4. VOLCANOES 



Volcanoes and Man's Environment 



In many parts of the world vol- 

 canoes are an important part of man's 

 environment. They are usually con- 

 sidered destroyers. But although vol- 

 canoes do a great deal of damage, and 

 have taken many thousands of lives 

 over the past few centuries, they are 

 benefactors in the long run. 



Volcanic regions, especially those 

 in which the surface has been covered 

 with volcanic ash, tend to be very 

 fertile. The effect is most marked 

 in tropical regions where leaching 

 rapidly removes plant nutrients from 

 the upper part of the soil; there, new 

 ash falls restore the lost materials. A 

 close correlation between population 

 density and soil type has been shown 

 in Indonesia, for example, with by 

 far the densest populations in areas 

 where very young or still active vol- 

 canoes have added ash to the soil. 

 World over, the agricultural popula- 

 tion clusters in the most fertile re- 

 gions; it is likely to do so increasingly 

 as population grows and food supplies 

 become less adequate. Yet some of 

 the most fertile areas, close to active 

 volcanoes, are the most subject to 

 volcanic destruction. Furthermore, 

 volcanoes that have been quiet for 

 centuries may still be active and may 

 erupt again. In order to continue to 

 make use of these badly needed rich 

 agricultural areas close to volcanoes, 

 we must learn to forecast volcanic 

 activity, and to deal with it when it 

 comes. 



A Brief Overview 



Volcanoes are places where molten 

 rock or gas, or usually both, issue at 

 the surface of the earth. As the 

 molten rock, known as magma, rises 

 from depth, it contains dissolved 



gases; but as the magma enters zones 

 of lesser pressure near the earth's sur- 

 face, some of the gas comes out of 

 solution and forms bubbles in the 

 liquid. The bubbles tend to escape 

 from the magma, but in order to do 

 so they must move to the upper sur- 

 face of the liquid and rupture the 

 surface. When the viscosity of the 

 magma is relatively low — as it is, 

 for example, at Kilauea Volcano, in 

 Hawaii — the bubbles escape easily; 

 but when the viscosity is high, they 

 escape less readily and accumulate in 

 the magma instead, their size and 

 pressure increasing until they are able 

 to burst their way free. This produces 

 an explosion. Thus, volcanic erup- 

 tions may consist of a relatively gentle 

 outwelling or spurting of molten rock, 

 which flows away from the vents as 

 lava flows, or of violent explosions 

 that throw shreds of the molten rock 

 or solid fragments of older rock high 

 into the air, or of any mixture of 

 the two. 



The fragments thrown out by ex- 

 plosions are known as pyroclastic 

 material. The large fragments are 

 bombs, blocks, scoria, or cinder; the 

 sand- to dust-size material is called 

 volcanic ash. Some eruptions dis- 

 charge mostly gas; and gas is given 

 off, sometimes copiously, by many 

 volcanoes between eruptions. 



The fact that a volcano has not 

 erupted for centuries does not make 

 it less dangerous. We have many ex- 

 amples of volcanoes that have been 

 dormant for hundreds of years, only 

 to return to life with catastrophic 

 eruptions. At the beginning of the 

 Christian era, Vesuvius had been quiet 

 for hundreds of years; but in a.d. 79 

 it erupted, destroying all the agricul- 

 tural land on its flanks and close to its 

 base, and the cities of Pompeii, Her- 



culaneum, and Stabia. The greatest 

 eruption of recent years, at Kam- 

 chatka in 1956, took place at a long- 

 inactive volcano that had been given 

 so little attention that it had not even 

 received a name. The name we use 

 for it today, Bezymianny, means "no 

 name." Many other examples could 

 be given, including that of Arenal, in 

 Costa Rica, in 1968. 



Within the U.S., the active vol- 

 canoes of Hawaii and Alaska are well 

 known. Familiar, too, is the line of 

 great volcanic mountains along the 

 Cascade Range, from northern Wash- 

 ington into northern California. Al- 

 though the latter are not usually con- 

 sidered to present any volcanic risk, 

 they really do. Several eruptions have 

 taken place in the Cascade Range in 

 the past 170 years, the latest at Lassen 

 Peak, California, during the years 

 1914 to 1919. Six thousand years ago 

 a tremendous eruption at the site of 

 the present Crater Lake, in Oregon, 

 covered hundreds of thousands of 

 square miles with ash and devastated 

 the area immediately around the 

 mountain. Other Cascade volcanoes 

 may behave similarly in the future. 

 Several appear to be in essentially the 

 same state as Mt. Mazama, at Crater 

 Lake, before its great eruption. 



Lava Flows 



Streams of liquid rock are lava 

 flows. Where the magma has low 

 viscosity and the supply is large, a 

 lava flow may spread for tens of 

 miles. Some flows in the Columbia 

 River lavas of Washington and Ore- 

 gon have been traced for distances of 

 more than 100 miles and over areas 

 of more than 10,000 square miles. 

 Since 1800, lava flows on the island 



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