of the mantle rocks to form the magma 

 below volcanoes. 



Once formed, the magma, because of 

 its buoyancy, will tend to rise towards 

 the surface, particularly if ruptures have 

 formed in the overlying rock. As the 

 magma rises, the pressure on it becomes 

 less and the dissolved gases begin to sep- 

 arate and to form bubbles, much as bub- 

 bles are formed when the top is taken off 

 of a soda-pop bottle. This makes the 

 fluid even more buoyant and serves to 

 drive it upward. The magma appar- 

 ently accumulates in a relatively shal- 

 low chamber located a few miles or less 

 beneath the volcano. Continued sepa- 

 ration of gas causes the magma to ex- 

 pand because the gas bubbles expand as 

 they move upward. Eventually, suffi- 

 cient pressures are produced to clear the 

 way for eruption to the surface. 



Very fluid magma, with relatively 

 small gas content, usually erupts quietly, 

 pouring out quantities of lava but little 

 ash. The Hawaiian volcanoes erupt in 

 this fashion. Spurts of gas-charged lava 

 are thrown periodically from the lava 

 pits in the craters thus giving rise to the 

 picturesque lava fountains. The solidi- 

 fied lava flow has two characteristic ap- 

 pearances: in one, called pahoehoe in 

 Hawaii, it is smooth and commonly 

 rope-like, containing many small ellip- 

 tical vesicles formed by gas bubbles. 

 The liquid lava in this case was hot, 

 mobile, and contained relatively little 

 gas, which escaped quietly. The other 

 kind of lava flow, known as aa, is rough, 

 blocky, and contains many large vesi- 

 cles. It solidified from a cooler but more 

 gas-rich lava, and the escape of the gases 

 caused the fragmentation and break-up 

 of the consolidating material. 



With increasing viscosity and greater 

 gas content, volcanic eruptions become 

 more explosive and spasmodic. Gas- 

 fragmented lava is discharged from the 

 craters, producing much ash and cinders 

 and volcanic bombs (clots of lava shaped 

 during fall). Lava flows may also break 

 out on the flanks of the volcano or at its 

 base. Such an eruptive pattern is called 

 Strombolian after the volcano of that 

 name. Vulcanian and Vesuvian erup- 

 tions are increasingly explosive and in 

 the strongest outburst there is a violent 

 release of tremendous quantities of gas, 

 which rush upward for thousands of feet 



Page 8 January 



and spread out like a mushroom cloud. 

 This is the Plinian type. The ultimate 

 in explosive discharge is the Pel<:an type, 

 named after the 1902 eruption of Mount 

 Pelee described above. The magma in 

 this case is very viscous, and as the gas 

 separates and expands a tremendous 

 pressure is built up beneath the volcano 

 until a weakened zone finally gives way, 

 often on the flank. The resulting sud- 

 den release causes enormous expansion 

 of the gas, and the magma froths up and 

 blows out with great force as a dense in- 

 candescent cloud of gas, crystals, and 

 tiny soft glass fragments. 



Other Volcanic 

 Phenomena 



Volcanoes often show a cyclical activ- 

 ity. Outbursts that become stronger 

 and more frequent culminate in a par- 

 oxysmal explosion followed by dwin- 

 dling eruptions or quiescence. During 

 dormancy or the final decline of a vol- 

 canic area, discharges of gas may be the 

 only signs of activity. Hot springs and 

 geysers (a term derived from an Ice- 

 landic name) also indicate the dying 

 stages of volcanism. 



The world-famous geysers of Yellow- 

 stone National Park owe their occur- 

 rence to a large cooling mass of igneous 

 rock which lies within the crust below 

 this once very active volcanic area. Most 

 of the steam and hot water is derived 

 from circulating ground water which is 

 heated by the hot gases (mainly water 

 vapor) given off from below. The spas- 

 modic behavior of a geyser arises from 

 the heating, by superheated steam, of 

 water contained in an underground 

 chamber. The weight of the water col- 

 umn elevates the boiling point so that 

 the deeper water becomes extremely hot. 

 Eventually it boils and pushes up the 

 overlying water, some of which, in con- 

 sequence of the reduced pressure, imme- 

 diately turns to steam. This great ex- 

 pansion forces the steam to the surface 

 together with the remaining hot water, 

 which is thrown high into the air until 

 the supply of water is exhausted. A new 

 cycle begins with the filling, again, of 

 the underground chamber with ground 

 water. 



Sources of underground steam are be- 

 ing utilized for energy production in 



Larderello, Italy, and Wairakei, New 

 Zealand, by boring into the ground. In 

 the future such geothermal energy will 

 also be obtained in California and 

 Kamchatka. 



Although there are many active volca- 

 noes in the world, there are many more 

 extinct ones which have not erupted in 

 historic times. A famous extinct vol- 

 canic area is in the Auvergne region of 

 central France. It was here that the 

 volcanic origin of certain kinds of rocks 

 and structures was first demonstrated in 

 the second half of the 18th century. This 

 led to the rejection of the idea that ba- 

 salt was a rock formed by precipitation 

 in the oceans. 



Many other areas, now so eroded that 

 the deep structures underlying volca- 

 noes are exposed, give witness to the 

 constancy of volcanism somewhere in 

 the world throughout geological time. 

 The study of these regions has not only 

 helped to elucidate the structures of vol- 

 canoes, but also has revealed how the 

 magma finds its way to the surface, the 

 nature of the variations of the lavas pro- 

 duced, and their relationships to other 

 internal processes of the earth such as 

 mountain-building. 



Large populations have settled near 

 both active and dormant volcanoes, one 

 of the reasons being that the breakdown 

 of volcanic lava and ash produces an ex- 

 ceedingly fertile soil which supports in- 

 tensive agriculture. Active centers for 

 the study of volcanism are maintained in 

 Japan, Hawaii, Italy, and Kamchatka, 

 with lesser ones elsewhere. More are 

 needed to keep the active volcanoes un- 

 der observation, particularly in Indo- 

 nesia and Latin America. Eruptions 

 that might cause loss of life can now be 

 predicted by several methods, including 

 the recording of earthquakes on seismo- 

 graphs, observing the tilt of the ground 

 as the magma swells in the chamber be- 

 low a volcano, and observing the tem- 

 perature and types of gas emitted. 

 Although the eruptions cannot be pre- 

 vented lives can be saved, and in some 

 favorable cases it may even be possible 

 to divert lava streams to less destructive 

 paths if advance warning is available. 

 Thus as our knowledge of volcanoes in- 

 creases, men become more able to live 

 with and survive the depredations of vol- 

 canism, earth's fiery activity. 



