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Chapter IX — THE MOVEMENTS AND DEFORMATIONS OF THE EARTH'S BODY (DIASTROPHISM) | Index | Chapter XI — STRUCTURAL (GEOTECTONIC) GEOLOGY |
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The term vulcariism is here used to include all movements of lava toward the surface of the earth, as well as certain other phenomena connected with these movements. The rise of lava assumes two general phases. The one includes movements by which lava reaches the surface, giving rise to eruptive or volcanic phenomena; the other, movements by which lavas intrude themselves into the outer formations of the earth and congeal underground. The first gives rise to volcanic rocks, and the second to plutonic rocks. The first are extrusive; the second, intrusive; the first constitute eruptions; the second, irruptions. The fundamental nature of the two phases of vulcanism is the same.
1. Intrusions
Fluid rock forced into fissures and solidified there forms dikes; forced into chimney-like passages, it forms pipes or plugs; insinuated between beds, it forms sills; accumulated in considerable bodies which arch the strata up over them, it forms laccoliths (Fig. 6) ; if the overlying beds are faulted up, bysmaliths, while in greater aggregations underground, it constitutes batholiths. Dikes (Fig. 289) and sills vary greatly in thickness, from inches to hundreds of feet. Batholiths may be many miles across, but their depths are not known. Laccoliths and bysmaliths may be looked upon as small batholiths with special features.
The heating of the adjacent rock by intrusions varies with the mass and temperature of the lava. Thin dikes and sills produce little effect, while greater masses metamorphose the adjacent rock notably. The metamorphism results in part from (1) the heat, in part from (2) the pressure incident to the intrusion, and in part [p. 368] from (3) the chemical changes stimulated by the heat, water, and gases issuing from the lava, and by pressure in the presence of ground-water.
The total amount of lava which has risen toward but not to the surface far exceeds all that has flowed out at the surface. Intrusions are usually seen only after erosion has removed the rocks which overlay them.
There appear to be certain cases where the intrusion comes so near the surface as to develop explosive phenomena without the extrusion of lava. From the nature of the case this is an inference rather than a demonstration. It is certain, however, that occasional violent explosions take place where no lava comes to the surface. The explosion may be due to the intrusion of lava, or it may be [p. 369] due to the penetration of surface-waters to hot rocks that have remained uncooled from previous volcanic action. The contact of the water with the hot rock may develop a volume of confined steam sufficient to cause the explosion. A case of this kind occurred in 1888 at Bandai-San in Japan, where there was a sudden and violent explosion which blew away a considerable part of the side of a volcanic mountain which had not been in eruption for at least a thousand years. The mass and violence of the exploded material was such as to fill the air with ashes and debris in a fashion altogether similar to a typical volcanic eruption. The eruption was confined to one explosion, and within a few hours the cloud of dust had disappeared and the phenomenon was ended. No lava was extruded.
Another illustration is perhaps furnished by Coon Butte, Arizona.[1] This “butte” consists of a rim of fragmental material encircling a crater-like pit from which the fragments were obviously ejected by an explosion. The pit is in sedimentary strata, and the material of the rim is composed of fragments of the sedimentary rock thrown out of the pit. The volume of the material in the rim is in keeping with the size of the pit. No igneous rocks appear in the pit or about it, though there has been igneous action in the vicinity. The cause of the explosion is not demonstrable, and it may be an error to connect it with an intrusion of lava below. Fragments of meteorites were found about the butte, but this association may be accidental or causal. It has been suggested that a large meteorite fell at the site of the butte, and penetrating the earth a few hundred feet, exploded.[2] This sequence of events would account for the pit and the rim.
2. Extrusions
When molten rock is forced to the surface’ it gives rise to the most intense and impressive of all geological phenomena. The energies acquired in the interior under great compression here find sudden relief. Enclosed gases often expand with extreme violence, [p. 370] hurling portions of lava to great heights and shattering them into fragments, special forms of which are called bombs, cinders, ash, etc., all of which constitute pyroclastic material. Much of the explosive violence of volcanoes has been attributed to the contact of the hot rising lava with ground-water, but the function of groundwater in the explosions has probably been exaggerated.
