Chapter X: The Extrusive Processes (1)

=Outward movements.=—In the preceding chapters movements toward the center have been considered. The complementary processes of outward movement now invite attention. Without doubt these are mainly but a resultant of the centripetal actions. For each pound of material moved outwards an equivalent is quite surely moved inwards. Notwithstanding this, the outward movements have a peculiar nature of their own, and serve a function of radical importance in the economy of the globe. Some minor phases have been incidentally considered, such as the upward flow of springs and deep-seated waters, but here the descending and ascending factors are alike, and are closely and obviously connected.

VULCANISM.

The great example of ascensive action is the movement of fluid rock from the interior outwards. The term vulcanism will be used to embrace not only volcanic phenomena in the narrower sense, but all outward forcing of molten material, whether strictly extrusive or merely ascensive.

The philosophy of this ascensive action, taken as a whole, is simple. In the effort at concentration under the powerful action of the earth’s gravity, the material of high specific gravity is urged more strongly toward the center, volume for volume, than that of less specific gravity, and as gravity is perpetually active, it follows that whenever any movement, molecular or molar, takes place which permits a readjustment of the positions of the two kinds of matter, the heavier sinks toward the center and the lighter rises, or at least tends to do so. So also where there are stress-differences, the mobile matter tends to flow from the regions of greater stress toward those of lesser stress. In so far as any portion of the interior becomes liquid, it is free to move up or down according to the balance of stress brought to bear upon it, and adapts itself to any line of least resistance available to it. As a natural result, therefore, the portion of the interior which becomes fluid most largely participates in the outward movement. In so far as molecular action permits a readjustment of material, there is a tendency, even in the solid state, for the lighter material to move upwards and the heavier downwards, and for the more stressed portions to move toward points of less stress; but this takes place with extreme slowness. In so far as the materials of the interior diffuse themselves through each other, the same laws hold good, but they are modified by the special principles that control diffusion. The outward diffusion of interior gases may be a factor of appreciable importance, but this cannot be affirmed at present.

=Phases of vulcanism.=—The forcing of fluid rock outward assumes two general phases, which, however, merge into each other; and these main phases take on various sub-phases. The first phase embraces those outward movements of fluid rock which do not reach the surface. The lavas, after ascending to the vicinity of the surface, intrude themselves into the outer formations of the earth and congeal underground (plutonic). The second phase embraces those outward movements in which the fluid rock reaches the surface and gives rise to eruptive phenomena (volcanic). The first is _intrusive_, the second _extrusive_; the first constitutes _irruptions_, the second _eruptions_.[275] The fundamental nature of the two is the same, but the extrusions usually take on special phases because of the relief of pressure at the surface of the earth, and because of the action of surface-waters in contact with the heated lavas. Just where the lavas come from, and how they find their way through the deep-lying compact zone below the zone of fracture, may better be considered later. When they reach the zone of fracture, they usually either take advantage of fissures already formed, or force passageways for themselves by fracture. There is little evidence that they bore their way through the rocks by melting, though they appear to round out their channels in some way into pipes, ducts, and other tubular forms when they flow through them for long periods of time.

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_; bunched under strata so as to arch them upwards, it forms _laccoliths_; massed in great aggregations underground, it constitutes _batholiths_, as already described (pp. 394 and 500). Lavas sometimes crowd aside the adjacent rocks so far as to cause them to take a concentric form about the intruded mass. This is not uncommon in the oldest formations, and is probably not infrequent in the deeper horizons where the pressures are very great. Some part of this may, however, be due to later deformations. Nearer the surface, usually, the beds are merely lifted as in forming the sills, or are bowed upwards, as in the laccoliths, or faulted as in _bysmaliths_ (p. 500).

