Volcaxo (Lat. Vulcanus The God Of Fire), an opening in the crust of the earth from which are ejected heated gases, steam, finely divided solid matter resembling ashes, cinders, masses of solid rock intensely heated, and currents of molten rock called lava. These materials in time build up a conical pile, which may attain a height of several thousand feet, forming a volcanic hill or mountain around the opening, and having in its upper part a depression called the crater of the volcano, which communicates with the source of the fiery matters below. The action of certain of these volcanic vents or openings is continuous or nearly so, one or all of the products named being daily ejected, while in others eruptions take place only at rare intervals. Those which are supposed to have ceased to be active are called extinct volcanoes. The name of mud volcanoes is given to openings which through the action of steam or gas throw up a pasty mixture of earth and water unaccompanied by any igneous manifestations. Volcanic vents sometimes appear on high lands, and in this way their cones may be built up on mountains of ordinary rocks, while at other times the whole elevation from the sea level is of volcanic origin.

They occasionally break out beneath the sea, forming submarine volcanoes, the matters ejected from which sometimes build up islands. Volcanic activities have been at work on the earth's surface from early geological times; but modern volcanoes are limited to certain regions, generally very distinct from those which were the seats of volcanic energy in past geological periods. On the American continent modern volcanoes are limited to the Pacific slope, along which they may be traced almost continuously from Cape Horn to Alaska. Great numbers of volcanoes occur throughout the Andes, where some attain immense heights, as Cotopaxi in Peru, about 19,500 ft. above the sea (according to Dr. Reiss, who ascended it in 1872). The volcanoes of Central America and of Mexico are numerous and conspicuous, and in California and Oregon we have Shasta, Hood, and St. Helen's, attaining heights of from 11,000 to more than 14,000 ft. Mt. St. Elias, in Alaska, is about 18,000 ft. high. Through the Aleutian islands the belt of volcanoes of western America is connected with those of Kamtchatka, which, with those of the Kuriles, of Japan, and of the Philippines, form interrupted chains of volcanic vents to the burning mountains of the Indian archipelago.

In southern continental Europe there are numerous extinct volcanoes; but with the exception of Vesuvius the active vents are now confined to the islands of the Mediterranean. Besides Etna and Stromboli, there are many volcanic islands in the eastern parts of the Mediterranean, while further eastward in continental Asia are lofty volcanic peaks about the Red sea and the Caspian, on the shores of which is Demavend, 20,000 ft. high. Beyond this in the Thian-shan mountains a small volcanic region is said to exist. In Africa there are numerous volcanic vents in the eastern ranges about the equator, besides which a single volcano is known on the western coast at the bight of Benin, near which are also some volcanic islands. In the Atlantic are Jan Mayen and Iceland, remarkable for volcanoes, the volcanic islands of the West Indies, and those of the Azores, Madeira, the Canaries, the Cape Verd islands, St. Helena, and Tristan d'Acunha. There are great numbers of volcanoes throughout the North and South Pacific, the most remarkable of which are those of the Hawaiian islands. Volcanoes occur in New Zealand, in southern Australia, in the Indian ocean, and further south in the Antarctic seas, where remarkable burning mountains have been seen.

It is impossible to determine with any degree of accuracy the number of existing volcanic vents. Humboldt fixed it at 407, of which 225 had been active within a century. Of the latter about half were supposed to be upon the Asiatic islands. It has since been estimated that the Indian archipelago alone contains over 900. A noticeable fact in the history of volcanoes is their general linear arrangement, which is particularly conspicuous in the ranges of volcanic islands of eastern Asia and in those of the western 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 volcanic vents, while the seas separating the northern and southern continents abound in them, as seen in the West Indies, the Mediterranean 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 materials ejected from them, and the continuous or intermittent character of their action.

For more than 2,000 years Stromboli in the Mediterranean 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 explosions of tremendous violence. In other cases centuries elapse between the eruptions of a volcano. Thus Vesuvius, though built up of volcanic matter, had 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 eruption of this mountain in 1794 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 Jökull in Iceland, which began in 1783 and continued for two years, gave rise to two lava streams, one 40 and one 50 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 containing molten rock, which is forced upward in the crater by the action of steam or of permanent 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 unmelted 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 intimately 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 ebullition, rising into jets of great height, while the projected portions harden into a glassy substance.

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 unfused matter 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 diameter. These ejections of ignited solid matter are seen in the ordinary eruptions of Vesuvius, 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 volcanoes is often borne away by the upper currents of the atmosphere, and may, as is occasionally seen, descend in showers many hundred 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 filled 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. The 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 13,000 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 stupendous 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 material, 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 vapors and gases thrown up into cones of considerable size. Volcanic eruptions are sometimes accompanied or preceded by earthquakes, but great outflows resulting from the rupture and discharge of huge craters filled with lava may take place without any convulsion 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 insignificant part of volcanic ejections, and it is doubtful whether the luminous appearances accompanying eruptions, which have given rise to the popular name of burning mountains, are dependent in any degree on combustion. They are most probably due solely to the intense ignition of the ejected matters. How far the movements of the lavas in the craters of volcanoes are dependent on local and external conditions, and how far on deepseated and occult agencies, is a question.