There are two phases of extrusion, and at their extremes they are strongly contrasted. The one is explosive ejection, often attended with great violence; the other a quiet out-welling of the lava. More or less closely related to these two phases of extrusion are two classes of conduits, the one, restricted openings, often pipes, ducts, or limited fissures, from which the amount of lava extruded is relatively small, and hence it congeals near the orifice, forming cones; the other, great fissures out of which the lava pours in great volume and from which it spreads over wide tracts, often in broad thin sheets. There is no fundamental difference between great fissure eruptions and the eruptions of restricted vents, and the two types blend. The extent of the spreading of lava into thin sheets is due more to the mass and the fluidity of the lava than to the form of the outlet. The stupendous outflows of certain geologic periods appear to have issued mainly from extended fissures.
a. Fissure Eruptions
The chief known fissure eruptions of recent times are the vast basaltic floods of Iceland; but at certain times in the past there have been prodigious outpourings of lava, flow following flow until formations thousands of feet thick and covering thousands of square miles, were built up. One of these occurred in Tertiary times in Idaho, Oregon, and Washington (Fig. 290), where about 200,000 square miles were covered with sheets of lava, aggregating in places some 2,000 feet in thickness. Still earlier, in the Cretaceous period, there were enormous flows on the Deccan, covering a like area to the depth of 4,000 to 6,000 feet. Still earlier, in the K< weenawan period, an even more remarkable succession of lavaflows in the Lake Superior region developed a series of igneous rocks of almost incredible thickness. In these cases there is little evidence of explosive or other violent action. There are but few [p. 371] beds of ash, cinders, or other pyroclastic material. The inference is, therefore, that the lavas welled out quietly, and flowed over the surrounding country. For the most part these wide-spreading flows are composed of basic material, which is more easily liquefied and more fluent at a given temperature than the acidic lavas. The latter are more disposed to form thick bodies near the point of extrusion.
Massive outflows of this class are the greatest examples of extrusions, though they are not now the dominant type. It has been thought that the volcanic type of extrusion followed the fissure eruptions as a phase of decline; but this view has not been substantiated.
b. Volcanoes
A volcano is a circumscribed vent in the earth’s crust, out of which hot rock, gases, and vapors issue. The material is generally built up into mounds or cones (Figs. 291-293). These cones are often called volcanoes, though they are really the results of volcanic activity. So long as a volcano is active there is likely to be a depression, or crater (Fig. 294), in the summit of its cone. The crater connects downward with the source of the lava at an unknown depth. Craters may be a mile or more across, but they are usually smaller, some much smaller. After sufficient erosion, [p. 372] extinct volcanoes show that the former passageways, leading down toward the sources of the lava, vary much in size and shape, and usually have diameters smaller than the craters.
The exact number of volcanoes now active cannot be stated precisely, because most volcanoes are in action only at more or less distant periods, and it is impossible to say whether a volcano that is now quiescent is extinct or only resting. It is quite safe to include at least 300 in the active list, and the number may reach 350 or more. The number that have been active so recently that their cones remain distinct is several times as great.
Distribution of Volcanoes
1. In time. In the earliest known ages, igneous action appears to have been very widespread. No great area of the oldest (Archaean) rocks is now known where the formations are not Largely igneous, either of the intrusive or of the extrusive kind. From the Paleozoic to the present, the distribution of volcanic action over the surface seems to have been, in a general way, much what it is to-day; that is, certain areas were affected at times by volcanoes, [p. 373] while other and larger areas had few or none. This is not equally true of all periods, as will be seen in the historical studies that follow. There were periods when volcanic activity was widespread and energetic, and other periods when it was limited in amount and in distribution. The known facts do not indicate a steady decline in volcanic activity, but rather a periodicity; at least this is so for the portion of the globe that is now known well enough to warrant conclusions. One of the greatest of the volcanic periods falls within the Cenozoic era, just preceding the present geological period, and the volcanic activity of the present is perhaps but a declining phase of the activity of that time.
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2. Relative to land and sea. The active volcanoes of the present time are located chiefly along the borders of the continents, and within the great oceanic basins (Fig. 295). On this account, the sea-water has often been supposed to have some causal connection with volcanic action, and the presence of chlorine in the volcanic gases has been urged in support of this view. Volcanoes, however, are not distributed so equably and exclusively about the several oceans as to give this conclusion force, though the basins, as basins, probably favor vulcanism. Volcanoes are numerous within and around the Pacific, the greatest of the oceans; but they are not especially abundant in or about the Atlantic, while they are numerous in and around the Mediterranean, a much smaller body of water. On the other hand, there are existing or very recent volcanoes in the interior of Asia, Africa, and America. If volcanoes were dependent upon proximity to the sea, they should have had close relations to it in the past, as much as now; but in recent periods there has been much volcanic activity in the plateau region and even in the plains region of western America, and in the heart of Asia and Africa, far from the ocean. In older periods, it is still less clear that there was any connection between volcanoes and surface waters.
