Volcanic crater

“ It would seem as if the small amount of rain or snow that fell would get pretty well dried out before it arrived at a point to create an eruption by contact with melted rock, or that such a quantity could keep up steam in such an immense cauldron. There is no reason to presume that a drop of rain water ever enters a volcanic crater, except what may fall into its open mouth, which will be impossible. ”

“ The mass which supported us had doubtless been shaken a thousand times, and was liable every hour to be shaken again; but being in the short curvature of the crater, like the key-stone of an arch, it could not easily be thrown from its position by any agitation that would naturally occur while this great safety-valve is kept open, or the numerous fissures round it, reaching to the very bowels of the mountain, convey harmlessly from unknown depths, gases and volumes of steam, generated where water comes in contact with intense volcanic heat. ”

“ However uniform they may appear when seen from the sea, or the plains below, nothing can be more diversified than their shape when we look from above into their craters, one side of which, as we have seen, is generally broken down. There are indeed, few objects in nature more picturesque than a wooded volcanic crater. ”

“ The highest of these hills had on its summit a regular volcanic crater, whence there have been thrown up great quantities of dross and lava, containing fragments of older rocks. The ashes were transported to immense distances, some of them having fallen on the houses at Queretaro, more than forty-eight leagues from Jorullo. The volcano continued in this energetic state of activity for about four months; in the following years its eruptions became less frequent, but it still continues to emit volumes of vapor from the principal crater, as well as from many of the ovens in the upheaved ground. ”

“ As shown by the map in the Report of the Royal Society, the group of islands which existed previous to 1883 were but the unsubmerged portions of one vast volcanic crater, built up of a remarkable variety of lava allied to the andesite of the Java volcanoes, but having a larger percentage of silica, and hence falling under the head of "enstatite-dacite." ”

“ In this manner we may suppose that ashes, scoriæ, and blocks of rock torn from the sides of the crater-throat, and hurled into the air, are piled around the vent, and accumulate into hills or mountains of conical form. After the explosion has exhausted itself, the molten lava quietly wells up and fills the crater, as in the cases of those of Auvergne and Syria, and other places. We may, therefore, adopt the general principle that in volcanic eruptions where water in large quantities is present, we shall have crater-cones built up of ashes, scoriæ, and pumice ”

“ The steep conical hill which overlooks the city presenting its dull, brick-red façade when viewed from nearly any direction is a ceaseless reminder of the volcanic origin of the place. It contains a large extinct crater, and is called on account of its peculiar formation the Punch Bowl. Its apex is about five hundred feet above the level of the harbor. At the top one looks down into a large concave,—a scooped-out, bowl-like cavity, partially filled with a débris of stones and cinders over and about which vegetation has freely grown, the earth being mixed with decomposed lava. ”

“ Mixed with the volcanic ashes and bombs there are often large broken pieces of sedimentary rocks which may have been crystallized and hardened by the heat and vapours emitted by the volcano. Sometimes great fragments of the walls have foundered or collapsed into the crater, and masses of non-volcanic rock, an acre or more in extent, may occur in a volcanic neck. In Arran, for example, there is a large neck which contains lumps of Cretaceous rocks inowhere else known to occur on the island ”

“ We cannot see beyond the bottom of the crater. We cannot tell anything about the subterranean ducts, or how the molten and fragmental materials behave in them. All the underground mechanism of volcanoes is necessarily hidden from our eyes. But much of this concealed structure has been revealed in the case of ancient volcanic masses, which have been buried and afterwards upraised and laid bare by denudation. ”

“ Such breached craters are to be found in all parts of the world, and are not confined to any one district, or even continent, so that they may be considered as characteristic of the class of volcanic crater-cones to which I am now referring. In the case of the moon, however, we fail to observe any decided instances [Pg 248] of breached craters, with lava-streams, such as those I have described. [13] In nearly all cases the ramparts appear to extend continuously round the enclosed depression, solid and unbroken ”

“ But the chief point of interest is the chain of volcanic crater-cones and dome-shaped eminences which rise from the plateau, amongst which the Puy de Dôme towers supreme. Their individual forms stand out in clear and sharp relief against the western sky, and gradually [Pg 95] fade away towards the south into the serried masses of Mont Dore and Cantal, around whose summits the evening mists are gathering. ”

“ If we go upon the supposition, that it is an independent volcano, and not merely an elevated volcanic mass, we may easily recognize in the top an extinct crater, levelled by the fallen walls of rock, such as we see even now on the eastern side, by lava and volcanic mud, wherein ants at present wallow, and by the crumbling fragments of projecting rocks. ”

“ The depth of the crater varies from eight hundred to one thousand feet, according as the molten sea below is at flood or ebb. Signs of volcanic activity are present more or less throughout its whole depth, and for some distance round its margin, in the form of steam-cracks, jets of sulphurous vapour, blowing cones, accumulating deposits of acicular crystals of sulphur, etc., and the pit itself is constantly rent and shaken by earthquakes. ”