=The heating action= on the adjacent rock varies greatly with the mass and temperature of the intruded lava. Thin dikes and sills often produce little effect, while greater and hotter masses notably metamorphose the adjacent rock. In some cases marked effects are due to a thin stream of lava flowing through a fissure for a long period, and so maintaining a high temperature. In the least effective cases, the adjacent rock usually shows some signs of baking. In the marked cases, there is more or less new crystallization. The surrounding rock commonly shows some evidence of material derived from the lavas; less often the lava shows some evidence of having received material from the adjacent rock. But since the lavas do not usually bore their way through the strata in the zone of fracture, nor melt the adjacent rock, the constitution of the lavas is not appreciably changed by the kinds of rock which they penetrate. On the other hand, the intrusions often show the effects of rather rapid cooling by contact with the adjacent rock, (_a_) by a less coarse crystallization near the rock-walls, and sometimes (_b_) in a segregation of the material.

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. Occluded gases often expand with extreme violence, hurling portions of the lavas to great heights and shattering them into fragments constituting “smoke,” ash, cinders, bombs, and other pyroclastic material. Much of the explosive violence of volcanoes has been attributed to the contact of surface-waters with the hot rising lava, but the function of this kind of action has probably been exaggerated.

There are two phases of extrusion often quite strongly contrasted. The one is explosive ejection, often attended with great violence; the other, a quiet out-welling of the lava, with little more than ebullition. More or less closely related to these differences are two classes of conduits, (_a_) the one, great fissures, out of which the lava pours in great volume and spreads forth over wide tracts, often in broad thin sheets; (_b_) the other, restricted openings, often pipes, ducts, or limited fissures, from which the extrusion is usually much less abundant, and hence it more largely congeals near the orifice, forming cones. Flows from the former constitute massive eruptions; those from the latter, the more familiar volcanic eruptions. There is no radical difference between them, and the two classes blend. The extent of the spreading of lava into thin sheets is due more to the mass and the fluidity than to the form of the outlet. The stupendous outflows of certain geologic periods appear to have issued mainly from extended fissures, doubtless because these better accommodated the outbursting floods.

=_a._ Fissure eruptions.=—The chief known fissure eruptions of recent times are the vast basaltic floods of Iceland. Most of the eruptions of historic times are of the volcanic type; but at certain times in the past there were prodigious outpourings, flow following flow until layers thousands of feet thick covering thousands of square miles were built up. One of these occurred in Tertiary times in Idaho, Oregon, and Washington, where some 200,000 square miles were covered with sheets of lava, aggregating in places 2000 feet or more in thickness. Earlier than this, in Cretaceous times, there were enormous flows on the Deccan plateau of India, covering a like area to a depth of 4000 to 6000 feet. Still earlier than this, in Keweenawan times, an even more prolonged succession of lava-flows covered nearly all the area of the Lake Superior basin, and extended beyond it, and built up a series of almost incredible thickness, the estimates reaching 15,000 to 25,000 feet. In these cases there is little evidence of explosive or other violent action. There are few beds of ash, cinders, and similar pyroclastic material. The inference is, therefore, that the lavas welled out rather quietly and spread themselves rather fluently over the surrounding country. For the most part these wide-spreading flows are composed of basic material, which is more easily fusible and more highly fluent at a given temperature than the acidic lavas. The latter are more disposed to form thick embossments near the point of extrusion.

Massive outflows of this class constitute by far the greatest phenomena of the extrusive type, though they are not now the dominant type. It has been sometimes thought that the more local volcanic type of extrusion followed the more massive fissure type as a phase of decline; but this has not been substantiated.

=_b._ Volcanic eruptions.=—In the types of eruption prevailing at the present time, the lavas are forced out through ducts or perhaps short fissures or sections of fissures, and build up cones about the vents, the eruptive action maintaining craters in the centers of the cones. The essential feature of a volcano is the issuance of hot rock and gas from a local vent. A mountain is the usual result, but the mountain is secondary and not usually present in the first stages; the localized eruption is the primary and necessary factor. The amount of rock matter ejected is not necessarily great. Compared to the massive extrusions of fissure eruptions, it is usually rather trivial; but the volcano makes up in demonstrativeness what it lacks in massiveness of product.