3. Relative to crustal deformations. The distribution of present and recent volcanoes is much more suggestively associated with those portions of the crust that have undergone movement in comparatively recent times, or are still moving. The great mountain belt stretching from Cape Horn to Alaska and thence onwards along the east coast of Asia is dotted with active and recently extinct volcanoes. The tortuous zone of mountainous wrinkles that borders the Mediterranean, and stretches thence eastward to the Polynesian Islands, is another notable volcanic tract. These two belts include the greater number of existing and recent volcanoes on the land.
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4. In latitude. The distribution of volcanoes appears to have no specific relation to latitude. Mounts Erebus and Terror amid the ice-mantle of Antarctica, and Mount Hecla in Iceland, as well as the numerous volcanoes of the Aleutian chain, give no ground for supposing that volcanoes shun the frigid zones, while the numerous volcanoes of the equatorial zone imply that they do not avoid the torrid belt.
5. In curved lines. In the Antilles, the Aleutian Islands, the Kurile Islands, and in some other tracts, there is a linear arrangement of volcanoes, with appreciable curvatures, the convexities of which are turned toward the adjacent ocean. In other cases there is a linear arrangement without appreciable curvature, as in the Hawaiian range. Less often, volcanoes are bunched irregularly, as in some of the groups of volcanic islands of the Pacific (Fig. 295).
The Relations of Volcanoes
A significant feature in connection with volcanoes is the apparent sympathy between adjacent vents in some cases, and their entire independence in others. The recent outbursts in Martinique and St. Vincent, and the concurrent symptoms of activity in other places, seem to point clearly to sympathy. On the other hand. the independence of neighboring vents is sometimes extraordinary, as those of Mauna Loa and Kilauea in Hawaii. These two volcanoes are only about twenty miles apart, the one on the top and the other on the side of the same great mountain mass. The crater of Loa is about 10,000 feet higher than that of Kilauea, and yet, while the latter has been in constant activity as far back as its history is known, the former is periodic. The case is the more remarkable because of the greatness of the ejections. The outflow of Mauna Loa in 1885 formed a stream 3 to 10 miles in width, and 45 miles in length, with a probable average thickness of 100 feet, and some [p. 377] of its other outflows were nearly as massive. Besides this massiveness, there were extraordinary movements of the lava within the crater, if the testimony of witnesses may be trusted. But throughout these great movements in the higher crater, the lavacolumn of Kilauea, 10,000 feet lower, continued its quiet action without sensible relation to its boisterous neighbor. No difference in specific gravity that could at all account for a difference in height of 10,000 feet has been observed or can be presumed. It seems a necessary inference, therefore, that the two lava-columns have no connection with each other, or with a common reservoir. The tops of some lava-columns stand about 20,000 feet above the sea, while others emerge on the sea-bottom far below sea-level. The total vertical range of emergence is between 30,000 and 40,000 feet, a difference which tells its own story as to their relative independence.
Trivial agencies. Eruptions seem to be somewhat more common when atmospheric pressure is high than when low, doubtless because the increased atmospheric weight on a large area of the crust, aids in forcing out the lava or the volcanic gases. This can only be effective when other forces have almost accomplished the result. Eruptions seem also to be more common when tidal strains favor them, for like reasons. In the same class are probably to be put the effects of heavy rains. Such factors are to be regarded as mere incidents, of no moment in the real causation of vulcanism, but of some value in determining the precise moment of eruption.
Periodicity. Most volcanoes are intermittent in their action, long periods of dormancy intervening between periods of activity. Volcanoes supposed to be extinct may renew their activity, occasionally with terrific violence. Their periodicity awaits an explanation, but the temporary quiet very likely means an exhaustion of the supply of gas or of lava, or of both, to which the active stage is due.
Products of Volcanoes
Pyroclastic material. The fragmental materials which are blown out of a volcano are, as a rule, portions of lava which solidified before ejection, or during their flight in the air. Masses of rock [p. 378] tons in weight are sometimes thrown out, and from masses of such size, the fragments grade down to minute particles of dust. The dust particles (often called ash) are thrown high into the air in some cases, and, caught by the winds, are shifted incredible distances, as already noted (p. 89). While, therefore, the fluid lava and the larger fragment al materials ejected from the volcano stay near the vent, the fine materials are scattered broadcast.