It is by some supposed that the atmospheric waters falling on a volcanic region, and sinking through the soil under the pressure of the column of water above, may penetrate the lavas and become an efficient agent in their elevation in the manner already pointed out; but there is good reason to believe that the force is in many cases far more deeply seated. - The nature of the earthy materials ejected from volcanoes, and their relations to the other rocky matters of the earth's crust, next demand our notice. Setting aside the volcanic ashes and tufas, in which the constituent mineral elements are much disguised, the solidified lavas are conveniently grouped in the two classes of feldspathic or trachytic and augitic or doleritic rocks, according as the minerals feldspar and augite predominate in them. The rocks of the former class are characterized by a comparatively low specific gravity, ordinarily from 2.4 to 2.7, a generally light color, a predominance of silica and alkalies, and a scarcity of iron oxide, lime, and magnesia; while the augitic rocks have a specific gravity of from 2.8 to 3.2, are dark-colored, and contain in abundance the three bases last named.

The amount of silica in the latter is generally from 40 to 50 per cent., while in the trachytic rocks it ordinarily ranges from 60 to 70 per cent. The analogies of the feldspathic class are with granitic rocks, into which the trachytes pass by insensible gradations, and they contain as essential elements feldspars allied to orthoclase, sanidine, and albite, sometimes with a little hornblende or mica, and often with an excess of silica over that required to form these minerals, which, instead of being separate in the form of visible quartz as in granite, is intimately blended with the base of the rock. These trachytic rocks are either coarsely crystalline, granular, and friable, or else fine-grained and compact with enclosed feldspar crystals. Dolerite, which may be taken as the most common form of the augitic rocks, consists of a mixture of augite and labradorite, generally with more or less disseminated magnetic or titanic iron. These rocks sometimes occur coarsely granular and crystalline, and at other times fine-grained or compact, constituting basalt. Other feldspars or related minerals, very distinct from those of the trachytic rocks, sometimes replace the labradorite, giving rise to various augitic rocks closely related to dolerite.

Two of the most noticeable of these contain the minerals leucite and nepheline in place of labradorite. Olivine or chrysolite, moreover, often forms an important element in these augitic rocks. As all of these minerals, augite included, contain a comparatively large proportion of basic elements, the rocks of this group are often designated "basic " rocks, in contradistinction to which those of the trachytic group, in which silica or silicic acid predominates, are spoken of as "acidic" rocks. The results of the subsequent action of water upon many of these rocks are supposed to have given rise to certain modifications in composition since their ejection. In this way the cells and interstices in the porous lavas have been filled with various hydrated minerals, such as zeolites, calcite, and chlorite, the results of a partial decomposition of the original mineral species. These two great groups and their subordinate varieties may be said to include all the volcanic rocks of both ancient and modern times. Mention should here be made of the vitreous variety known as ob sidian, which is a glass generally formed from a trachytic lava, while pumice is a highly inflated or vesicular form of the same.

The difference between the crystalline and the vitreous state of rocks is due to the rate of cooling from fusion. The slags resulting from the fusion of the copper slates of Mansfeld in Germany, which closely resemble in composition some doleritic rocks, form a glass when rapidly cooled, and a similar glass may be produced from the melting of basalts; but when slowly cooled the one and the other assume a crystalline structure. - It was formerly supposed that the volcanic rocks of the present day and of the later geological periods differed widely from those of more ancient times; but the results of careful microscopical and chemical study during the past few years go far to show that the constitution of these rocks, even as far back as palaeozoic times, is identical with that of recent ones, so that the volcanic activities which in former periods exhibited themselves over different regions of the earth's surface must have presented conditions similar to those of our own time. Although erosion has in most cases effaced the cones of the more ancient volcanoes, yet dikes and sheets of lava and beds of volcanic tufa still remain from remote geological periods.

It would appear that some of these former outbursts of igneous rocks were on a grander scale than anything known in the historic period, and differed somewhat in the mode of their eruption. Thus at the beginning of the Cambrian period were poured out the enormous floods of doleritic, lavas which, with volcanic tufas and interstratified sandstones, form the copper-bearing or Keweenaw series of Lake Superior, which have a thickness of many thousand feet, and probably once occupied the entire breadth of the lake. Great outbursts both of trachytic and doleritic rocks occurred in the palaeozoic in the province of Quebec; and later, in the mesozoic period, were erupted the immense masses of doleritic rooks which along the bay of Fundy, in the valley of the Connecticut, and from the Palisades of the Hudson S. W. through Pennsylvania, are associated with the red sandstones of that period. In the later tertiary time occurred the enormous eruptions which, extending from northern California and Nevada to British Columbia, cover an area of over 200,000 sq. m. with an average thickness of 2,000 ft., and in one place with a thickness of 4,000 ft., consisting of alternate layers of trachytic and doleritic rocks.

