Volcanic eruption

“ Volcanic eruptions.—In the types of eruption prevailing at the present time, the lavas are forced out through ducts or perhaps short fissures or sections of fissures, and build up cones about the vents, the eruptive action maintaining craters in the centers of the cones. The essential feature of a volcano is the issuance of hot rock and gas from a local vent. A mountain is the usual result, but the mountain is secondary and not usually present in the first stages ”

“ When conditions occur by which it can outwardly manifest itself, it follows the natural law and escapes where the crust of the earth is thinnest.
But something more than the mere presence of internal fire is necessary to account for volcanic action, although it may in a large degree account for minor seismic convulsions in the form of an earthquake. The elements which enter into the source of volcanic eruption are fire and water. The characteristic phenomenon of a volcanic eruption is the steam which issues from the crater before the appearance of the molten lava, dust, ashes and scoria. ”

“ We can only say that observations on the height of volcanic ejections are extremely desirable; they can probably only be made from a balloon. An ascension thus made beyond the cloud disk which the eruption produces might bring the observer where he could discern enough to determine the matter. Although the movements of the rocky particles could not be observed, the colour which they would give to the heavens might tell the story which we wish to know. There is evidence that large masses of stone hurled up by volcanic eruption have fallen seven miles from the base of the cone. ”

“ We can hardly conceive what would be the scale of a volcanic eruption attending upon so colossal a disturbance of the terrestrial crust. But the eruptions which have taken place within the memory of man have been the accompaniments of no such disturbance. Although they have been many in number and sometimes powerful in effect, they have seldom been attended with any marked displacement of the surrounding parts of the terrestrial crust. ”

“ The presence of lightning is also a prominent feature during volcanic eruptions. When Kattleguia, in Iceland, now extinct, was last in a state of eruption, lightning proceeded from it and killed a farmer and his servant, together with some horses and cows. We cannot, therefore, exclude the consideration of volcanoes as producers of earthquake, sometimes by direct action, at others through the medium of electric disturbance. ”

“ These shocks are, however, most commonly in evident connection with volcanoes; and it is in the neighborhood of the latter that they are especially frequent. As is well known, every volcanic eruption is announced by precursory earthquakes, the violence of which is stilled when an outlet is opened for the vapor of water which is successively the cause of the subterranean agitations and the projecting agent of all the eruptions. The tension of the vapor in the volcanic reservoirs must be very high. ”

“ The heavier particles, however, soon begin to fall, and before long the earth's surface both around the volcano, and often at considerable distances from it, is covered with a thick layer of ashes.
The sounds accompanying a volcanic eruption are often terrifying. Amid shakings and tremblings of the earth's crust, known as earthquakes, there are occasionally heard noises like the explosion of huge guns. ”

“ If, however, we desire to form some adequate idea of the part which volcanic action has played in the past history of the earth, we should be misled were we to confine our attention to the phenomena of the eruptions of the present day. An attentive examination of any modern volcano will convince us that of some of the most startling features of an eruption no enduring memorial remains. The convulsive earthquakes that accompany a great volcanic paroxysm, unless where they actually fissure the ground, leave little or no trace behind them. ”

“ Suppose, then, that a volcanic eruption takes place in a region of the earth's surface where successive layers of limestone, of clay, and of sandstone, have been previously deposited by the action of water. If such an eruption has force enough to break through these beds, the hot, melted masses will pour out through the rent, flow over its edges, and fill all the lesser cracks and fissures produced by such a disturbance. What will be the effect upon the stratified rocks? ”

“ This could hardly be possible if all volcanoes had a common source. The really essential and important part of a volcanic eruption is the escape of gases, which are or soon become largely steam. This forms the clouds which overhang a volcano, and descends in time of violent eruptions in torrential rains, such as buried Pompeii in mud. ”

“ Earthquakes, sometimes local, sometimes extending over vast areas, often precede volcanic eruptions. The subterranean movement and the eruption return again and again, at irregular intervals of time, and with unequal degrees of force, to the same spots. The action of either may continue for a few hours, or for several consecutive years. Paroxysmal convulsions are usually followed, in both cases, by long periods of tranquillity. Thermal and mineral springs are abundant in countries of earthquakes and active volcanoes. ”

“ A volcanic eruption is generally preceded by low, rumbling sounds, and trembling of the earth's surface. Then follows greater activity of the volcano, from which dense volumes of smoke and steam issue, and fire and molten lava make their appearance. ”

“ There has been much difference of opinion as to the immediate cause of volcanic action, but there can be little doubt that the energy which is manifested in a volcanic eruption has been originally derived in some way from the contraction of the primæval nebula. The extraordinary vehemence that a volcanic eruption sometimes attains may be specially illustrated by the case of the great eruption of Krakatoa. ”

“ There are strong grounds for the gravest doubts that there exists any real connection of a physical character between Volcanic Eruptions, and Earthquakes more or less approximately coincident only, in time of occurrence ”

“ Should the volcano again become active, earthquakes and volcanic explosions would trigger avalanches and mudflows that would rush down the mountain. Molten rock would melt snow and ice at the volcano’s summit and send floods of water down the volcano’s flanks. These indirect effects of an eruption would be much more hazardous than lava flows and pumice, if eruptions are on a scale similar to that of the past 10,000 years. ”

“ There are two classes of volcanoes—those which have explosive eruptions, like Vesuvius and Krakatoa, and this latest one, and those of no explosive nature, like Mauna Loa and Kilauea, in Hawaii, which boil up and flow over. It is the explosive eruption which brings widespread destruction, and it is astonishing to learn of the tremendous power one of those eruptions unleashes.”
Professor John Milne, of London, the highest authority in the world on volcanic explosions, classifies eruptions into two grades: Those that build up very slowly. Those that destroy most rapidly. ”