Liquid rock, lava. The term lava is applied to all kinds of liquid rock which issue from a volcano, and also to the solid rock formed when this congeals. Lava never flows so freely as water, and it is sometimes very stiff or viscous. The distance to which it flows depends on its liquidity, its amount, and the slope of the surface on which it is poured out. The more fluid the lava, the greater its amount, and the steeper the slope on which it flows, the farther it will move.
As lava flows, its upper surface may cool so much as to become [p. 379] hard while the interior is still fluid. The fluid part may then break out at the side or end of the hardened shell and flow away, leaving a hollow crust of solidified lava. On further cooling, the shell contracts and cracks, and often caves in. The hardened surface of a lava-flow may be broken by the movement of the fluid lava below, and the solid fragments be displaced and upturned so as to give the surface a jagged appearance.
Lava takes on various phases as it becomes solid. If it hardens under little pressure, or at the surface, the gases and vapors which it contains may expand so that it is converted into a sort of solidified rock froth, called scoria; or if the pores are very small, pumice. If the lava solidifies quickly without becoming frothy, it usually makes volcanic glass or obsidian. If the lava cools slowly under pressure, the substances of which it is composed usually crystallize into various minerals. The kinds and proportions of the minerals depend chiefly upon the composition of the lava.
Gases and vapors. The gases and vapors which issue from volcanoes are of many kinds. Among the commoner ones are those of water (H2O), carbon dioxide (CO2), chlorine (Cl), hydrochloric acid (HCl), sulphur dioxide (SO2), and hydrogen sulphide (H2S); but with these more important ones there are many others. Some of the gases are poisonous, and, as in the case of Pelee, their temperature is in some cases so high as to be destructive to life.
Formation of Cones
Lava-cones. The lava usually flows away from the vent in streams which solidify before running far. As the lava-streams flow in different directions at different times, the total effect is a low cone formed of tongues of lava radiating from the point of exit. The streams often congeal before they reach much beyond the base of the cone, and not rarely while they are yet on its slope. So far, therefore, as the volcanic cone is formed of lava, it has a radiate structure made up of a succession of congealed lava-streams. In these cases the slopes are low, because the fluidity of the lava prevents the development of high gradients. It is, however, the exception rather than the rule, that the cone is made up mainly of lava-streams, though the great Hawaiian volcanoes are of this class.
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The form of the cone, when composed chiefly of lava, is also affected by the mass of the outflow and by its fluidity. Other things being equal, the larger the outflow at a given time, the more widely it distributes itself, and the flatter the cone. As a rule, basic lava cones are flatter than the cones of acidic lavas.
Cinder-cones. The larger portion of the lava blown into the air by the expanding gas-bubbles falls back in the immediate vicinity of the vent and builds up a cinder-cone. From the nature of the case, this fragmental matter is often disposed symmetrically, making a cone with steep slopes (Fig. 291).
Small or temporary vents formed as offshoots from the main vents often give rise to secondary or “parasitic” cones. These are sometimes numerous, as in the case of Etna, and they may be so important that a volcanic mountain becomes a compound cone. A still more subordinate variety consists of “spatter-cones” formed about small vents that eject little dabs of lava which form chimneys, cones, domes, etc. Spatter-cones (Fig. 297) often arise from the surface of the lava-flows themselves.
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From most existing volcanoes both lava-flows and fragmental ejecta are given forth, and the resulting cones are composite in material. The lava breaks through the side of the cone more frequently than it overflows the summit, and this gives rise to irregularities of form and structure. The cones are also subject to partial destruction both by outbursts of lava and by explosions.
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As a result, many volcanic regions show old, partially destroyed craters, as well as new and more perfect ones.
In violent eruptions, the steam, accompanied with much ash, is shot up to great heights, often rolling outwards in cumulus or cauliflower-like forms (Fig. 298). In the more violent explosions, these columns are projected several miles. In the phenomenal case of Krakatoa, the projection was estimated at seventeen miles. The steam, by reason of its great expansion as it rises, and by its contact with the colder air, is condensed quickly, and prodigious floods of rain frequently accompany an eruption. This rain, carrying down a portion of the ash and gathering up much that had previously fallen, gives rise to mud-flows, which in some cases constitute a large part of the final deposit. These mud-flows lodge chiefly on the lower slopes of the cone or adjacent to its base.
