 volcanoes has led to the use of wrong descrip- 

 tions of them. A volcano is not a "burning 

 mountain;" what appears to be fire is the re- 



THE CRATER OF VESUVI 



flection of the heated mass in the crater. 

 Neither is the cloudlike mass caused by the 

 eruption of smoke. It is formed by condensed 

 steam and minute particles of rock, falsely 

 termed ash. This ash often falls over large 

 areas and to depths varying from a mere cov- 

 ering to many feet. The city of Pompeii was 

 buried by a fall of these ashes in A. D..79. As 

 tin steam is condensed in the cool upper air, 

 showers accompany the eruption Sim*- the 

 rain brings down more or less of th< a>h. 

 these are frequently showers of mud; indeed, 

 sometimes large quantities of hot mud and 

 boiling water are thrown out. It was an ava- 

 lanche of hot mud that destroyed the inhabit- 

 ants of Saint-Pierre. Many eruptions arc 

 accompanied by a flow of molten rock which 

 destroys everything before it. 



Classes of Volcanoes, Volcanoes which are 

 frequently in a state of eruption are known as 

 active ; those that arc in eruption at long inter- 

 vals, as dormant, and those whx-h h.ive passed 

 th< stage of eruption, aa extinct. There are 