_c._ =Intermediate phenomena.=—On the border-line between the intrusive and the extrusive phenomena there are special cases of interest. There appear to be certain instances in which 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 interpretation rather than a demonstration. It is certain, however, that occasional violent explosions take place where no lava comes in sight. This sometimes occurs in old volcanic formations, and sometimes in regions of undisturbed horizontal strata. In the former case the phenomena may be due to the intrusion of a fresh tongue of lava below, or it may be due to the penetration of surface-waters to hot rocks that have remained uncooled from previous volcanic action, and the development, by such contact, of a volume of confined steam sufficient to produce the explosion. A case of this doubtful kind occurred at Bandai-San in Japan in 1888, 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 débris in a fashion altogether similar to a typical volcanic eruption. A large tract of adjacent country was devastated, and many lives lost. The whole action, however, was concentrated in the initial explosion, and within a few hours the cloud of ashes had disappeared and the phenomenon was ended. An examination of the disrupted area revealed no signs of liquid lava.

An example of the latter class is Coon Butte in Arizona.[276] This consists of a rim of fragmental material encircling a crater-like pit from which the fragments were obviously forced by violent explosion. The pit is in ordinary sedimentary strata, and the material of the rim is composed of the disrupted fragments of the sedimentary rock ejected from the pit. There are no signs of igneous material, but there was igneous action in the vicinity. Fragments of a meteorite were found on the rim and in the vicinity, but this association appears to be accidental. Computation shows that the volume of the material of the rim closely matches the size of the pit. The source of the explosion is not demonstrable, and it may be an error to connect it with an intrusion of lava below; but since intrusions rise to various degrees of nearness to the surface, and in innumerable cases reach the surface, there is every reason to entertain the conception of a class of intrusions which develop explosive phenomena by close approach to the surface, without actually reaching it.

=Lunar craters.=—There are grounds for thinking that the remarkable craters of the moon, assuming that they are truly volcanic,[277] may belong to this class, for they are very similar to the Coon Butte pit. The capacities of the lunar craters, so far as they can be estimated, seem to equal, if they do not in many cases exceed, the volume of matter in their rims. They do not appear usually to be great cones of accumulated material with relatively small craters, like the typical products of terrestrial volcanoes. Besides, there are no clear evidences of lava-streams. The radiating tracts once interpreted as such have been shown by increased telescopic power and the resources of photography to be at least something other than lava-streams. They are vaguely defined tracts which run over heights and depths indifferently, and are plausibly interpreted as lines of débris projected to extraordinary distances because of the absence of a lunar atmosphere, and because of the low force of the moon’s gravity. Since the moon now has no appreciable atmosphere or surface-waters, and since it is doubtful whether it ever possessed either on account of its probable inability to hold atmospheric gases or the vapor of water in the form of an envelope about it, owing to its low gravity, there is reason to suppose that the external matter of the moon derived from the explosions of the multitude of lunar volcanoes would remain in a loose, incoherent condition, from the absence of dissolving and cementing agencies. It is reasonable to suppose that lava-tongues arising from the deeper interior would have a higher specific gravity, even in their heated condition, than this porous covering of the moon, and that therefore they would almost universally become intrusions rather than extrusions, or at most they would not rise beyond the bottom of the craters they had produced by explosion. This seems to furnish at least a plausible explanation of the prevailing differences between the large lunar craters encircled by mere rims and the much smaller terrestrial craters seated in relatively large cones.

VOLCANOES.

=Number of volcanoes.=—It is impracticable to state exactly the number of volcanoes that are active at the present time, because most volcanoes are periodic, and become active at more or less distant periods, and it is impossible to say whether a given volcano that may be now quiescent has really become extinct or is only enjoying its customary period of rest. It is quite safe to include at least 300 in the active list, and the number may reach 350 or more. The numbers that have been active so recently that their cones have not been entirely worn away is several times as great.