A portion of the finer exploded material floats away in the air to greater or less distances, and forms widespread tufa-deposits. In some cases, beds of volcanic ash many feet in thickness (as those of Nebraska) are found far from any known volcanic center. The extremely fine ash from the great explosion of Krakatoa floated several times around the earth in the equatorial belt, and spread northward into the temperate zones.
Lavas
Their nature. The nature of lavas and of the rocks derived from them was discussed in chapter II. In view of prevalent misconceptions, it may be repeated that lavas are solutions of mineral matter in mineral matter, rather than simply melted rock. Gases, as well as rock materials, enter into this mutual solution. The distinction between such solutions and molten rock is not very sharp, but it is essential to know that the order in which the minerals crystallize from lavas is not dependent simply on their melting temperatures. It appears rather to depend on the order in which the solution becomes saturated with the constituents of each of the several minerals. For example, quartz, which has a very high melting-point, often crystallizes out from the lava much later than minerals which have lower melting temperatures. The solutions are exceedingly complex, and include a wide range of chemical [p. 384] substances. Chief among them, as already stated, are silicates of aluminum, potassium, sodium, calcium, magnesium, and iron, with minor ingredients of nearly all known substances. The stages at which saturation for certain compounds is reached vary somewhat widely.
The old idea that lava is melted rock, is not, however, to be abandoned wholly. Lava sometimes solidifies much as water freezes. Thus when lava is suddenly cooled, the congelation is essentially the solidification of a melted substance. The result is a glass, every part of which has essentially the same composition that the liquid had. Even in this case, however, some of the gases escape. If the cooling is slower, the various substances in the mixture crystallize out into minerals in the order in which they severally reach saturation. This involves the principle that solubility is dependent on temperature, and that as the temperature sinks the degree of solubility declines, and the saturation-point for some constituents of the solution is reached earlier than that for others. With sufficiently slow cooling, all the material passes into the solid state by the crystallizing of the several minerals in succession. This does not mean that two or more minerals may not be forming at the same time, but it does mean that some minerals may be crystallized out while the surrounding material is still fluid. In most igneous rocks, nearly perfect crystals of certain minerals are common, while other minerals, crystallizing later, adapt themselves to the space left. This conception is supported by the fact that lavas, while still in the fluid condition, often contain wellformed crystals, and these crystals sometimes make up a considerable part of the flowing mass, very much as water in certain conditions may be filled with crystals of ice.
The temperature of lava. Accurate determinations of the temperatures in the centers of lava-columns, where they have been least reduced by contact with the rock-walls, have not been made; but it is clear from the white heat of some lavas that their temperatures are often appreciably above the melting-point. This is also a necessary inference from the length of time lavas remain fluid. notwithstanding the great surface of contact of the column in its miles of ascent, the conversion into steam of the water in the rock [p. 385] through which the lava passes, and the expansion and escape of the gases. In cases where determination has been practicable, it has been found that the melting-points of silver (about 960° C.) and copper (about 1,060° C.) are reached. From these and other facts it is probably safe to assume that the original temperatures of the lavas as they rise to the surface are sometimes considerably above 2,000° Fahr. (1,093° C). Even such a temperature must be somewhat below the original temperature of the lava, because some heat must be lost in rising, both by contact with the walls of the colder rocks, and by the expansion of the gases within them. If any considerable part of these gases is derived from waters which join the lava in its upward course, the energy consumed in raising the water to the high temperature of the lavas must be subtracted from the original heat, and must be a further source of reduction of temperature. It seems probable that temperatures as high as those necessary for ordinary fusion, and perhaps even higher, are attained by the lavas at their sources.
Depth of the source of lavas. Attempts have been made to determine the depth from which lavas rise, by calculations based on the earthquake tremors that accompany eruptions; but such calculations really tell very little concerning the true point of origin of the lava. At most they probably tell merely where the ascending lava begins to rupture the rock through which it passes, and rupture may not be possible below the zone of fracture, which is probably not more than six miles deep. In the zone of flowage below, where the pressure is too great to permit fracture, the lava not improbably makes its way by some boring or fluxing process, which might not be capable of giving rise to seismic tremors. The tremors perhaps compel us to place the beginning of movement of lava at least as low as the bottom of the fracture zone, but they probably offer no sufficient ground for limiting the lava’s origin to this or any other specific depth.
