THE ERUPTION OF AIOUXT KATMAI 



143 



reservoir of molten material deep in the 

 earth's crust. The vent usually reaches 

 the surface at the summit of a mountain, 

 composed of material thrown out in ear- 

 lier eruptions and terminates above in the 

 opening known as the crater. When the 

 volcano is not in eruption this vent is 

 closed by material fallen in from above 

 and by material which was not fully ex- 

 pelled during- preceding eruptions and 

 solidified there. 



WHAT CAUSES AN ERUPTION 



An eruption is preceded by a long 

 continuing and gradually increasing ac- 

 cumulation of pressure from the reser- 

 voir of molten material. The eruption is 

 caused by this pressure becoming at last 

 sufficient to overcome the resistance of 

 the material which chokes the vent, or 

 by a sudden relief of pressure by fault- 

 ing or some other cause. The conditions 

 preceding the eruption include a gradual 

 rise of lava in the vent, accompanied by 

 an increase in the temperature of the 

 surface rocks, an increase in both tem- 

 perature and volume of the waters and 

 gases given out. and by earthquakes and 

 minor explosions. 



The initial outburst is accompanied by 

 the final clearing of the vent and break- 

 ing up and expulsion of the detritus and 

 solidified lava by which it had been 

 closed and sometimes by the destruction 

 of the mountain. This suddenly relieves 

 the underlying liquid lava of an enor- 

 mous pressure, and results in the rapid 

 giving off of the steam and other gases 

 which the lava contained. 



Hot molten lava, especially when un- 

 der pressure, has the capacity to dissolve 

 great volumes of gas. It is in the con- 

 dition of water under pressure and 

 charged with gas. The uncorking of the 

 volcano has therefore the same effect as 

 the uncorking of a bottle of any other 

 liquid charged with gas : the gas rushes 

 out, carrying part of the liquid material, 

 chiefly in the form of coarse spray. 



It is this frothing of the lava which 

 creates pumice, which is nothing but 

 lava blown full of holes and projected 

 in a liquid condition into the air. where 

 it cools. The explosion which began 

 is then continued in great force with the 

 ejection of a stream of lava-spray or 



liquid pumice, which is kept up as long 

 as the imprisoned gases last. 



This action is usually intermittent, the 

 volcano behavmg somewhat as a geyser 

 and being subject to the effects of' re- 

 peated accumulation and sudden release 

 of pressure from below. In most cases, 

 after the larger part of the gas is given 

 off. the lava flows quietly out in response 

 to the pressure back of it. 



HOW SHOWERS OF ASHES ARE FORMED 



The column of steam and lava-spray, 

 after being blown out of the crater, ex- 

 pands until it is in so rarefied a condition 

 that it floats freely in the air and is 

 known as volcanic smoke. This grad- 

 ually cools, and in so doing becomes un- 

 able to support the solid particles which 

 gradually fall in a shower of ash and 

 dust. These finer materials are com- 

 posed chiefly of the smaller particles of 

 spray as they solidified, together with 

 detrital material made up of fragments 

 broken from the walls of the crater by 

 the passing blast, and of pumice dust 

 made by the larger pieces breaking as 

 they struck each other or cracking as 

 they cooled. 



The eruption of Mount Katmai was 

 doubtless preceded by the conditions de- 

 scribed above, yet in this case, probably 

 because of the absence of near-by ob- 

 servers, none of them except the earth- 

 quakes were recorded. Earthquakes 

 were felt at Katmai for at least five days 

 prior to the eruption, while more severe 

 shocks were felt on June 4 and 5 at 

 Kanatak, Uyak, and Nushagak. These 

 places are 65, 58, and 130 miles from 

 ]\Iount Katmai to the southwest, south- 

 east, and northwest respectively. 



It seems highly probable that the vol- 

 cano began to throw out large volume^ 

 of gases on the 5th. since observers at 

 Cold Bay noted that the northern sky in 

 the direction of the volcano "looked 

 black and storming" late that night, in 

 spite of the fact that there was fair 

 weather on the coast. 



THE EXPLOSIONS THAT WERE HEARD QOf 

 MILES AWAY 



Early in the afternoon of June 6 the 

 volcano passed into a state of violent 

 eruption. There must have been fre- 



