Page:The American Cyclopædia (1879) Volume XVI.djvu/420

 400 VOLCANO era part of the American continent, along both shores of the Pacific. It is however to be noticed that the regions bordering on the Atlantic, with the exception of a single point on the coast of Africa, are destitute of vol- canic vents, while the seas separating the northern and southern continents abound in them, as seen in the West Indies, the Mediter- ranean basin, and the Indian archipelago. Volcanoes differ greatly among themselves, not only in dimensions but in the degree of their activity, the quantity and quality of the ma- terials ejected from them, and the continuous or intermittent character of their action. For more than 2,000 years Stromboli in the Medi- terranean has been constantly discharging lava; and Sangai in Peru, 17,000 ft. high, has for 150 years been in continuous action, ejecting every few minutes fiery cinders, with explo- sions of tremendous violence. In other cases centuries elapse between the eruptions of a volcano. Thus Vesuvius, though built up of volcanic matter, bad remained dormant for ages previous to the beginning of our era, when its discharges of lava and ashes buried the cities of Pompeii and Herculaneum. A single erup- tion of this mountain in 1704 is supposed to have yielded 46,000,000 cubic feet of lava, and one of Etna in 1669 more than twice that amount. The great eruption from the Skapta Jokull in Iceland, which began in 1783 and continued for two years, gave rise to two lava streams, one 40 and one 60 m. long, with breadths of 7 and 15 m. respectively. A large part of the lava current was 100 ft. thick, and in some of the valleys it attained 600 ft., while its total bulk was estimated at not less than 21 cubic miles. The phenomena of volcanoes may be best understood by considering that they are openings connected with spaces con- taining molten rock, which is forced upward in the crater by the action of steam or of per- manent gases, or in some cases probably by movements of the earth's crust. This material is sometimes in a state of complete fusion like glass, but oftener consists in great part of un- meltod grains mingled with a sufficient quantity of liquid matter to give fluidity to the mass. It is moreover charged with water and with various gases, all of which are probably inti- mately combined with the molten mass under the great pressure which exists below, and in many cases aid materially in giving it fluidity, but, as the lava ascends and the pressure is thus removed, assume the gaseous state and escape. One result of this process appears in the very fluid lava of the great crater of Kilauea in Hawaii, where a surface of molten lava 1,000 ft. in diameter is sometimes seen in active ebul- lition, rising into jets of great height, while the projected portions harden into a glassy sub- stance. But if, as is generally the case, the lava is in a state of less perfect fusion, it swells up greatly, forming huge bubbles, from the bursting of which the grains of unftised mat- ter which it contains, as well as the interposed liquid portion, are scattered in the shape of ashes or cinders, sometimes with masses of solid unfused rock, often several feet in diam- eter. These ejections of ignited solid matter are seen in the ordinary eruptions of Vesu- vius, and in one instance the fiery cinders from this mountain were estimated to ascend to a height of nearly two miles from the crater. In such cases the lighter material from volca- noes is often borne away by the upper cur- rents of the atmosphere, and may, as is occa- sionally seen, descend in showers many hun- dred miles away. The heavier materials fall in the shape of cinders or ashes in the vicinity of the crater, and by their accumulation help to build up the cone. When, as very often happens, there is a precipitation of water due to the condensation of the immense amount of steam given off during the eruption, the wetted cinders constitute a kind of mud called volcanic tufa. Not unfrequently the swelling up within the crater will cause the lava to overflow ; or else the pressure of the column of liquid matter may cause a breach in the side of the mountain; in either of which cases a lava current is formed. These currents, as we have seen, are sometimes of great volume, and sheets of such molten rock contribute with the cinders to build up the mountain cone, the two being often interstratified. The fissures in the mountain side resulting from the action of the volcanic forces do not always give rise to lava currents, but may become tilled up with the more or less liquid mass. This, hardening within them, gives rise to great walls or dikes of rock, which intersect the beds of lava and of cinders, giving stability to the mass. Tho surface of the lava stream is rough cinder, light and porous, but at a little depth the lava hardens to a solid rock. The great volcanoes of Hawaii, rising with an average slope of 5 or 6 to heights more than 18,000 ft. above the sea, have been built up mainly of lava. This volcanic region has within the past 40 years been the seat of some of the most stu- pendous volcanic eruptions on record. The outbursts of lava issue from the volcanic mass at various elevations, from near the summit down almost to the sea level. When lavas break out near the sea or beneath its waters, the action of the water on the molten lava produces a granular and disintegrated mate- rial, which like the moistened cinders is known as volcanic tufa, and is sometimes spread out in beds in the sea, or from the action of va- pors and gases thrown up into cones of con- siderable size. Volcanic eruptions are some- times accompanied or preceded by earth- quakes, but great outflows resulting from the rupture and discharge of huge craters filled with lava may take place without any convul- sion of the earth. The gaseous products of volcanoes appear to be chiefly carbonic acid, chlorohydric acid, and sulphur in the forms of sulphuretted hydrogen and sulphurous acid. Combustible gases form at best but an insig-