Page:The New International Encyclopædia 1st ed. v. 20.djvu/251

* VOLCANO. 203 VOLCANO. perhaps still larger. Again, the size of the crater in no wa' measures the intensity of an eruption. Krakatoa. Vcsuviiis, and Polfe, at the times of their greatest paroxysms, had craters of only moderat(: dinieiisions, that of I'elee being ahout 2500 feet in diameter. The Building Up of a Volcano. It has al- ready been remarked that probably niueli the greater number of volcanoes have made their own masses through the simple accunuilation in time of their ejected products: in other words, the heaping up about the point or points of ac- tivity of lava, cinders, or ashes, most generally, perhaps, of all three of these products. Inas- much as these are thrown out with a certain amount of alternation or regularity, the moun- tain itself acquires a regvilar internal structure, which, in section, would appear in alternating layers (strata) of deposition. Some voU^anocs, whose eruptive energies arc of a greater kind, and are not expressed in steam eruptions, are built up almost entirely of lava, and their outer dress, depending upon the viscosity or liquidity of the outflows, is either steep (like the iname- lons of the island of Bourbon, and some of the puys of tile Auvergne region of France) or very gentle, descending in a gradient of from 15° to 5° or less, as in some of the Hawaiian volcanoes. Other volcanoes are constructed almost wholly of cinders or scori.T, but the composite cone is the one of most general occurrence. It was at one time held that volcanoes, in- stead of being in the main accumulations of ejected material, were massive upheavals or swellings of the earth's surface, caused by the pent-i:p force within acting upon the depressing surface. The more accurate stiuly of volcanic phenomena has not given proof of the existence of 'craters' or 'cones of elevation,' as these as- sinned structures have been called. At the same time it is by no means improbable that in the initial formation of some (or even many) vol- canoes a preparatory step has been the upheaving of the deeper strata through niagmatic intru- sions, a condition that is fairly well indicated by the uplifts which are recognized in or asso- ciated with the structures known as laccoHtes — intruded lava-masses which have forcibly domed up the strata lying over the ai'eas of u|iward passage of escaping lava. The Henry llountains of southeastern Utah, the Eugancan Hills (or portions of them) in Northern Italy, are familiar examples of this type of structure. The prodi- gious force of this uprising lava is measured by the thousands of feet of thickness of sedimentary strata which it has bodily uplifted. The ]I.tebial Ejected. The quantity of ma- terial ejected in the course of eruption is or- dinarily dependent upon the force or violence of the eruption, and the period of its continuance. Skaptar jiikull, in its great eruption of 17S3, threw out a mass of lava the cubical contents of which are thought to have equaled the bulk of the Mont Blanc massif. In the historic eruption of Vesuvius of the year 79 there was no lava- ilow. but the discharge of fraginental products (cinders and ashes) was very large, burying., as is well known, a considerable portion of Cam- pania with debris several feet in thickness. The cataclysm of Krakatoa in 1883 was responsible for the extrusion of material which, it has been thought, could not have measured less than 4.3 Vol. XX.— 14. cubical miles; and this again, we are informed by some geologists, could hardly have been more than the one-hundredth part of the material that was thrown out by Temboro, on the island of Sumbawa, in 1815. It seems not unlikely that on certain days of its eruptions in May and August, 11)02, the ejecta of PehV, ehielly ashes, more than equaled in bulk the quantity of sediment discharged Ijy the Mississippi lliver in the course of a full year. The projectile force of eruption is such that the lighter materials of eruption are thrown many miles into the air, where they may be kept .suspended for months or even years, as in the case of the Krakatoa eruption, moving about with the u))per atmospheric currents, and giving rise, through the sifting out and reflection of the sun's rays, to those remarkable phenomena which are known as the 'red glows' or 'volcanic after- glows.' These, made beautiful by their intense illumination, and impressing the beholder at times with the aspect of a vast conflagration, usually follow the true sunset by the period of about a half-hour (20-40 minutes), and rise to a ])Osition in the sky possibly exceeding 45° above the horizon. The afterglows of the Kraka- toa eruption, which were the first to be carefully observed, were thought to have been deflected from dnst particles sailing at a height above the earth's surface of from 30 to HO or even 70 miles, and were continued for a period of con- sidcrabl.v over a year. The afterglows following, some of the Antillean eruptions were less bril- liant and less continuous, and were seemingly thrown from a height not exceeding 20 to 25 miles, and at times, possibly, of only 8 to 15 miles. In both eases the phenomena were ob- served over widely removed portions of the earth's surface, which led to the conclusion that they were visible over the entire circumfer- ence of the globe, appearing at different localities at different times, depending upon the rate of travel of the suspended dust particles. Those from Krakatoa traveled with a general velocity of from 50 to 70 miles per hour, or more than twice that which impelled the particles from PelCe. The course of travel was in both cases from east to west. Electric, Magnetic, and Acoustic Dlstueb- ANCES. The ordinary electric disturbances which are associated with volcanic eruptions have long been recognized. These are the lightning flashes and peals of thunder which appear or are gener- ated (perhaps as the result of friction) in the furious steam-cloud which is shot out from and overhangs the vent of the active volcano. In addition to the commoner or true flash-lightning, other forms of electric or p.yro-electric discharge have been observed in association with volcanic discharges — balls or 'stars' of electric lumi- nosity appearing in the volcanic cloud, as has several times been noted in the ease of the Vesu- vian eruptions, and again noticed at the time of the eruption of Tarawera. in New Zealand, in June, 1880. But the most wonderful displays were those which were associated with the out- breaks of Pel^e. notably those of the evenings of May 2Gth and August '30th. On the latter date the 'electric' figures appeared as clustered or rocket stars, moving in straight and serpent lines (sometimes in parallel series), normal circles, apd circles with zigzag streamers. These were observed simultaneously with the ordinary zigzag