Volcanic Gases
One of the most distinctive features of volcanoes is the explosive action arising from the gases and vapors pent up in the lava. The precise way in which they are held has not been determined. It has [p. 386] been thought that lava spontaneously absorbs gases, especially when the gases are under great pressure, and that as the pressure ifi relieved and the lava cooled and solidified, the larger part of the gases escape. 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 off after the lava has been extruded and is about to congeal. The gases are then given off slowly and quietly. If, however, the lava is surcharged with gases, and if their escape is retarded by the viscosity of the lava, they gather in large vesicles or bubbles 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 the explosion reduces a portion of the lava to the fineness of dust, — the “ash” and “smoke” of the volcano. Portions of the lava may be inflated by the gases without being blown to bits, forming pumice and scoria, as already noted. Masses of lava that have solidified into more or less rounded masses in the crater are hurled out as bombs, and not infrequently portions of the walls of the crater or of the duct below are broken off 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) 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; (3) probably a main occasion of violent explosions lies in the fact that the lavas have begun to crystallize while yet in the volcano. When the crystals form in the magma;, they exclude the gases which were in the substance from which they are developed, and this excluded gas overcharges the remainder of the lava. This view is supported by the fact that the pumice and ash of such extraordinarily explosive eruptions as those of Krakatoa and Pele*e contain many small crystals which had certainly formed before the explosion took place. Incipient crystallization does not, however, appear to be a universal accompaniment of explosive action.
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The discharge of the gases is often spasmodic, and usually consists -of a succession of distinct explosions. In some cases the explosions follow one another at rather constant and frequent intervals, as in Stromboli, where they occur at intervals of three to ten or more minutes. In others the spasms are distant and irregular.
Kinds of gases. Steam is the chief volcanic gas. Free hydrogen and oxygen are present also, and are perhaps the result of the dissociation of steam at the very high temperature of the lava. Carbon dioxide is probably next in abundance, and carbon monoxide is present. Sulphur gases (sulphuretted hydrogen, sulphurous acid, and perhaps sublimated sulphur) are common accompaniments of volcanic eruptions. All of the sulphurous gases are liable to pass into sulphuric acid by oxidation and hydration. Chlorine and hydrochloric gases are also common, particularly at high temperatures. Certain gases, such as hydrogen, chlorine, hydrochloric acid, and some of the sulphurous gases, are especially associated with high temperatures, and are perhaps dependent on them. Sulphuretted hydrogen, on the other hand, is commoner at lower temperatures. Oxygen, nitrogen, and probably carbon dioxide or carbon monoxide are present throughout all ranges of temperatures. The gases mentioned above are the more abundant ones in lavas, but the list is not exhaustive.
Gases in volcanic rocks. Igneous rocks contain gases, often in large quantities.[3] When the lavas lodge underground without free communication with the surface, there is reason to think that they retain a larger percentage of their original gases than the lavas which are freely exposed at the surface. At any rate, deep intrusive rocks contain notable quantities of gases. Recent surface lavas also contain gases of similar kinds, but not in equal amount, so far as available analyses show.
Source of the gases. One of the outstanding problems of geology is to determine (a) how far the material of the gases had the same origin as the material of the lavas, and (b) how far the material for the gases penetrated from the surface.
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The peculiar proportions of the rock-gases, among which hydrogen and carbon dioxide so greatly preponderate, seem to imply that they are not derived chiefly from surface waters or the atmosphere; they appear to be original constituents of the rocks in the main, and when given forth they appear to constitute real additions to the atmosphere.
The Cause of Vulcanism
The explanation of the extraordinary facts involved in volcanic phenomena is wrapped up in that of the origin of the earth, for the agencies which the earth inherited from its birth are beyond doubt factors in vulcanism. This phase of the subject will be treated only briefly here.[4]
The explanation of vulcanism involves two essential elements, (1) the origin of the lavas, and (2) the forces by which they are expelled. The . current explanations of vulcanism fall into two general classes: (1) those which assume that the lavas are residual portions of an original molten mass, and (2) those which assign the lavas to the local liquefaction of rock.
The view that the lavas are residues of an original molten globe formerly prevailed, but is found to encounter grave difficulties because of the independent action of vents which are closely adjacent to one another. When the lava columns vary thousands of feet in height on the same mountain mass, as in the Hawaii volcanoes, even a resort to the hypothesis of local residual reservoirs is not altogether satisfactory. Another view which has had much currency supposes that surface water and its absorbed gases penetrate to heated rock and are absorbed by it, rendering the whole liquid, and that the lava thus formed is then forced to the surface. The progress of investigation, however, does not support the belief that water penetrates from the surface to depths below the zone of fracture, and hence is far from reaching highly- heated rocks. The gases of volcanoes and of igneous rocks are not sufficiently like those of water to support this view.
