VOLCANIC GASES.
The most distinctive feature of volcanoes is the explosive action arising from the gases and vapors pent up in the lava. There is not a little explosive action of a secondary character arising from the mere outer contact of surface-waters with lavas or with the hot rocks of the crater walls, or with the hot ashes and rocks thrown out; but these are incidental, not essential, features.
The precise nature of the occlusion or absorption of gases and vapors has not yet been determined. It is thought that lava spontaneously absorbs such gases when at high temperatures, and especially when the gases are under great pressure, and that as the pressure is relieved and the lava is cooled and solidified, the larger part of the gases escapes. In those cases in which the eruption is quiet, the escape of the gases is but partial while the lava is in the crater, and much gas remains to be given out from the molten material after it has been extruded and is about to congeal. The gases are then given off with relative slowness and quietness. If, however, the lavas are surcharged with gases, and if these are restrained from free escape by the viscosity of the lavas, the gases gather in large vesicles in the lava in the throat of the volcano, and on coming to the surface explode, hurling the enveloping lava upwards and outwards, often to great distances. The violence of projection reduces a portion of the lava to a finely divided state constituting the “ash” and “smoke” of the volcano. Other portions less divided are inflated by the gases disseminated through them, and form “pumice” and “scoria,” according to the degree of inflation, while masses of lava that have already solidified into more or less rounded masses in the crater are hurled forth as “bombs”; not infrequently portions of the walls of the crater or of the duct below are also disrupted and shot forth.
Differences in gas action.—The causes of the differences of gas action in different volcanoes are undetermined, but the following suggestions may point to a part of the truth: (1) Doubtless some lavas contain more gases than others, and hence are predisposed to be more explosive; (2) some are more viscous than others and hence hold the gases more tenaciously until they accumulate and acquire explosive force, while the more liquid lavas allow their gases to escape more freely and easily; (3) some are hotter than others, and hence hold their gases until after they have escaped from the crater, when they give them off from their expanded surfaces in the open air, where there is no restraint to develop explosiveness; (4) some flows are so massive that they cool to the chief gas-discharging point only after they are spread out on the surface, when quiet escape is possible; (5) probably a main occasion of the very violent explosions lies in the fact that the lavas have begun to crystallize while yet in the duct of the volcano. The crystals, in forming in the magma, exclude the gases from themselves, and this excluded portion overcharges the remaining portion of the lava. This process continues as the lava rises and grows cooler until the gases acquire great volume and explosive force. This view is sustained by the fact that the pumice and ash of such extraordinarily explosive eruptions as those of Krakatoa and Pelée contain many small crystals which had certainly formed before the explosive inflation took place. Incipient crystallization does not, however, appear to be a universal accompaniment of explosive action.
Spasmodic action.—The discharge of the gases is spasmodic, and usually consists of a succession of distinct explosions. Sometimes these succeed one another at rather constant and frequent intervals, as in Stromboli, where the explosions follow one another at intervals of three to ten or more minutes. In many others the outbursts are rhythmic, while in others the spasms are distant and irregular.
