One of the greatest explosions on record was that of Katmai volcano, several thousand feet high, on the coast of Alaska, in June, 1912. Not only was the top of the mountain completely blown off, but also a great crater pit, three miles wide across the top and several thousand feet deep, was developed in the stump of the former mountain. Volcanic dust fell to a depth of several feet within twenty-five to fifty miles of the mountain. Dust accumulated to a depth of nearly a foot in the village of Kodiak, 100 miles east of the mountain, where total darkness prevailed for more than two days. A lake of very hot water now occupies the bottom of the great new crater. The noise of the explosion was heard for at least 750 miles.
Fig. 18.—Diagrammatic vertical or structure section through a portion of the earth illustrating the common modes of occurrence of igneous rocks. P, deep-seated (plutonic) igneous rock; S, strata; D, dikes; M, mass of igneous rock forced between strata bending them upward; F, feeding channel of volcano; V, volcano; L, lava sheets. (By the author.)
One of the most frightful volcanic catastrophes of recent years was the eruption of Mont Pelée, island of Martinique, West Indies, in 1902. In this case, also, no lava was poured out, but violent explosions sent great clouds of very highly heated gases and vapors, mingled with incandescent dust, thousands of feet into the air. One of these great clouds rushed down the mountain at hurricane speed and destroyed the city of St. Pierre with its 30,000 inhabitants. After the main eruption a spine or core of hard rock began to rise out of the crater and it slowly grew to a height of 1,000 feet in several months, after which it began to crumble away. This spine probably represented nearly frozen lava which solidified as it was gradually forced out of the mountain.
Of special interest to us, though not of great importance is the only active volcano in the United States. In May, 1914, Mount Lassen (or Lassen Peak), a long inactive volcano in northern California, suddenly burst forth explosively and during the next several years hundreds of eruptions occurred. Little or no lava appeared, but great clouds of steam and dust often shot into the air from one to three miles above the top of the mountain, which lies over 10,000 feet above sea level. ([Plate 10.]) Great quantities of dust have accumulated for miles around the mountain. At this writing (October, 1920) the mountain is again active.
It should not be presumed, however, that all, or nearly all, volcanoes are of the explosive type. Others of the more quiet type are well exemplified by the two great Hawaiian volcanoes, Mauna Loa and Kilauea. Any but relatively very minor explosions rarely, if ever, occur, the product of such volcanoes being almost wholly lava, which flows down the mountainsides in molten streams. The Hawaiian Islands have, in fact, been almost entirely built up by successive eruptions of lava, the building-up process having begun well below sea level. Mauna Loa rises to nearly 14,000 feet above the sea, but, due to the fact that the streams of lava have spread so far, the mountain has an exceptionally low angle of slope which makes it difficult to realize that it is so high. Considering its submarine portion, Mauna Loa really rises nearly 30,000 feet above the sea floor. Although Kilauea lies nearly 4,000 feet above sea level on the flank of Mauna Loa, and only twenty miles distant from it, the two volcanoes are singularly independent in regard to their eruptions. Each mountain has a crater irregularly oval in shape, nearly three miles long, bounded by almost vertical walls of hard lava, in some cases arranged in terraces. The floors of the great crater pits are relatively level, and consist of black lava in which are lakes of molten and even boiling lava. The black lava floor is, in each case, only a frozen or hardened crust upon a great column of molten lava extending down into the mountain. Prior to an eruption of Mauna Loa the lava column rises hundreds of feet in the crater, but during recent years the lava seldom, if ever, flows out over the crater rim. Instead, it breaks through the mountainsides at various altitudes, the great flow of 1919 having started at an altitude of about 8,000 feet. This stream of liquid rock, fully one-half of a mile wide, flowed for weeks down the mountainside and into the ocean, the waters of which, in contact with the highly heated lava, were thrown into terrific commotion. In 1885 a stream of lava several miles wide flowed forty-five miles. In one case, lava traveled the first fifteen miles in two hours, but this is an unusually great rate of speed. Lava streams in general seldom move faster than one or two miles per hour, and as the liquid rock gradually cools and becomes more and more viscous, the speed diminishes to zero. Almost incredible volumes of steam emanate from streams of molten lava.
In 1840 an outflow of lava took place from the side of Kilauea Mountain and ran into the sea. Since that time the floor of the great crater pit (quoting Professor W. H. Hobbs) “has been essentially a movable platform of frozen lava of unknown and doubtless variable thickness which has risen and descended (hundreds of feet) like the floor of an elevator car between its guiding ways. The floor has, however, never been complete, for one or more open lakes are always to be seen, that of Halemaumau, located near the southwestern margin, having been much the most persistent. Within the open lakes the boiling lava is apparently white hot at a depth of but a few inches below the surface, and in the overturnings of the mass these hotter portions are brought to the surface and appear as white streaks marking the redder surface portions. From time to time the surface freezes over, the cracks open and erupt at favored points along the fissures, sending up jets and fountains of lava, the material of which falls in pasty fragments that build up driblet cones. Small fluid clots are shot out, carrying threadlike lines of lava glass behind them, the well-known “Pelée’s hair.” Sometimes the open lakes build up congealed walls, rising above the general level of the pit, and from their rim the lava spills over in cascades to spread out upon the frozen floor.”
In some regions, like the Columbian Plateau of the northwestern United States and the Deccan of India, each covering about 200,000 square miles, vast quantities of lava have been poured out layer upon layer to depths of even thousands of feet. Distinct volcanic cones or mountains in those regions are either absent or too scarce to look to as sources of so much lava. Such lava floods were probably mostly erupted from great fissures in the earth’s crust, the fluidity to spread many miles.