Volcanism

“ We have already seen that when an explosive volcanic eruption occurs there is suddenly thrown out of the crater of the volcano a huge column of various substances [Pg 181] that rises sometimes as high as 30,000 feet or even more. The smaller fragments of lava are quickly cooled and form volcanic ashes, sand, cinders, or dust. These are rapidly spread out by the wind in the form of a black cloud, that not only covers the mountain but reaches out over the surrounding country, completely shutting off the light of the sun. ”

“ Black blobs of shadow become distinct craters; volcanoes rose slowly to meet them, to drift aside and rise above as they sank to the floor of a valley. They came to rest upon a rocky floor.
On all sides their windows showed a waste of torn and twisted rock. Volcanic mountains towered to the heights, their sides streaked with masses of lava, frozen to stillness these countless years from its molten state. The rising sun, its movement imperceptible, cast long slanting rays between the peaks. It lighted a ghostly world, white with thick hoar-frost of solid carbon dioxide. ”

“ Sometimes these fires burst forth from the crater of the volcano with tremendous force. The smoke becomes thick and black, and lurid flames shoot up to a height of hundreds of feet, making a scene of amazing grandeur.
With the flames there are thrown out stones, ashes, and streams of melted rock, called lava. This lava flows down the sides of the mountain, and, being red-hot, destroys every thing with which it comes in contact. At such times, a volcano is said to be in eruption. ”

“ Now and then, by the explosions from a volcano at the present day, a single stone may be ejected at such an angle and with such force as to fall to the ground at a long distance from the vent. In like manner, among the volcanic records of former periods, we may occasionally come upon a single block of lava imbedded among tuffs or even in non-volcanic strata. Where such a stone has fallen upon soft sediment, it can be seen to have sunk into it, pressing down the layers beneath it, and having the subsequently deposited layers heaped over it. ”

“ The mountains composed of volcanic materials, but which have lost through denudation the external form of volcanoes, are still more numerous, and the smaller temporary openings which are usually subordinate to the habitual vents that have been active during the periods covered by history and tradition, must be numbered by thousands. There are still feebler manifestations of the volcanic forces—such as steam-jets, geysers, thermal and mineral waters, spouting saline and muddy springs, and mud volcanoes—that may be reckoned by millions. ”

“ In the basal wrecks of volcanoes, of which we have spoken, we usually find only small and fragmentary remains of the great accumulations of loose and scoriaceous materials which originally constituted the bulk of the mountain mass. In the centre of the ground-plan - 142 - of such a denuded volcano we find great masses of highly crystalline or granitic rock, which evidently occupy vast fissures broken through the sedimentary or other rocks upon which the volcanic pile has been reared. ”

“ VOLCANIC NECKS. The pipe of a volcano rises from far below the base of the cone,—from the deep reservoir from which its eruptions are supplied. When the volcano has become extinct this great tube remains filled with hardened lava. It forms a cylindrical core of solid rock, except for some distance below the ancient crater, where it may contain a mass of fragments which had fallen back into the chimney after being hurled into the air. ”

“ And a most wise arrangement of the Supreme Intelligence it is, which has left open to King Pluto these ready means of forcing an outlet; and man ought to feel rather thankful when he beholds the flaming head of the Fire King towering above the crater of some volcano. Earthquakes surely are much more terrible and destructive than volcanic eruptions.
A volcano may be defined as a perpendicular tunnel in the earth’s crust, through which heated matter from below is thrown up to the surface. The matter thrown up may be in the form of lava, scoriæ, ashes, mud, &c. ”

“ 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. ”

“ Our usual impression of a volcano is indicated in the title of “burning mountain,” so often employed, a great fire-spouting cone of volcanic debris, from which steam, lava, rock-masses, cinder-like fragments, and dust, often of extreme fineness, are flung high into the air or flow in river-like torrents of molten rock. ”

“ On its shore are evidences of volcanic action, which must from the earliest period have aroused wonder and fear, and stimulated the myth-making tendency to account for them. On the eastern side are impressive mountain masses which have been thrown up from old volcanic vents; mineral and hot springs abound, some of them spreading sulphurous odours; earthquakes have been frequent, and from time to time these have cast up masses of bitumen; concretions of sulphur and large formations of salt constantly appear. ”

“ The renewal of action in an old volcano, after a long period of repose, may be looked upon as exhibiting, in a considerable degree, the phenomena which accompany the first origin of a volcanic vent. Earthquakes, subterranean noises, the bursting forth of new springs, and the suppression of old sources, are symptoms of a particular kind of subterraneous disturbance, of which they record the violence, and in some degree moderate the effects. Volcanic forces are in action wherever such phenomena appear ”

“ Mountains are thereby forming by upheavals from beneath, and the corresponding surface valleys are consequently filling up, either by the slow deposit of the matter from the saline water of hot springs, or by the sudden eruption of a new or presumably extinct volcano."
"What would happen if a crevice in the bottom of the ocean should conduct the waters of the ocean into a deposit of metallic bases?"
"That often occurs," was the reply; "a volcanic wave results, and a volcano may thus rise from the ocean's depths."
"Is there any danger to the earth itself? ”

“ 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. ”

“ On the other hand, the similarity of volcanic lavas over wide regions is strong evidence that they are drawn from [Pg 226] one continuous magma, consisting of molten matter beneath the solid exterior crust.
(a.) Lines of Volcanic Action.—It has been shown in a previous page that volcanic action of recent or Tertiary times has taken place mainly along certain lines which may be traced on the surface of a map or globe. One of these lines girdles the whole globe, while others lie in certain directions more or less coincident with lines of flexure, plication or faulting. ”

