The year 1902 was made memorable by a series of excessively violent explosive eruptions, in some instances accompanied by frightful destruction and loss of life, in Central America and the Lesser Antilles. In Nicaragua there was an unimportant eruption of Masaya (June 25), and Isalco in Salvador, after a pause of more than a year, began erupting on May 10, but this eruption was not of the explosive type and produced streams of lava. Far more violent was the outbreak of Santa Maria in Guatemala, a volcano which had been regarded as extinct. The eruptions began October 24 and were repeated, with diminishing energy, for more than a year, and were accompanied by an incredible quantity of ashes, which covered several hundred thousand square miles and, for a long distance around the vent, destroyed a great amount of property.
Fig. 12. - Crater Lake, Oregon. The small island is a cone of eruption, built up after the formation of the crater ring.
Fig. 13. - Gorge 200 feet deep filled by ash from La Soufriere, St. Vincent, eruption of 1902. (E. O. Hovey, courtesy of the American Museum of Natural History).
The islands of St. Vincent and Martinique in the Lesser Antilles were devastated by a series of fearful and nearly simultaneous eruptions, which in certain important respects differ from those of any other known volcanoes. The volcano of St. Vincent, known as La Soufriere (the last violent eruption of which had been in 1812), began to show signs of activity in February, 1901, by a succession of earthquakes, which were repeated, with longer or shorter intervals, until April, 1902, in the latter part of which they increased in number and violence. The actual outbreak began on May 6, 1902, in a series of tremendous steam explosions; May 7 the eruption became continuous and on the same day occurred the dreadful, descending " hot blast," a cloud of superheated steam and other gases, mingled with red-hot particles of ash, which rushed down the mountain and destroyed 1400 human lives. The eruptions, which were repeated at varying intervals and with different degrees of violence for considerably more than a year, were characterized by the absence of lava and by the vast quantity of finely divided ash ejected by the explosions.
Fig. 14. - Spine of Mt. Pelee. (Photograph by Heilprin).
The eruptions of Mont Pelee in Martinique were actually less violent, but far more destructive to life than those of St, Vincent. The previous outbreaks of Mont Pelee within historic times had been those of 1792 and 1851, both of which occurred with the same suddenness as the awful catastrophe of 1902. In the latter year slight earthquakes were noted on April 23, and on the 25th a heavy cloud of "smoke" appeared over the volcano. On May 2 the ejections of ash became heavier and more frequent, increasing until the 8th, when a descending cloud of hot vapours and glowing ash swept with terrible velocity down the ravine of the Riviere Blanche upon the city of St. Pierre, which, together with its 30,000 inhabitants, was instantly annihilated. The velocity of the air set in motion by the descending cloud was sufficiently great to hurl from its pedestal the great iron statue of Notre Dame de la Garde, weighing several tons, to a distance of more than 40 feet.
Mont Pelee had a long succession of subsequent eruptions of varying violence, especially on May 19, 20, and 25, June 6, July 9 and 13, August 25, 28, and 30, September 3, 1902; January 25, March 26, September 12 and 16, 1903; the last hardly less violent than the first terrible outbreak of May 8, 1902.
It is the descending clouds which lend such an exceptional character to the eruptions of St. Vincent and Martinique, but Mont Pelee also displayed certain other peculiar phenomena. While no lava streams were produced, very stiff and viscous lava appeared at the summit, filling up the old crater and forming a steep cone, through which protruded a lofty obelisk or spine, which, thrust up from below, grew irregularly in height, as it continually lost material by scaling off of the top and sides; eventually it fell altogether.
In all eruptions of the explosive kind, a few typical examples of which are described above, the active agency is obviously exploding masses of intensely heated and compressed steam, and all such eruptions are accompanied by gigantic steam-clouds, which, condensing in the atmosphere, fall in rains of torrential volume and violence. The hot water thus produced mingles with the volcanic ash in the air and on the ground, forming streams of hot mud, which are often more destructive than the lava flows themselves. When cold, the mud sets into quite a firm rock, called tuff.
Fig. 15. - Crater-floor of Kilauea, showing the lava lake, Hale-mau-mau. (Photograph by Libbey).
