PART II— DYNAMICS OF THE SOLID EARTH 



disturbance of the natural regime may 

 have surprising results! 



Direct damage to fruit and nut trees 

 can be reduced by shaking the ash 

 from the branches; and collapse of 

 roofs of dwellings under the weight 

 of ash can be reduced by shoveling or 

 sweeping off the ash. Much addi- 

 tional research is needed on ways to 

 reduce other damage from ash. 



Light ash falls are beneficial. The 

 ash acts as a mulch, and helps to 

 retain water in the soil for plant use 

 and to supply needed plant foods. 

 Within a few months after the erup- 

 tion, areas covered with a thin layer 

 of ash commonly look as though they 

 had been artificially fertilized. The 

 fertility probably could be further in- 

 creased by proper treatment of the 

 ash-covered ground. 



Fragmental Flows 



Glowing avalanches ("nuees ar- 

 dentes") are masses of red-hot frag- 

 ments suspended in a turbulent cloud 

 of expanding gas. The main portion 

 of the mass travels close to the 

 ground and is closely guided by 

 topography, but above it is a cloud 

 of incandescent dust that is much less 

 restricted in its spread. The ava- 

 lanches are exceedingly mobile; they 

 may travel as fast as 100 miles an 

 hour. Some glowing avalanches are 

 caused when large volumes of hot 

 debris are thrown upward nearly ver- 

 tically by explosions and then fall 

 back and rush down the slopes of 

 the volcano. This happened, for in- 

 stance, on the island of St. Vincent, 

 in the Lesser Antilles, in 1902. The 

 results were disastrous; thousands of 

 people died. The glowing avalanches 

 of Mt. Pelee, Martinique, in the same 

 year, appear to have originated from 

 low-angle blasts at the edge of a 

 steep-sided pile of viscous lava (a 

 volcanic dome) that grew in the crater 

 of the volcano. They devastated the 

 mountain slopes, destroyed the city of 

 St. Pierre, and took over 30,000 



human lives. Still other glowing ava- 

 lanches result from collapse of the 

 side of the dome after it has grown 

 beyond the crater, or from collapse 

 of thick lava flows on the slope of 

 the volcano. Those formed by col- 

 lapse of a summit dome are common 

 on Merapi Volcano, in Java. 



The association of glowing ava- 

 lanches with domes is so common 

 that any volcano on which a dome 

 is growing or has grown should be 

 suspect. Particularly where a growing 

 dome has expanded onto the outer 

 slope of the volcano, the area down- 

 slope is subject to glowing avalanches 

 and probably should be evacuated 

 until some months after the dome has 

 stopped growing and achieved appar- 

 ent stability. 



Glowing avalanches are guided by 

 existing valleys, and their courses can 

 be predicted to some extent. The 

 upper parts of big ones may override 

 topographic barriers, however. St. 

 Pierre was destroyed by the upper 

 part of a big avalanche that continued 

 over a ridge while the main mass of 

 the avalanche turned and followed a 

 valley. 



Ash flows resemble glowing ava- 

 lanches in being emulsions of hot 

 fragments in gas. They are also ex- 

 ceedingly mobile and travel distances 

 as great as 100 miles or more so 

 rapidly that, when they finally come 

 to rest, the fragments are still so hot 

 they weld themselves together. An 

 historical example occurred in the 

 Valley of Ten Thousand Smokes, 

 Alaska, in 1912. Older ones cover 

 many thousands of square miles in 

 western continental United States. A 

 fairly recent example is the Bishop 

 tuff in California. 



The great speed of glowing ava- 

 lanches and ash flows probably makes 

 effective warning impossible once 

 they have started; and their great 

 mobility and depth appears to make 

 control by means of walls unfeasible. 

 The only hope of averting future dis- 



asters seems to be in recognizing the 

 existence of conditions favorable to 

 their generation, and issuing a long- 

 range warning in advance of their 

 actual initiation. 



Mudflows are slurries of solid frag- 

 ments in water. Not all of them are 

 volcanic, but volcanic ones (lahars) 

 are common. They may be either hot 

 or cold, and they may originate in 

 various ways: by the ejection of the 

 water of a crater lake, by rapid melt- 

 ing of ice or snow, or, most com- 

 monly, by heavy rains. The water 

 mixes with loose pyroclastic or other 

 debris on the sides of the volcano 

 and the mud rushes downslope, with 

 speeds of up to 60 miles an hour, 

 sweeping up everything loose in its 

 path. In the last several centuries, 

 mudflows have probably done more 

 damage, and taken more lives, than 

 any other volcanic phenomenon. 

 They were, for instance, the principal 

 cause of damage during the 1963 

 eruption of Irazu, in Costa Rica. 



At Kelut Volcano, in Java, explosive 

 eruptions repeatedly ejected the water 

 of the crater lake, causing mudflows 

 on the flanks that took thousands of 

 lives and destroyed plantations and 

 rice paddies in the rich agricultural 

 area near the base of the volcano. 

 In 1919 alone, an area of 50 square 

 miles of arable land was buried and 

 about 5,100 persons were killed. In 

 an effort to improve the situation, 

 Dutch engineers drove a series of 

 tunnels through the flank of the vol- 

 cano and lowered the level of the 

 crater lake to the point that the vol- 

 ume of water remaining would be 

 insufficient to cause big mudflows. 

 This was effective. During the big 

 eruption of 1951 only seven persons 

 were killed, all on the upper slopes 

 of the volcano, and no damage was 

 done to the agricultural land at the 

 base. The eruption destroyed the 

 tunnel entrances, however, and they 

 were not reconstructed in time to pre- 

 vent a new disaster in 1966. A new 

 tunnel, completed in 1967, has again 

 drained the lake to a low level. As 



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