This photograph of Paricutin in eruption is from a series of photographs showing the birth 

 and death of this famous .Mexican volcano. The series is part of the Museum 's featured exhibit 

 for January on volcanism. The exhibit also illustrates the many different kinds of volcanic 

 action and explains their causes. A diorama reconstructing an active volcanic region sets the 

 stage for displays of actual materials ejected during an eruption, including volcanic bombs, ash, 

 and many varieties of lava. 



Located in Hall 34, the exhibit on volcanism was constructed under the direction of the late 

 Dr. Sharat K. Roy, the Museum's former Chief Curator of Geology. For many years Dr. Roy 

 was actively interested in volcanoes, particularly those in Central America. During his field 

 research he made many daring climbs to study the craters of "live" volcanoes. 



ash, these forms rise to great heights of 

 conical majesty. Many well-known vol- 

 canoes are of this type, such as Vesuvius 

 in Italy, Etna in Sicily, Cotopaxi in 

 Equador, Mount Rainier in Washing- 

 ton, and Mount Fuji in Japan. 



When the erupting lava is very vis- 

 cous, it is hardly able to flow and so 

 frequently builds up into dome-shaped 

 masses, being pushed higher and higher 

 from within. Lassen Peak, in Lassen 

 Volcanic National Park, is an example 

 of such a lava dome. Great out-pour- 

 ings of lava also take place from long 

 fissures in the earth's crust, from which 

 the molten rock spreads out to form a 

 large sheet. Iceland has experienced 

 such events in historic time; one in 1783 

 caused great destruction and famine. 

 Central volcanoes commonly form at 

 one or more points along such a fissure. 

 At various times during the earth's his- 

 tory, repeated fissure eruptions have 

 built up thick masses of basalt, one of 



the main varieties of lava, over vast 

 areas of the world. Such plateau basalts 

 are extensive in India, Brazil, South Af- 

 rica, and in the North Atlantic region, 

 where they embrace parts of Greenland, 

 Iceland, Scotland, and Ireland. A part 

 of one basalt spread forms the famous 

 Giant's Causeway on the northeastern 

 coast of Ireland. Washington and Ore- 

 gon also claim one of the world's larger 

 spreads of plateau basalt. 



Another type of sheet deposit is to be 

 found in Yellowstone Park. Here, by 

 far the greater part of the volcanic rocks 

 is composed of highly siliceous or rhyo- 

 litic material that was thrown out as 

 glowing clouds of gas and ash particles 

 in great explosive fissure eruptions. Be- 

 cause of the heat, the ash was welded 

 together to form a rock, known as ignim- 

 brite, which is very similar to a lava. 



Many volcanoes are surmounted by 

 large craters called calderas. Smaller 

 craters may be formed when the tops 



of volcanoes are destroyed by explosion; 

 larger ones, such as Crater Lake, Ore- 

 gon, which is six miles across, are formed 

 chiefly by collapse of a part of the whole 

 volcano. It has been estimated that in 

 the case of Crater Lake the volume of 

 the caldera plus the peak that must have 

 surmounted it was about 17 cubic miles, 

 of which less than two cubic miles are 

 accounted for by surrounding debris. 

 The remainder collapsed downward in- 

 to the chamber occupied by the molten 

 rock, or magma. Space for this debris 

 was provided in part by the volcanic 

 eruptions that preceded the collapse; 

 during these eruptions it is estimated 

 that some six cubic miles of magma were 

 ejected. Additional space for the unac- 

 counted-for volume was evidently made 

 available by the magma draining away 

 elsewhere within the earth. 



What Causes Eruptions? 



An obvious question about volcanoes 

 is : what causes them? We now have a 

 great deal of information about the na- 

 ture of the volcanic process, but we are 

 still far from understanding all its aspects, 

 particularly the basic cause. The ap- 

 pearance of large quantities of molten 

 rock, or magma, at the earth's surface 

 led first to the belief that beneath the 

 crust the earth was molten. Later it was 

 thought that there was a continuous 

 liquid shell beneath the crust. Study 

 of earthquake waves and other consider- 

 ations have led to the rejection of both 

 of these ideas; now the region known as 

 the mantle, which lies between the crust 

 and the earth's core 1,800 miles deep, is 

 regarded as solid. 



Toward the center of the earth, the 

 temperature increases at a rate of about 

 one degree centigrade per hundred feet, 

 but below a few miles we have only esti- 

 mates. However, it is considered that 

 the melting point is never reached in the 

 upper mantle regions because the high 

 pressure raises the temperature of melt- 

 ing. Earthquakes originate some 25 

 miles below the volcanoes of Hawaii and 

 it is likely that magma is generated at 

 this level. It seems, therefore, that extra 

 heat must be brought up from deeper 

 levels within the earth to bring about 

 partial or, less likely, complete melting 

 (Continued on next page) 



JANUARY Page 7 