“ If the lava from the last eruption has consolidated as a plug in the throat of the volcano, the conduit may be practically closed, and hence the first effort of the renewed activity is to expel this obstruction. The hard mass becomes shattered by the explosions, and the angular fragments so formed are hurled forth by the out rushing stream of vapour. When the discharge is violent, the vapour, as it rushes impetuously up the volcanic duct, may tear fragments of rock from its walls and project them to a considerable distance from the vent. ”

“ The true cause of volcanic eruption, he says, is that air is driven into the pores of the earth, and when this comes into contact with lava and flint which contain atoms of fire, it creates the explosions that cause such destruction. ”

“ Though on the first formation of the volcano, masses of non-volcanic rock may be torn from the chimney or pipe of the mountain, only slightly fused externally owing to the bad conducting power of most rocks, and hurled to a distance; and though at the beginning of a subsequent eruption the solid plug of rock which has cooled at the bottom of the crater, or, in fact, any part of the volcano, may be similarly blown up, the bulk of the solid particles of which the volcano itself is composed is derived from the lake of lava or molten rock which seethes at the orifice. ”

“ They tear off great masses of stone, which the resultant energy disturbances, pressure, gas, and friction, redden with heat. The mixture of gases from the decomposed water is in large amount, is burning and exploding, and in this fiery furnace amid such convulsions as have been described, the adjacent earth substance is fused, and even clay is melted, and carried on with the fiery blast. Finally the current reaches the earth's surface through the funnel passage, the apex of which is a volcano—the blast described a volcanic eruption. ”

“ In fact, when a volcanic outburst has forced a convenient passage for the vapor, the exit of liquid rock seems of secondary importance, for generally the terrific explosions, earthquakes, and subterranean thunder that accompany the first stage of eruption are more or less absent, or at least much diminished during the welling up of the fluid rock. If, as in the latter case, a cone of some considerable size has been formed, the lava will rise and occupy the whole of the crater-cavity. ”

“ It is dangerous to form conclusions as to the general course of volcanic action anywhere, for science is very much in the dark as to the causes of eruptions and earthquakes, as to the condition of the interior of our globe, whether fluid or solid, and also as to whether the lavas poured out during an eruption have been fluid since the earth was formed, or have been suddenly melted either as cause or effect of what we call an eruption. ”

“ It now appears that liquid lava is a gas froth, that Kilauea and Mauna Loa are all one system, that hydrogen is the most elemental gas in eruption, that a gas-free paste is the residue of flowing foam both in pits and lava flows, that earthquakes and vibrations are a function of this paste wedging up cracks and sinking back underground, and that the rise and fall is in tides and cycles, short and long. These things are not guesses, but measurements.
The earthquake problem at volcanoes is misunderstood in geology. The superstition that volcanic quakes are small is wrong. ”

“ These phenomena grew more and more alarming until August 27th, when four explosions of fearful intensity shook earth and sea and air, the third being “far the most violent and productive of the most widespread results.” It was, in fact, perhaps the most tremendous volcanic outburst, in its intensity, known in human history. It seemed to overcome the obstruction to the energy of the internal forces, for the eruption now declined, and in a day or two practically died away, though one or two comparatively insignificant outbursts took place later. ”

“ It is not difficult meanwhile to assign the very probable mechanism of those comparatively petty repercussions which are experienced in close proximity to volcanic vents when in eruption, and which, though certainly seismic in their nature, and powerful enough, as upon the flanks of Etna, to crack and fissure well-built church-towers, can scarcely be termed Earthquakes. ”

“ The third general fact is the repetitive nature of volcanic eruptions. A single outbreak with none following is an exceedingly rare phenomenon. Many eruptions, going often into the thousands, occur before the climax is reached and the decline of activity follows. The reason why a volcano, when its vent is once open, does not discharge all the material in its reservoir in one stupendous belch and then close up forever will be shortly brought up. ”

“ The powerful agency of steam or aqueous vapor in volcanic eruptions leads us to compare its power of propelling lava to the surface with that which it exerts in driving up water in the pipe of an Icelandic geyser. Various gases also, rendered liquid by pressure at great depths, may aid in causing volcanic outbursts and in fissuring and convulsing the rocks during earthquakes. ”

“ The chief effect of volcanic eruptions on the Geological structure of our Globe consists in the accumulation of cinders and molten rock, either upon the Surface of the Earth, or in the crevices and caverns that abound within its solid Crust. Sometimes, indeed, the operations of an active Volcano are accompanied by a movement of upheaval or of subsidence. ”

“ From this cloud particles of red hot ashes, cinders, sand, etc., begin to fall, the largest particles near the crater of the volcano, and the smaller particles at much greater distances. In very powerful explosive volcanic eruptions such as Krakatoa, the finer dust may be carried to practically all parts of the world. ”

“ The largest and most explosive volcanic eruptions eject tens to hundreds of cubic kilometers of magma onto the Earth’s surface. When such a large volume of magma is removed from beneath a volcano, the ground subsides or collapses into the emptied space, to form a huge depression called a caldera. ”

“ The quiet volcanic eruptions are attended by the escape of steam or gases from the molten rock, but the lava being in a highly liquid state, the steam and gases dissolved in it escape quietly and without explosions. If, however, the molten rock is less completely fluid, or in a viscous condition, the vapors and gases contained in it find difficulty in escaping, and may be retained until, becoming concentrated in large volume, they break their way to the surface, producing violent explosions. ”