Kinds of gases.—Steam is the chief volcanic gas. Its constituents, hydrogen and oxygen, are also present in the free state, and are perhaps the result of the dissociation of the steam at the very high temperatures of the lavas. Carbon dioxide is probably next in abundance. No positive statement as to the relative amounts of the subordinate gases can be made because of the obvious difficulties of obtaining anything like a representative analysis of the gases concerned in the great volcanic eruptions. The materials for the analyses which have been made were derived chiefly from little secondary or “parasitic” vents, or from side-wall crevices, through which the volcanic gases rise. Such vents probably derive their gases from the very border of the main mass, where it is most subject to the influence of waters and gases from the adjacent walls, and it is uncertain how far they are truly representative of the gases in the interior of the lava itself. The data now at command seem to indicate that carbon dioxide increases greatly in relative abundance as volcanic action dies away. Great quantities of this gas are often given forth long after all signs of active vulcanism have disappeared. Such gases have been attributed to the action of the lavas on buried beds of limestone or other carbonates, but in many cases the geology of the region offers no special support to this hypothesis. It does not seem inherently probable that the heat of the lava would be sufficient to decompose limestone at a period very long after the active eruption. An alternative suggestion is that the stronger volcanic acids mentioned below are gradually conveyed into the adjacent rocks and there act on limestones or on partially carbonated crystalline rocks, setting free carbon dioxide. Whatever may be true with regard to secondary gases of this kind, it is quite certain that the lavas themselves contain large quantities of carbon dioxide, and also of carbon monoxide, doubtless reduced from the dioxide. Sulphur gases are very common accompaniments of volcanic eruptions. They take the forms of sulphuretted hydrogen and sulphurous acid and perhaps of sublimated sulphur, all of which are liable to pass by oxidation and hydration into sulphuric acid. Chlorine and hydrochloric gases are also common, particularly at high temperatures. Fluorine and other gases are occasionally present. Certain gases, such as hydrogen and chlorine, are especially associated with high temperatures and energetic action, and are probably dependent on them. Hydrochloric acid and the sulphurous gases are also mainly associated with high temperatures, while sulphuretted hydrogen is commoner at lower temperatures. Oxygen, nitrogen, and probably carbon dioxide or carbon monoxide are present throughout all ranges of temperature. Nitrogen is a rather frequent but not very abundant constituent of the volcanic gases. How far it results from admixture of the atmosphere and how far it is original, is not determined. It is, however, one of the gases found in volcanic rocks after they have cooled, and is presumably original in part. A large series of secondary vapors naturally arise from the volatilization of substances contained in the lavas, such as the oxides, chlorides, and sulphides of the metals, etc.
Residual gases in volcanic rocks.—Some light upon the vital question of the original, as distinguished from the secondary gases of lavas may be found in the analyses of the gases that remain in the lavas after they are solidified. When the lavas lodged underground without free communication with the surface, there is reason to think that they retained a larger percentage of their original gases in solidification than in cases of free exposure at the surface; at any rate, such rocks contain notable quantities of gases occluded in some way within themselves. Recent surface-lavas also contain gases of similar kinds, but not in equal degree, so far as available analyses show. The gases are in part held in numerous small cavities within the constituent minerals, especially in the quartz. This is perhaps due to the fact that quartz usually crystallizes late in the process of solidification, and its mother-material becomes crowded with gases excluded by the previous crystallization of other minerals. Analyses of twenty-five crystalline rocks of various kinds from many typical localities by Tilden,[285] gave an average volume of gas, under ordinary atmospheric pressure, four and a half times that of the containing rock. This shows the condensed condition in which the gases are held. Of these gases, the chief is hydrogen, which much exceeds all the rest. Next in order of abundance is carbon dioxide, followed by carbon monoxide, marsh gas (CH4), and nitrogen. Water is frequently present and free oxygen almost universally absent. The average ratio of hydrogen to carbon dioxide by volume in these analyses is about 70 : 30. Five complete analyses gave the following averages: H2, 52.134; CO2, 34.104; CO, 8.422; CH4, 3.224; N2, 2.072. It will be seen that the gases contained in these rocks are in proportions radically different from those of the atmosphere, and it is doubtful whether they can be reasonably assigned to any other source than the lavas from which the rocks were formed. It is to be noted, however, that some sedimentary and meta-sedimentary rocks, such as quartzite and quartz-schist, contain similar gases, but this may be because the granules of the original rock retain them, notwithstanding the secondary processes through which they have passed. Analyses of meteorites show essentially the same gases in much the same proportions. If evidence of this kind can be trusted, the standard original gases of lavas are the elements or compounds of hydrogen, carbon, and nitrogen, in the order named, while the chlorine and sulphur gases are to be regarded as accessory. Because of their intensely energetic and noxious character, these latter gases make themselves disproportionately manifest in the vicinity of active volcanoes. That they are really not preponderant seems to be implied by the fact that the volcanic rains, which are extremely copious, are usually fresh, and only in rare cases is the presence of the hydrochloric or sulphurous elements sufficient to produce noxious effects. Volcanic and meteoric data seem to indicate that steam is held less tenaciously than the other gases in the magmas as they solidify into rocks.