These great outflows of volcanic material from the lower Cambrian time were on a far more extended scale than any modern eruptions, and probably issued from vast fissures in the earth, due to widespread and deep-seated forces connected with the folding of strata and continental movements. - The hypotheses which have been proposed to explain the origin of volcanoes are manifold, but for the most part can be briefly discussed. That of Davy and Daubeny was based upon the assumption that the earth's interior holds in an unoxidized condition silicon and the metallic bases of the earths and alkalies, which if brought in contact with the water of the ocean would react violently with it, generating a great amount of heat and giving rise to the elements of the silicated minerals which make up the volcanic rocks, at the same time liberating in gaseous compounds the chlorine and the sulphur of the sea salts, together with free hydrogen. These, with aqueous vapors, by their elasticity would rend the earth's crust, and thus account for the mechanical phenomena of earthquakes and volcanoes. This once celebrated hypothesis is now generally rejected as altogether baseless.

Others, while rejecting the theory of an unoxidized nucleus, have sought to explain the origin of the volcanic heat by chemical reactions set up in the sedimentary deposits; a view which the study of the thermal relations of chemical change shows to be untenable. Most geologists, adopting the view that the earth was originally in a fused condition, and that the interior is still intensely heated, and probably consists, at least to a great depth, of oxidized mineral matters not very unlike in composition to those at the surface, have sought therein an explanation of volcanic phenomena, though from this point there are several diverging hypotheses. Some have supposed that these materials immediately beneath- a crust of no great thickness still remain in a state of igneous fusion, and are arranged in two layers, the lighter of feldspathic matter corresponding to the trachytes, and the heavier and inferior layer having the composition of the dolerites. From these two liquid strata it is imagined that the various volcanic rocks are ejected by movements in the earth's crust, allowing the escape from time to time of portions of the one or the other, or of admixtures of the two; while the intervention of sea water, as already explained, was also possible, giving rise to gaseous products and producing greater or less modifications in the composition of the rocks erupted.

Such, in general terms, is the hypothesis adopted by a great many modern geologists, whether they admit the internal fluidity of the earth as a whole, or suppose it, though solid from the centre, to include beneath its surface still unsolidified lakes of molten rock, which may underlie the present volcanic regions. But the supposed separation of the cooling globe into two layers is a gratuitous hypothesis, and there are many chemical reasons for supposing that the upper layer of the earth's surface must have originally had a constitution not far removed from that of the dolerites, and moreover that the materials of the granitic and trachytic rocks were derived from the action of water upon this surface, and are not to be expected among the results of a simple igneous fusion. (See Geanite.) It is also shown that certain anomalous types of eruptive rocks, which exhibit in their composition pretty wide divergences from both of the types above mentioned, are met with among water-formed rocks, and are doubtless of aqueous origin, as are also some augitic rocks.

Another hypothesis in accordance with these facts has therefore been advanced, which is that the source of the volcanic and eruptive rocks of all ages is to be sought in the softening and melting of portions of the solid crust, including both the primitive doleritic layer and the various results of its alteration by aqueous agencies, embracing a great variety of rocks of sedimentary origin. In fact, we find among aqueous crystalline rocks the two types already described as characterizing the igneous masses, and these types have been shown to be the natural results of chemical and mechanical forces always at work at the surface of the latter. The fusion of these and of the various heterogeneous materials which make up the sedimentary deposits, in the presence of the water and saline matters with which the rocks are always impregnated, would seem to explain all the chemical phenomena of voleanicity. The heat necessary to produce this result has been sought for either entirely in that transmitted from the earth's interior to the deeply buried portions of the crust, or in that mechanically evolved by the crushing of the deeply buried strata during the contraction of the earth's crust and the consequent conversion of motion into heat.

In reality the two influences must concur in producing this result. In this connection it has been pointed out that the great volcanic regions of modern times, wherever circumstances permit us to determine their geological relations, appear to be those in which have occurred both great deposition of sediments burying to considerable depths the older rocks, and great movements of the earth's crust in comparatively recent geological periods. - See Von Buch, Physicalische Beschreibung der Canarisclier Inseln (1825); Daubeny, "The Geology and Chemical Phenomena of Volcanoes" (1824), and "Description of Active and Extinct Volcanoes" (1826; 2d ed., 1848); Scrope, "Considerations on Volcanoes " (1825; enlarged ed., 1862); Sir H. Davy, on the "Phenomena of Volcanoes," in the " Philosophical Transactions" (1828); Lyell, "Principles of Geology" (1830; 11th ed., 1873), and on "Etna" in the " Philosophical Transactions " (1858); Hopkins, "Researches in Physical Geography" (London, 1839-'42; also "Philosophical Transactions," 1839), and " On the Phenomena and Theory of Volcanoes" (report of the British association, 1847); Von Waltershausen, Atlas des Aetna (1848-'59); Dana, " Geology of the American Exploring Expedition " (1849); Kallmann, Geognosie (2 vols., 1850-53); Phillips, "Vesuvius" (1869); Hunt, "Chemical and Geological Essays" (1875); and Humboldt's " Cosmos," " Travels," and " Treatise on Books".