The relief of pressure, which lowers the melting point of rock, [p. 389] when felt by rocks already heated above what would be their melting points at lowered pressures, has been held to be a possible cause of vulcanism. Such relief of pressure is assigned to faulting and denudation. But many volcanoes are located in the bottom of the ocean, where denudation does not take place, and faulting that would give relief of pressure is not always related to vulcanism in any clear way. Melting by crushing has been suggested, but in the deeper parts, crushing involves increase of pressure, which opposes melting. Depression to the zone of high temperature, under accumulated sediments, is also assigned as a cause of melting, but there is very little sedimentation in the ocean far from land where many volcanoes are situated.
If the earth grew up by slow accessions of matter, and if its interior heat is due chiefly to the internal compression resulting from growth, the distribution of internal temperature would be as shown in Fig. 288, p. 362. With like conductivity, the flow of heat from the deep interior to the middle zone of the earth would be greater than the loss from this zone to the superficial shell. The middle zone might thus rise in temperature. This zone is, under this view, supposed to be composed of various kinds of matter, mixed as they happened to fall in. If the temperature rises, the fusion-points of some of these constituents will be reached sooner than those of others. A fusion or solution of the more soluble portions may thus take place while the rest of the rock remains solid. To the liquid part the gases and volatile constituents in the original material would obviously unite, as being also liquefiable parts. With a continued rise of temperature, the liquefaction would extend itself until adjacent pockets or threads of lava found means of uniting, and the lighter portions of the fluid would be forced upwards and work their way toward the surface by fusing and fluxing.
As these portions rise, the pressure upon them becomes less and less, and hence the temperature necessary for liquefaction gradually falls, leaving them a constantly renewed margin of temperature available for melting their way through the upper horizons. Thus it is conceived that these fusible and fluxing selections from the middle zone might thread their ways up to the zone of fracture, [p. 390] and thence, taking advantage of fissures and fractures, reach the surface (Fig. 301). It is conceived that such liquefaction and extrusion would carry the excess of temperature received by the middle zone from the deeper interior, out toward the surface, or even to it. The outward movement of the lava would tend to regulate the temperature of the middle zone, forestalling general liquefaction, and keeping the zone as a whole solid. The independence of volcanoes is assigned to the independence of the liquid threads that work their way to the surface. Nothing like a reservoir or molten lake enters into the conception. The prolonged action of volcanoes is attributed to the slow feeding of the liquid threads from the middle zone, which is liquefied in spots only. The frequent pauses in volcanic action are assigned to temporary deficiencies of supply, and the renewals to the gathering of new supplies after a sufficient period of accumulation. The distribution of volcanoes in essentially all latitudes and longitudes is assigned to the general nature of the cause. The special surface distribution is assumed to be influenced, though not altogether controlled, by the favorable or unfavorable conditions for escape presented at different places. The persistence of volcanic action in time is attributed to the magnitude of the interior source, to its deepseated position, and to the slowness of conduction of heat from the earth’s interior. The force of expulsion is found in the stressdifferences in the interior, particularly the periodic tidal and other astronomic stresses, and in the slow pressure brought to bear on the slender threads of liquid by the creep of the adjacent rock. The violent explosions are due to the included gases, of which steam is chief. Little efficiency is assigned to surface-waters, and that little is regarded as secondary and incidental. The true volcanic gases are regarded as coming from the deep interior, and as being after expulsion, accessions to the atmosphere and hydrosphere. The standing of the lavas in volcanic ducts for hundreds and even thousands of years with only little outflow, as in some of the bestknown volcanoes, is regarded as an exhibition of an approximate equilibrium between the hydrostatic pressure of the deep-penetrating column of lava, and the flowage-tendency of the rock-walls, the outflow being also conditioned on the slow supply below, and on the periodic stress-differences of the interior.
[p. 391]
For the present, volcanic hypotheses must be left to work out their own destiny, serving in the meantime as stimulants of research. All but the last have been long under consideration. The recent discovery of the heating effects of radioactivity has given rise to the hypothesis that the origin of lavas is due to this cause. It seems clear that this must at least be a cooperative agency. It is too early in the new investigation to decide whether it can wisely be regarded as the sole cause or even an essential one.