_Distribution of Volcanoes._

1. =In time.=—In the earliest known ages igneous action appears to have been very general, if not practically universal. No area of the earliest (Archean) rocks is now known which is not formed chiefly of rocks that appear to have been either intruded or extruded. Rocks which can reasonably be assigned to the hypothetical molten globe, if there be such, are not here included. It is probable that the surface of the early earth was as thickly occupied with points of extrusion as the surface of the moon appears to be. In the ages between the Archean and 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 volcanically active at times, while other and larger areas were measurably free from any outward expressions of igneous action. This is not equally true of all ages, as will be seen in the historical studies that follow. There were periods when volcanic activity seems to have been widespread and energetic, and others when it was limited both in amount and 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 well enough known geologically 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 that time.

2. =Relative to land and sea.=—At present the active volcanoes are chiefly distributed about the borders of the continents, and, less notably, within the great oceanic basins. On this account the sea has often been supposed to have some connection with volcanic action, and the presence of chlorine in the volcanic emanations has been cited in support of this position. When critically examined, however, the argument from distribution is not very strong; for the volcanoes are not distributed equally or proportionately about the several oceans, as if dependent on them. Volcanoes are especially numerous around and within the Pacific, the greatest of the oceans, and this might seem a favorable instance, but they are also numerous around and within the Mediterranean, a relatively small body of water. Volcanoes are not especially abundant in or about the margins of the Atlantic.

If volcanoes were dependent upon proximity to the sea, the relation should be close in the past as well as in the present, but this does not seem to be true. There has recently been much volcanic activity in the plateau region of western America at long distances from the Pacific basin. Even on the plains east of the Rocky Mountains notable volcanic action took place. There were also volcanoes in the interior of Asia and of Africa.

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 notable changes_ in position in comparatively recent times. The great “world-ridge” stretching from Cape Horn to Alaska and thence onwards along the east coast of Asia is a striking instance, for it is dotted throughout 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, while the great basins associated with them embrace the chief oceanic volcanoes.

There is perhaps some significance in the fact that the most active regions of vulcanism to-day lie _at the angular junctions of the great earth-segments_. The Antillean and Central American volcanic region, that has recently been so demonstrative, lies where the southern angle of the North American continental block joins the northern angle of the South American continental block, and where the western angle of the North Atlantic abysmal segment closely approaches one of the eastern angles of the great Pacific abysmal segment. The complex and very active Java-Philippine volcanic region lies where the southeastern angle of the great Asian segment projects toward the Australian block, and where the western angle of the Pacific block approaches the northeastern angle of the Indian oceanic segment. The active Alaskan volcanic area lies at the angles of the North American, Asian, Pacific, and Arctic segments. The Mediterranean volcanic area falls less notably under this generalization, but it lies where the continental blocks of Europe and Africa come into peculiar relations to each other on either side of the remarkable Mediterranean trough. The eastern angle of the North Atlantic segment is near by, but not in very close relations. The Icelandic region, small but vigorous, lies near the junction of the North American, European, North Atlantic, and Arctic segments, and the New Zealand volcanic region is somewhat less closely related to the approach of the Australian, Antarctic, Pacific, and Southern oceanic segments. Nearly all of these angular conjunctions involve two depressed segments joining two relatively elevated segments. This relationship suggests a causal connection between the intensified movements at these angular conjunctions and the intensified volcanic action of these regions. There are enough volcanoes, however, that do not fall into these groups, or apparently into any other grouping, to suggest that the development of volcanoes is not wholly dependent on any surface relationship, but that it is connected with deep-seated causes that are indeed modified, but not wholly controlled, by surface conditions, or even by the movements of the master segments of the earth’s crust.