The source of the gases.—As already noted, it is one of the outstanding problems of geology to determine (a) how far the gases of lavas were possessed by them from their origin, whatever that may be, and (b) how far they have been acquired in the lava’s ascent to the surface. It is recognized that lavas have the power of absorbing gases, and one of the views entertained is that surface-waters, percolating through the rocks and coming in contact with the ascending column of lava, are converted into steam, which is absorbed into the lava and rises with it to the surface. There are two phases of this view. (1) The more conservative one supposes that the water merely penetrates the fracture zone of the surface of the earth through the ordinary means of passage of underground-waters, and so makes a comparatively short circuit. Under this view, the steam and other gases given forth are not a contribution to the atmosphere and hydrosphere, but merely a restoration to them of water and dissolved gases previously carried down from the surface. An even narrower view is sometimes entertained which supposes that the larger part of the water descends through the volcanic cone itself, or immediately about its base. The presence of chlorine gases in the volcanic emanations and the nearness of most existing volcanoes to the sea have been the basis for the idea that sea-water, penetrating to the lava, is a chief source of the volcanic gases. (2) The broader phase of the view assumes that the waters penetrate not only the outer fracture zone of the earth, which is probably limited to five or six miles in depth, but that they diffuse themselves through the continuous unfractured zone down to depths where temperatures of fusion prevail, and that they there enter into combination with the lavas or with hot rock to form lavas. It is well known that aqueous vapor facilitates the fusion, or more accurately, the mutual solution of the minerals. This view is a part of one of the hypotheses concerning the origin of the lavas themselves.
The opposing view supposes that the gases were in the main original. Of this view there are two phases: (1) One supposes that the lavas are remnants of an original molten globe which absorbed gases from the primitive atmosphere and retained them till the time of their eruption. The possible absorption of steam and air into the supposed molten globe has been much neglected in current conceptions of early conditions. If the lavas of the supposed molten globe absorbed proportionately as much water-vapor as the volcanic lavas often contain, it would probably take forty or fifty times the present ocean and atmosphere to supply them. Any remnants of these original lavas might well be supposed to hold gases. Even rocks derived from them by deep-seated solidification might retain much gas. (2) The other phase of the view assumes that the gases were entrapped when the globe was built up of meteoroidal or planetesimal matter, as assumed in the accretion hypothesis.
Under either of the last two views the gases may be said to be primary, and genetically connected with the origin of the lavas themselves. Such gases would be a contribution to the atmosphere and the hydrosphere. This view does not exclude the idea that as the lava rises through the surface-rocks, other gases are formed by contact, and that they may be absorbed into the rising column. On the contrary, the view recognizes the possibility that a tongue of lava rising into the upper formations may encounter bodies of water, or masses of thoroughly water-soaked rock, from which great quantities of steam may be generated, and that this accessory steam may be a large factor in the initial explosion which often accompanies the development of a new volcano, or the new eruption of an old one after a long period of quiescence.
A decision on the vital question whether the volcanic gases are largely primary, or are essentially secondary, has not yet been reached; but it will doubtless be reached when a sufficient number of really representative analyses of volcanic gases have been made, and when the phenomena of the gases occluded in igneous rocks have been thoroughly investigated.
The peculiar proportions of the rock-gases, in which hydrogen and carbon dioxide so greatly preponderate, seem to imply that they are not derived from the atmosphere; at least if they were so derived, there must have been a selective absorption of a most remarkable kind, because hydrogen is present in the atmosphere in exceedingly small quantities, while carbon dioxide is a very minor constituent. At the same time, as already remarked, no free oxygen is usually found in these absorbed gases.
The question as to whether the larger part of the volcanic gases is original or is merely a special form of convective circulation, has an important bearing on the supply of the atmosphere, which is constantly being depleted by the oxidation of the rocks and by the formation of carbonates and carbonaceous deposits. This vital phase of the subject will receive further consideration. While recognizing the lack of decisive proof, it would seem that the preponderance of evidence lies in favor of the view that a notable portion, at least, of gaseous volcanic emanations is derived from the interior of the earth, and is really a contribution to the atmosphere and the hydrosphere. The hydrogen, on coming in contact with the atmosphere, ignites and adds itself to the hydrosphere. The carbon dioxide is in part decomposed by plants, and adds to the supply of atmospheric oxygen. The nitrogen, being comparatively inert, doubtless gradually accumulates in the air and has thus come to be its preponderant constituent.