“ The extinct craters are very interesting. They have their broken-down side wall, through which the last great effort of volcanic life was poured out. They also have crater cones and sometimes lava flows of small extent on the floor left by the great eruption. These were the picturesque last throbs of life as a volcano died. Occasional spasmodic efforts were made in both earthquake and lava flow until the fire cooled in the submarine chambers. ”

“ 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. ”

“ Rugged buttresses of dark volcanic rock, with steep snow slopes between, jut out at intervals, and support the rim of this second crater, which reaches an altitude of fully 11,400 feet. From the north edge of this crater the ground seemed to ascend, at first gradually, then somewhat abruptly to the third crater, now active, further south. It is above this last crater that there continually floats a huge steam cloud. At the time of Ross’ Expedition this cloud was reddened with the glow of molten lava, and some thought they saw lava streams descending from the crater. ”

“ Within this same crater, strata of coarse tuff, chiefly composed of fragments of lava, abut, like a consolidated talus, against the inside walls. They rise to a height of between one hundred and one hundred and fifty feet above the surface of the internal brine-lake; they dip inwards, and are inclined at an angle varying from thirty to thirty-six degrees. They appear to have been formed beneath water, probably at a period when the sea occupied the hollow of the crater. ”

“ In later times, however, the volcano appears to have exploded and destroyed the northern part of the crater rim, out of the break in which there flowed a tongue-shaped mass of lava which now constitutes the northern half of the island. The last volcanic activity issued from a small ash-cone in the cup of the old crater, and then the island lapsed into its unbroken quietude.
A luxuriant growth of cocoanut palms, and forest trees now covers the old lava, while coarse grass grows rankly over the ash slopes of the crater. ”

“ It is often of large dimensions, and sometimes of vast depth. Some volcanoes consist of a crater alone, with scarcely any mountain at all; but in the majority of cases the crater is situated on the top of a mountain, which in some instances towers to an enormous height. The part of the mountain which terminates in the principal crater is usually of a conical form—much like a glass-house chimney, and is therefore named the cone. It is generally composed of loose ashes and cinders, with here and there masses of stone, which have been tossed into the air by the volcanic forces. ”

“ 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. ”

“ Almost all volcanic cones exhibit craters, but in those which are formed entirely by the outwelling of viscid lavas the central depression is often slight and inconspicuous, and occasionally altogether wanting. It frequently happens, however, that eruptive action has ceased at the centre of a volcano, and its summit-crater may by denudation be entirely destroyed, while new and active craters are formed upon its flanks. ”

“ Near the summit, the traveller passes the crater of an extinguished volcano, having long perceived, throughout the whole district, the marks of volcanic eruption, and after a difficult climb of the extreme and precipitous ridge, gains the level of the plain above mentioned. ”

“ The snow was anything but slippery; the surface was scattered with dust, and it bristled with a forest of dwarf earth-pillars, where blown volcanic sand preserved the ice. After a slow hour and a half, we reached the crater of ’45, which opened at 9 A. M. on September 2, and discharged lava till the end of November. It might be passed unobserved by the inexperienced man. The only remnant is the upper lip prolonged to the right; the dimensions may have been 120 by 150 yards, and the cleft shows a projecting ice-ledge ready to fall. ”

“ The summit of this hill now nearly equals in height the crateriform ridge; and before having been denuded, it was probably higher than this ridge, from which it is separated by a broad and much lower tract of country; we here, therefore, see that the lower extremities of a set of lava-streams have been tilted up to as great a height as, or perhaps greater height than, the crater, down the flanks of which they originally flowed. I believe that dislocations on so grand a scale are extremely rare in volcanic districts. ”

“ This huge excavation, which I take to be the extent of the crater, for it has evidently been all formed by the irruption of volcanic matter, is divided into two parts, a broken fragment of a mountain now lying between them; and the smaller of these two has lost all volcanic appearance. It is a good deal covered with bush and scrubby forest trees, and seems to have no remaining connection with sulphur and brimstone. ”

“ These occur in the form of extinct craters, sometimes filled with water, and thus converted into circular lakes; or of extensive sheets and conical hills of tuff; or, finally, of old necks and masses of trachyte and basalt, sometimes exhibiting the columnar structure. The Eternal City itself is built on hills of volcanic material which some observers have supposed to be the crater of a great volcano ”

“ 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. ”

“ Where the growth of a volcanic mound takes place from within, as in certain steep-sided trachytic cones, there may be no perceptible crater or external outlet. Again, there are many volcanoes which have no crater at the summit, because the eruptions always take place from lateral outlets. Even when a terminal pit is present, the lava may issue from the body of the mountain, and in some cases it exudes from so many vents or cracks that the volcano has been described as “sweating fire.” ”

“ These when ignited may remain in a slight slumbering condition and burn for ages, but water will be the constant aggressor and from time to time will manifest itself by coming in contact with these burning forces thus producing the volcanic eruptions and in time will be the conqueror, and the crater of the volcano will become a lake, of which evidences exist all over the Earth. That volcanoes are only local, the same as fires in our houses, is fully evident from the fact that they burn, and go out. ”