Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/259

Rh VOLCANIC ACTION.] form the prevailing rocks there, and through which the various volcanic vents have been opened. Evidently in these cases elastic vapours only forced their way to the surface; and we see what probably often takes place in the early stages of a volcano’s history, though the frag- ments of the underlying disrupted rocks are in most instances buried and lost under the far more abundant sub- sequent volcanic materials. Sections of ancient volcanic necks or pipes sometimes afford an excellent opportunity of observing that these oriﬁces were originally opened by the blowing out of the solid crust and not by the forma- tion of ﬁssures. Many examples occur in Scotland among volcanic rocks of Old Red Sandstone, Carboniferous, and Permian age. The oriﬁces are there ﬁlled with fragmentary materials wherein portions of the surrounding and underly- ing rocks form a noticeable proportion. been opened, either by ﬁssuring or explosion, between the heated interior and the surface, fragmentary materials seldom fail to be ejected from it. These may consist at first mainly of the rocks through which the oriﬁce has been opened, as has just been explained. But if eruptive energy continues, they soon appear in larger quantities, and consist of thoroughly volcanic substances. In a great eruption vast numbers of red-hot stones are shot up into the air, and fall back partly into the crater and partly on the outer slopes of the cone. But instances are known where large stones, ejected obliquely, have described huge parabolic curves in '-the air, and fallen at a great distance. Stones 8 lb in weight occur among the ashes which buried Pompeii. The volcano of Antuco in Chili is said to send stones ﬂying to a ' distance of 36 miles, and Cotopaxi is reported to have hurled a 200—ton block 9 miles. But in many great eruptions, besides a constant shower -of stones and scorize, a vast column of exceedingly ﬁne dust rises out of the crater, sometimes to a height of more than a mile, and then spreads outwards like a sheet of cloud. So dense sometimes is this dust-cloud that the sun is obscured, and for days together the darkness of night reigns for miles around the volcano. In 1822 this was the case at Vesuvius, the ashes not only falling thickly on the villages round the base of the mountain, but travelling as far as Ascoli, which is 56 Italian miles distant from the volcano on one side, and as Casano, 105 miles on the other. But probably the most stupendous outpouring of volcanic ashes on record was that which took place, after a quiescence of 26 years, from the volcano Coseguina, in Nicaragua, during the early part of the year 1835. On that occasion utter darkness prevailed over a circle of 35 miles radius, the ashes falling so thickly that, even 8 leagues from the mountain, they covered the ground to a depth of about 10 feet. It was estimated that the rain of dust and sand fell over an area at least 270 geographical miles in diameter. Some of the ﬁner materials, thrown so high as to come within the inﬂuence of an upper air—current, were borne away eastward, and fell four days afterwards at Kingston, in Jamaica—a distance of 700 miles. An inquiry into the origin of these showers of fragmentary materials brings vividly before us some of the essential features of volcanic action. We ﬁnd that bombs, slags, and lapilli may be thrown up in comparatively tranquil states of a volcano, but that the showers of ﬁne dust are discharged with violence, and only appear when the volcano becomes more energetic. Thus, at the constantly, but quietly, active volcano of Stromboli, the column of lava in the pipe may be watched slowly rising and falling with a slow rhythmical movement. At every rise the surface of the lava swells up into blisters several feet in diameter, which by and by burst with a sharp explosion that makes the walls of the erater vibrate. A cloud of steam rushes out, carrying with GEOLOGY
 * S'Izowers of Dust and Stones.—A communication having
 * enormous quantities of fragmentary matter.

245 it hundreds of fragments of the glowing lava, sometimes to a height of 1200 feet. It is by the ascent of steam through its mass that a column of lava is kept boiling at the bottom of a crater, and by the explosion of successive larger bubbles of steam that the various bombs, slags, and fragments of lava are torn off and tossed into the air. It has often been noticed at Vesuvius that, after each great concussion, a huge ball-like cloud of steam rushes up from the crater. Doubt- less it is the sudden escape of that steam which causes the explosion. The violence of the explosion will depend greatly upon the viscidity of the lava, and the consequent resistance offered to the upward passage of the steam. Explosions and accompanying scoriac are abundant at Vesuvius, where the lavas are comparatively viscid; they are almost unknown at Kilauea, where the lava is remark- ably liquid. The steam, collecting into larger or smaller vesicles, works its way upward through the substance of the molten lava. As the elasticity of this compressed vapour overcomes the pressure of the overlying lava, it escapes at the surface, and there the lava is thus kept in ebullition. But this com- paratively quiet operation, which may be watched within the craters of many active volcan oes, does not produce clouds of ﬁne dust. The friction of the millions of stones ascend- ing and descending through the air in the dark column above the crater, though it must doubtless cause much dust and sand, can give rise to b11t an insigniﬁcant proportion of what is actually reduced to the condition of extreme subdi- vision necessary to produce widespread darkness and a thick far-reaching deposit of ashes. The explanation now ac- cepted calls in the explosive action of steam as the imme- diate cause of the trituration. A sudden and powerful explosion of steam, it is maintained, will blow the top of the lava column into dust, like water shot out of a gun. We must remember that the aqueous vapour by which many lavas are so largely impregnated must exist inter- stitially far down in the lava-column, under an enormous pressure, and at a white heat. The sudden ascent of lava so constituted will relieve the pressure rapidly without sensibly affecting the temperature of the mass. Conse- quently the white-hot steam will at length explode, and reduce the molten mass containing it to the ﬁnest powder. Evidently no part of the operations of a volcano has greater geological signiﬁcance than the ejection of such In the ﬁrst place, the fall of these loose materials round the oriﬁce of discharge is one main cause of the growth of the volcanic cone. The heavier fragments gather around the vent, and there too the thickest accumulation of ﬁner dust takes place. Hence, though successive explosions may blow out the upper part of the crater-walls, and prevent the mountain from growing so rapidly in height, every eruption must add to the diameter of the cone. In the second place, as every shower of dust and sand adds to the height of the ground on which it falls, thick volcanic accumulations may be formed far beyond the base of the mountain. In these are entombed trees and other kinds of vegetation, together with the bodies of many animals, as well as the works of man. Hence new geological formations arise which, in their component materials, not only bear witness to the volcanic eruptions which produced them, but preserve a record of the land-surfaces over which they spread. In the third place, besides the distance to which the fragments may be hurled by volcanic explosions, or to which they may be diffused by the ordinary aerial movements, we have to take into account the vast spaces across which the ﬁner dust is sometimes borne by upper currents in the atmosphere. An instance has already been cited where ashes from Coseguina fell 700 miles away, having been carried all that long distance by a high counter-current of air, moving apparently