The opposite extreme of volcanic activity from the explosive type is to be found in the volcanoes of the Sandwich Islands, such as Mauna Loa and Kilauea. Here the eruptions are usually not heralded by earthquakes; the lava is remarkably fluid and simply wells up over the sides of the crater, pouring down the sides of the mountain in streams which flow for many miles. More commonly the walls of the crater are unable to withstand the enormous pressure of the lava column, and the molten mass breaks through at some level below the crater, rising through the fissure in giant fountains, sometimes 1000 feet high. Even in the ordinary activity of Kilauea jets of 30 and 40 feet in height are thrown up. Hardly any ashes or other fragmental products are formed; and though the clouds of steam, the invariable accompaniments of volcanic outbursts, are present, yet the quantity of steam is relatively less than in those volcanoes in which explosions occur.
Between such extremes as the Hawaiian volcanoes on the one hand, and the explosive East Indian type (Krakatoa), on the other, we may find every intermediate gradation. The comparatively gentle operations of Stromboli, one of the Lipari Islands, northwest of Sicily, give an opportunity to observe directly the essential phenomena of a volcanic eruption. Though occasionally breaking out with violence, Stromboli has been in a state of almost continuous activity for more than 2000 years, and is, for long periods, in such exact equilibrium, that barometric changes have a marked effect upon its activity and the Mediterranean sailors make use of it as a weather signal.
The crater-floor is formed by hardened lava, the cracks in which glow at night from the heat of the molten mass below, and which is perforated by various openings. From some of these steam is given out, from others molten lava wells up occasionally. In openings of a third class the lava may be seen rising and sinking, until a great bubble forms on its surface and bursts with a loud roar, scattering the hardened lava scum about the crater in fragments of various sizes, some very fine, others coarse. The bubble is of steam, and when set free, the steam globule rises to join the cloud which always overhangs the mountain. The bursting of the bubble is followed by a rush of steam through the mass of the lava, the pressure is relieved, and the lava column sinks down out of sight, until the steam pressure again accumulates and the performance is repeated.
Evidently, one active agent in these phenomena is imprisoned steam in its struggles to escape. Different as are the manifestations at other volcanoes, steam is an important cause of the eruption in all cases, though the conditions under which it acts vary widely. Little or no combustion is involved, and that not as a cause, but as an effect of the activity.
In the modern eruptions of Vesuvius essentially the same phenomena may be observed, but on a far grander and more terrible scale. Earthquakes usually announce the coming eruption, increasing in force until the outbreak occurs. Terrific explosions blow out fragments of all sizes, from great blocks to the finest and most impalpable dust. The finer fragments arise chiefly from the scattering of the partly hardened lava by the force of the explosion, but in part also from the crashing together of the blocks as they rise and fall through the air. Inconceivable quantities of steam are given off with a loud roar, which is awe-inspiring in its great and steady volume. The condensation of such masses of vapour produces torrents of rain, which, mingling with the "ashes" and dust, gives rise to streams of hot mud that flow for long distances. Great floods of molten rock, or lava, issue from the crater, or burst their way through the walls of the cone, and pour down the mountain side, until they gradually stiffen by cooling.
Fig. 16. - Crater of Vesuvius in moderate eruption.
During historic times Vesuvius has had long periods of dormancy and the violence of the subsequent outbreak has been, in a general way, proportionate to the length of the dormant period, though one of the most notable eruptions, that of 1906, occurred after quite a short period of rest. With the exception of a moderate outbreak in 1500, the mountain was quiet between 1139 and 1631; one of the three most violent recorded eruptions took place in the latter year.
Radically different as the various types of volcanic activity appear to be, they are all connected together in one continuous series. In all cases, steam of very high temperature and under enormous pressures is an important agent, while the differing results are due to varying degrees of pressure, quantity of imprisoned steam, amount of resistance to be overcome, the character of the lava, and similar factors. The intermediate, or Vesuvian, type of eruption is the most frequent.
Several instances of submarine eruptions have been actually observed, and there is much reason to believe that the number of vents on the ocean-floor is very large. Volcanic islands are merely submarine volcanoes which have built their cones above sea-level and these represent a great proportion of the vents now active. The durability of volcanic islands depends upon the materials of which they are constructed. Cones built of loose masses, or of ash and tuff, are speedily destroyed by the sea when the activity ceases, and cut down into reefs and shoals, while masses of solidified lava resist destruction for very long periods.