Modes of Reaching the Surface
All views that locate the origin of the lavas deep in the earth must face the difficulty of the passage of lava through the zone of the earth below the fracture zone. Near the surface, the lavas usually take advantage of bedding-planes, or of fissures already existing, or made by themselves. There is little evidence that they bore their way through the zone of fracture by melting, though they round out their passageways into pipes as they use them, much as streamlets on glaciers falling into crevices round out moulins. But this use of fissures and bedding-planes for passage is probably merely a matter of least resistance where the lavas are [p. 392] relatively cool, and their capacity for melting is low, or perhaps even gone.
In the denser and warmer zone below, the alternatives seem to be (1) melting or fluxing, or (2) mechanical penetration without fracture. As rocks “flow” in this zone by differential pressure without rupture, an included liquid mass may be forced to flow through the zone by sufficient differential pressure. If local differential pressures at the surface are neglected as probably incompetent, there only remain the stress-differences of the interior, and the differences of hydrostatic pressure between the lava-column and the surrounding solid columns. The latter would not be great until a column of liquid of much depth was formed, and the former would probably not be concentrated on the liquid in such a waxas to force it bodily through the solid rock. Probably fusing or fluxing its way with the aid of stress-differences is the chief cause of the rise of lava below the zone of fracture. In this it may be supposed to be assisted by its gases, by its selective fusible and fluxing nature, by its very high temperature if it comes from very great depths, and by the stress-differences which attend tidal strains in the deep interior. In ascending from lower to higher horizons, the lava would be constantly invading regions of lower melting-point, because of lesser pressure, and thus always have an excess of heat above the local melting temperature until it invaded the external, cool zone. From that point on, the rising lava must constantly lose portions of its excess of temperature by contact with cooler rocks. If its excess of temperature is insufficient to enable it to reach the zone of fracture, the ascending column is arrested and becomes plutonic rock. If it suffices to reach the zone of fracture, advantage may be taken thereafter of fissures, and the problem of further ascent probably becomes chiefly one of hydrostatic pressure, in which the ascent of the lava-column is favored by its high temperature and its included gases. The hydrostatic contest is here between the lava-column measured to its extreme base, and the adjacent rock-columns measured to the same extreme depth. The result is, therefore, not necessarily dependent on the flowage of the outer rocks, but may be essentially or wholly dependent on the deep-seated flowage of the rock of the lower horizons. The ascending column may reach hydrostatic equilibrium before it reaches the surface, and may then form underground intrusions of various sorts without superficial eruption, or it may only find equilibrium by coming to the surface and pouring out a portion of its substance and discharging its gases.
[p. 393]
References on vulcanism. G. P. Scrope, Volcanoes, London, 1872. C. E. Dutton, Geology of the High Plateaus of Utah, U. S. Geog. and Geol. Surv., 1880. The Hawaiian Volcanoes, Fourth Ann. Rept., U.S. Geol. Surv., 1883. Judd, Volcanoes, 1881; The Eruption of Krakatoa (Com. of the Roy. Soc), 1888. J. D. Dana, Characteristics of Volcanoes, 1890. Milne and Burton, The Volcanoes of Japan, 1892. J. P. Iddings, The Origin of Igneous Rocks, Bull. Phil. Soc, Washington, Vol. XII, 1892. A. C. Lane, Geologic Activity of the Earth’s Originally Absorbed Gases, Bull. Geol." Soc. Am., Vol. V, 1894. A. Geikie, Ancient Volcanoes of Great Britain, London, 1897. I. C. Russell, Volcanoes of North America, 1897. T. G. Bonney, Volcanoes, Their Structure and Significance, New York (and London), 1899. A. Heilprin, Mont Pelee and the Tragedy of Martinique, Philadelphia (and London), 1903. Further accounts of the same volcanoes are found in the Nat’l Geog. Mag., Vol. XIII, 1902 (Russell, Hill, Hovey, Diller, and Hildebrand).
Map work. Plates CLV to CLXIV, of Professional Paper 60, U. S. Geological Survey, illustrate various topographic effects of vulcanism. The Structure Section Sheets of the folios of the Survey, and the maps of various Survey Reports show the many and diverse relations of igneous rocks.
Chapter IX — THE MOVEMENTS AND DEFORMATIONS OF THE EARTH'S BODY (DIASTROPHISM) | Index | Chapter XI — STRUCTURAL (GEOTECTONIC) GEOLOGY |