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. On the other hand, the numerous volcanoes of the equatorial zone do not imply that they avoid the torrid belt. Their distribution appears to be independent of latitude. This is not cited because of any supposed effects of external temperature, for that must be trivial, but because it bears on the question whether strains are now arising from the supposed _slackening of the earth’s rotation_, which have any connection with volcanic action. If the oblateness of the earth is decreasing, the equatorial belt must be sinking and growing shorter, and hence must be under lateral pressure, while the polar caps must be rising, and increasing their curvatures, and should be under tension. These conditions, if real, might be supposed to have something to do with the extrusion of lava. Nothing in the present or the past distribution of igneous action seems to afford much support to this hypothetical inference.

5. =In curved lines.=—In the Antilles, the Aleutian Islands, the Kurile Islands, and in other instances, there is a notable linear arrangement of volcanoes with appreciable curvature. It has been noted that the convexity of the curves is turned toward the adjacent ocean. In some cases, however, there is a notable linear arrangement without appreciable curvature, as in the Hawaiian range, in the recently extinct line of cones of the Cascade Range, and in others. Less often, volcanoes are bunched irregularly, as in some of the groups of volcanic islands of the Pacific (Fig. 460).

_Relations of Volcanoes._

1. =Relations to rising and sinking surfaces.=—So far as observations cover this point, the area immediately adjacent to active volcanoes is rising (Dutton). This is shown by raised beaches, terraces, coral deposits, etc. Whether this is wholly due to the expansional effect of the heating of the subterrane by the rising lava, or whether it has a wider significance, is not known. If a broader view is taken, it does not appear that there are sufficient data to connect volcanic action exclusively with either the rising or the sinking of the general surface. It is certain that the great mountain ranges and plateaus in which so much of the more recent volcanic action has taken place have been recently elevated relatively, but they have also undergone more or less of oscillation, involving some relative depression. The question whether the Pacific basin as a whole has been relatively elevated or depressed in modern times is a mooted one. Darwin[278] and Dana,[279] as the result of their early studies on its coral deposits and on other phenomena, concluded that the Pacific was a sinking area, but this view has been recently challenged by Murray[280] and Agassiz[281] with at least some measure of success. From the fiords on the borders of the Pacific and other physical phenomena, the inference has been drawn that relative sinking of the land has recently taken place. Raised beaches on the coasts are interpreted as indicating a relative rise of the land or a sinking of some ocean basin, for the withdrawal of the waters can only be the result of increasing the capacity of the oceanic basin as a whole. The most probable view is that the general areas of present and recent volcanic action are partly rising areas and partly sinking areas, and that movement of either kind may be connected with the extrusion of the lavas. The rising and sinking are but complementary phases of a deformation of the earth’s body, and involve a readjustment of stresses within the body of the earth. These stresses are possibly an essential factor in eruptions.

2. =Relations to one another.=—A most significant feature of volcanic action is the degree of concurrence or of independence of action in adjacent volcanoes. In some instances they act as though in sympathy, as in the recent outburst in Martinique and Saint Vincent, and the concurrent symptoms of activity in other places. On the other hand, the independence of neighboring vents is sometimes extraordinary. The group of volcanoes near the center of the Mediterranean, of which Vesuvius and Etna are the most conspicuous examples, usually act with measurable independence of one another, an eruption in the one not being habitually coincident with an eruption in the others. But the most conspicuous instance of independence is found in the great craters of Mauna Loa and Kilauea in Hawaii. They 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 Mauna Loa is about 10,000 feet higher than the crater 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 from three to ten miles in width, and forty-five miles in length, with a probable average thickness of 100 feet, and some of its other outflows were of nearly equal greatness; indeed its outflows are among the most massive that have issued from volcanoes in recent times. Besides this massiveness there have been 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 lava-column of Kilauea, 10,000 feet lower, continued its quiet action without sensible effects from its boisterous neighbor. The bearing of such extraordinary independence upon the sources of volcanic action is very cogent, for the lavas are of the same type, both being basalts, that of Mauna Loa being notably basic and probably as high in specific gravity as that in Kilauea. No difference in specific gravity that could at all account for a difference in height of 10,000 feet can be presumed, unless their ducts remain separate to extraordinary depths. Nor does it appear possible that a superior amount of gas within the column of Mauna Loa could account for such an extraordinary difference in height, for the hydrostatic pressure of such a column is not far from 10,000 pounds to the square inch. Even if the difference in the heights of the columns could be explained by differences in specific gravity, the agitation of the one should be communicated to the other, and an outflow of the one, particularly an outflow by a breakage through its walls sufficient to lower its surface hundreds of feet, as has repeatedly occurred in Kilauea, should change the surface of the other proportionately, if they were in hydrostatic equilibrium. 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 is, therefore, probably between 30,000 and 40,000 feet, a difference which tells its own story as to their relative independence.

