HEAT. 291 



part of Mount Kea the extraordinary density of 3-7, for which volcanic science has as yet 

 no explanation. 



Haleakala ended its work in throwing up over the bottom of the great crater many 

 cinder-cones 500' to 900' high, and Mount Kea gasped out its life in making similar cones, 

 but as summit peaks. 



Extinction, besides being due to a dovraward withdrawal of the conduit lava, may 

 take place in consequence of the cooling of these lavas from the outside, when in contact 

 with the solid rocks. The supply conduit of Kilauea has probably been as large in cross 

 sections as the crater (page 276) ; and perhaps much larger, this being suggested by an 

 outside range of bluffs on its north and northeast side. It may be as large now ; but the 

 confinement of the recent eruptions to the basin, Halemaumau, in its southwestern part, 

 suggests the possibility that cooling has already reduced it to less than a third of its former 

 size. The reduction may, however, be restricted to the upper portion of the conduit ; in 

 which case it may regain its former size at another great eruption. At Vesuvius, the 

 modern active cone is partly surrounded by the walls of a former crater, of far wider 

 extent, called Somma ; and the relative sizes of the modern and ancient craters probably 

 indicate the amount of contraction that has taken place in the lava-conduit. Teneriffe 

 and some other large volcanic mountains have extensive amphitheaters marking the limits 

 of the ancient crater, and a cone of relatively small size within it representing the later 

 condition of the fires. 



7. Explosive eruptions. — Besides the ordinary eruptions above described, 

 there may be, in all kinds of volcanoes, true explosive eruptions. The 

 projectile action ivithin the crater in such an eruption, instead of ceasing 

 at the commencement of a discharge of the lavas, as described above, 

 becomes at once enormously increased, and projectile discharges of terrific 

 violence are produced, with destructive shakings, violent thunder storms, 

 and copious cinder-ejections over a wide reach of country. The stones 

 thrown out are often of great size. At one such eruption in 1883, that of 

 Krakatoa, an island off western Java, the finer ashes ascended 50,000 feet, 

 and are supposed to have been carried around the world, and to have 

 caused the red sunset-glows of the autumn following. The end came as 

 suddenly as the beginning. The eruption began early one morning, made 

 day into night by its gray and black cinder-ejections, and left the sky clear 

 by the close of the next day. ISTo outflow of lavas took place. Another 

 such eruption occurred in the Tarawera region. New Zealand, in 1886. 

 The eruption was of extreme violence, yet it was ended, and the ashen sky 

 cleared, in six hours. But it destroyed villages and their inhabitants, and 

 deluged with mud-eruptions the beautiful geyser terraces of the region. 



Kilauea had such an eruption in 1789 (or about that time). The borders 

 of the crater for one to two miles in breadth, especially to the south and 

 southwest, are covered with the blocks of lava (some of 100 cubic feet), 

 scoria, and ashes of the eruption, and a larger region with the finer material. 



For such explosive eruptions water in large volumes must gain suddeii 

 access to the interior of a lava-conduit, — that is, to the liquid lavas of the 

 lava-column ; for the projectile force of the abruptly generated vapors is 

 enormous, and all is quick work, as in an explosion. The stones ordinarily 

 come up from the throat of the volcano, the region of hot rocks ; and this 