“ Even when every other trace of the volcano has been swept away, erosion will not have passed below this great stalk on which the volcano was borne as a fiery flower whose site it remains to mark. In volcanic regions of deep denudation volcanic necks rise solitary and abrupt from the surrounding country as dome-shaped hills. ”

“ We can only form theories on the point, except that everything shows that heat and volcanoes have in some way done the work, though no one has ever yet clearly proved that volcanic eruptions have taken place in our time. We must look back to ages long gone by for those mighty volcanic eruptions which hurled out stones and ashes from the great crater of Tycho, and formed the vast seas of lava in Copernicus and Plato. ”

“ As has been already remarked, many of the most prominent phenomena of a modern volcano are only of transient importance. The earthquakes and tremors, and the constant disengagement of steam and gases, that play so conspicuous a part in an eruption, may leave no sensible record behind them. ”

“ Although the violent and paroxysmal outbursts of volcanic mountains arrest the attention, and powerfully impress us with a sense of the volcanic activity going on beneath the earth's surface, yet it may well be doubted whether the quantity of heat, which the earth gets rid of by their means, at all approaches in amount that which is quietly dissipated by means of the numerous 'stufas,' gaseous exhalations, and thermal springs which occur in such abundance all over its surface. ”

“ By the study of volcanoes and their products we learn that the fused substances are not everywhere precisely the same and do not remain absolutely uniform, even in the same volcanic region. But in what manner and from what causes these variations arise is still unknown. We are further aware that the molten magma, under a centre of volcanic disturbance, manifests from time to time energetic movements which culminate in eruptions at the surface. ”

“ From this general survey of the present condition of the earth's surface, as regards the volcanic operations going on over it, and a comparison with those of a preceding period, we are driven to the conclusion that, however violent and often disastrous are the volcanic and seismic phenomena of the present day, they are restricted to comparatively narrow limits; and that even within these limits the volcanic forces are less powerful than they were in pre-historic times. ”

“ Those who have seen the interior and surroundings of a terrestrial volcano after a recent eruption, and before atmospheric agents have exercised their dimming influences, must have been struck with the colours of the erupted materials themselves and the varied brilliant tints conferred on these materials by the sublimated vapours of metals and mineral substances which have been deposited upon them. ”

“ The action which we call 'denudation' serves as a scalpel to dissect volcanic mountains for us, and to expose their inner recesses to our view. Many portions of the earth's surface are complete museums crowded with volcanic 'subjects,' exhibiting every stage of the process of dissection. In some, rains and winds have stripped off the loose covering of cinders and dust, and exposed the - 115 - harder and more solid parts—the skeleton of the mountain. ”

“ Thus we are led to the conclusion that all volcanic mountains are nothing but heaps of materials ejected from fissures in the earth's crust, the smaller ones having been formed during a single volcanic outburst, the larger ones being the result of repeated eruptions from the same orifice which may, in some cases, have continued in action for tens or hundreds of thousands of years. ”

“ The very finely-divided volcanic dust is often borne to enormous distances from the volcano out of which it has been ejected. The force of the steam-current carrying the fragments into the atmosphere is often so - 72 - great that they rise to the height of several miles above the mountain. ”

“ Amid so much quiet beauty, where every feature seems to be eloquent of long generations of undisturbed repose, it must surely stir the imagination to be told that underneath these orchards, meadows and woodlands lie the mouldering remnants of once active and long-lived volcanoes. ”

“ The final manifestation of volcanic activity in such a region may be the discharge of heated waters, which have ascended from the deep-seated magma far below the surface, and make their appearance as groups of hot springs; these springs persist long after the volcanoes which give rise to them have become quite extinct. ”

“ What augments the interest naturally attached to problems regarding the long duration and varying energy of volcanic fires, is the completeness of the series of phenomena which, taken collectively, they present. New vents are opened in every few years to show us the origin of volcanic accumulations on the land or in the sea; an hundred ignivomous mountains bring up to the surface abundant examples of substances most instructive on points which otherwise could only be sources of vain conjecture ”

“ Towards the middle, as the ground rises, extinct volcanoes make their appearance, sometimes scattered and solitary, with indistinct cones and craters, completely covered with volcanic cinders, and sending off sheets of lava in all directions ”

“ In others, the work of destruction has proceeded still further, and slowly wearing rivers or the waves of the sea may have cut perfect, vertical sections of the mountain-mass. Sometimes the removal of the materials of the volcanic mountain has gone on to such an extent that its base and ground-plan are fully exposed. ”

“ All around, the plain is of distinctly volcanic formation. Indeed, we have now entered a vast volcanic region, extending for several thousands of square miles. Many of the mountains that we shall see, some of them giant peaks, and some only little hills, are extinct volcanoes of other ages. They were young and active while man was in his barbaric infancy on this weary globe, perhaps even before that. ”

“ Even within the present century, the dominant geological school in Europe taught that volcanoes were mere accidents, due to the combustion of subterranean beds of coal casually set on fire by lightning, or by the decomposition of pyrites. ”

“ After all theories relating to the cause of volcanoes have been considered, the one that is most tenable and is sustained by the largest number of scientific men is that which traces volcanic effects back to the old accepted cause of internal fires in the center of the earth. ”