3. =Unimportant coincidences.=—Eruptions seem to be somewhat more liable to occur at times of high atmospheric pressure than at low, doubtless because the increased atmospheric weight on a large area of the adjacent crust aids in forcing out the lava or the volcanic gases. This can only be effective when other forces have almost accomplished the result, and would doubtless have completed it a little later had not the atmospheric wave supplied the little remaining pressure needed. Eruption seems also to be more common when the tidal strains favor it, for like reasons. In the same class are probably to be put the effects of heavy rains, whether they act by gravity or by giving rise to steam. Such agencies 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 action. This is not to be understood as inconsistent with the view that the periodic stresses of the body-tides of the earth are important factors in vulcanism, as elsewhere explained, but merely that the special time of surface-eruption is only incidentally connected with the water-tides.

=Periodicity.=—Most volcanoes are periodic in their stages of action. Long dormant periods intervene between eruptive periods. Volcanoes supposed to be extinct occasionally awaken with terrific violence. Sometimes also they awaken quietly. This larger periodicity yet awaits an explanation, but it very likely means a temporary exhaustion of the supply of gas or of lava, or of both, to which the active stage is due.

_Formation of Cones._

=Lava-cones.=—The lava usually flows away from the vent in short 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 radiating tongues surrounding the point of exit. Occasionally the streams run a dozen or a score of miles, but such cases, except in the gigantic volcanoes of Hawaii and a few others, are rare. Often the streams congeal before they reach much beyond the base of the cone, and quite often 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, rather the exception than the rule, that the cone is made up mainly of lava-streams, though the great Hawaiian volcanoes are of this class.

=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 often takes on a beautiful symmetry and assumes a steep slope (Fig. 465). The ragged cinders lend themselves readily to the formation of an acute cone, quite different from the flatter cone formed by lavas. Sometimes the cinders are still plastic when they fall, and weld themselves together and hold their places even on very steep slopes, but usually they have already hardened before they reach the surface.

=Subordinate cones.=—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 the mountain becomes a compound cone. A still more subordinate variety consists of “spatter-cones” formed by small mildly explosive vents that spatter forth little dabs of lava which form chimneys, or cones, and sometimes completely curved domes over vents (Figs. 466 and 467). Spatter-cones often arise from the lava-flows themselves.

=Composite cones.=—From most existing volcanoes there issue both lava-flows and fragmental ejecta, and the resulting cones are composite in material. The lava more frequently breaks through the side of the cone than overflows its summit, and this gives rise to irregularities of form and structure. The cones are also subject to partial destruction both by the outbursts of lava and by the explosions, and perhaps also by migration of the vents. As a result, many volcanic regions show old, partially destroyed craters, together with new and more perfect ones, and the history of volcanic action in a region may often be read in the succession of cone formations.

The form of the cone, when composed chiefly of lava, is also affected by the mass of the outflow and by its fluidity. The larger the outflow at a given time, other things being equal, the wider it distributes itself and the flatter is the cone. As a rule, the basic lavas are more fluid than the acidic, and the cones of basic lavas are flatter than the cones of acidic lavas.

