THE ERUPTION OF KRAKATOA IN 1883 233 



Volcanic dust rose to a great height and, floating away, gave rise 

 to the wonderful optical phenomena noted throughout the civilized 

 world. Some of the air waves generated caused the cracking of walls 

 100 miles away, and others less violent, though of greater dimensions, 

 travelled nearly four times round the earth. The terrible explosions 

 gave rise to vast sea waves that stranded vessels, and devastated the 

 shores of Sumatra and Java, causing the death of over 36,000 human 

 beings. These explosions formed a notable feature of the eruption, 

 and were probably due to the peculiar situation of the volcano. Owing 

 to the proximity of the ocean, the water, at times, must have rushed in 

 on the molten material. This would at first cause an evolution of 

 steam and then a diminution of activity in the volcano, thus causing a 

 "check," only to be followed by an outburst or "rally" of greater 

 violence. In this way the alternations of the "check and rally" are 

 supposed to have caused the unprecedented explosions. 



The eruption continued until the 28th or 29th of August, with 

 perhaps a slight outbreak on the 10th of October of the same year. 

 With this last sign of activity the subterranean forces seem to have 

 exhausted their energy, and Krakatoa once more relapsed into a period 

 of rest. Such is a general view of the eruption. We will now examine 

 a few points a little more in detail. 



Materials Ejected 



The materials ejected from Krakatoa were lava, pumice, and vol- 

 canic dust. The lava presents two distinct forms, characterized as 

 porphyritic obsidian and porphyritic pitchstone. Both contain the 

 same crystalline elements which constitute about 10 per cent of the 

 rock; these crystalline elements are feldspar, pyroxenes, and magnetite 

 scattered irregularly throughout the ground mass. In the obsidian 

 the crystals of feldspar have perfectly sharp outlines, while in the pitch- 

 stone they are very much corroded, probably by the action of the fluid 

 material which once surrounded them. The most striking difference 

 between these two rocks is shown when they are placed in the flame 

 of a blast-jet. The pitchstone is found to decrepitate and to undergo 

 fusion with the greatest difficulty ; the obsidian, on the other hand, fuses 

 with comparative ease, and during the process forms "cauliflower-like 

 masses" that are identical with the pumice thrown from the volcano. 