=Extra-cone distribution.=—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. 458). 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 and its contact with the colder regions of the upper air, is quickly condensed, and prodigious floods of rain frequently accompany the 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 chiefly lodge on the lower slopes of the volcano or adjacent to its base, and give rise to rather flat cones, sometimes designated as _tufa-cones_ to distinguish them from cinder-cones formed by the direct fall of fragmental material. Mud-flows appear also to be formed by the ejection of mud and water that had gathered in quiescent craters during intervals between stages of eruption.

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 of appreciable thickness (as those of Nebraska)[282] 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.

Illustration: +Fig.+ 468.—Mt. Shasta, a typical extinct cone, furrowed
by erosion, but retaining its general form. (Diller, U. S. Geol.
Surv.)]

LAVAS.

=Their nature.=—In the chapter on the Origin and Descent of Rocks, the nature of lavas and of the rocks derived from them has been discussed (Chapter VII). In view of prevalent misconceptions, it may be repeated, for the sake of emphasis, that lavas are mutual _solutions_ of mineral matter in mineral matter, rather than simply melted rock. Into this mutual solution there enter not only rock materials, but gases. The distinction between mutual solutions and simple molten rock cannot be rigorously made, but it is at least essential to know that the minerals do not necessarily crystallize from lavas in the order of their melting temperatures, or in any uniform order, but rather in the order in which saturation of the several mineral constituents happens to be reached in the given mutual solution. Thus quartz, which has a very high melting-point, is often one of the last minerals to crystallize. The mutual solutions are exceedingly complex, embracing a wide range of chemical substances, but the chief of them, as already stated, are silicates of aluminum, potassium, sodium, calcium, magnesium, and iron, with minor ingredients of nearly all known substances, in greater or less proportion. The old idea of lavas as simply melted rock is not, however, wholly to be abandoned. The mode of solidifying is often simply that of molten matter freezing. If lava be suddenly cooled, the congelation is essentially the solidification of a melted substance. The result is a glassy body, every part of which has essentially the same composition that the liquid had. Usually, however, even in this case, the gases escape in part. 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 for others. With sufficiently slow cooling, all the material will pass 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, for crystals often interfere with each other’s growth. It does, however, involve the doctrine that some substances may complete their crystallization while the surrounding material is yet in the fluid condition. In most igneous rocks nearly perfect crystals of certain minerals are common, while other minerals, crystallizing later, are compelled to adapt themselves to the space left. This conception is supported by the fact that lavas, while still in the fluid condition, often contain well-formed crystals, and these crystals sometimes make up a considerable percent. of the flowing mass, just as water in certain conditions may be filled with crystals of ice. So also crystals after having been formed may be redissolved in part, doubtless because of changes in the nature of the magma due to undetermined conditions which may arise in the process of crystallization, or from the accession of gas, or from new material dissolved from the walls of the passageway.

=Consanguinity and succession of lavas.=—The lavas that are poured forth at different stages in the succession of eruptions of a given region are usually not the same, as might naturally be expected, but form a curious series the members of which are related to one another. Iddings has called this relation _consanguinity_.[283] No universal law of succession has yet been established, and perhaps none exists; but Richthofen[284] many years ago announced a definite order for the Tertiary flows of western America which seems to hold fairly well in its general aspects, though not everywhere completely realized, so far as surface observation goes. Richthofen’s order is: (1) lavas of neutral types, (2) lavas of acid types, (3) lavas of basic types, (4) lavas of more acid types, and (5) lavas of more basic types. The special varieties of rock vary, and even the general order is often apparently defective. The defects are sometimes assigned to the concealment of some of the outflows. While this may be true in some cases, it is not unlikely that in others there is a real failure of the sequence. At any rate, the sequence can only be regarded as a rough generalization. It is supposed to be due to magmatic differentiation caused by the differences of temperature to which the different parts are subjected underground, by differences of specific gravity and fluidity which result from changes of temperature, and probably by other causes.

=Temperatures of lavas.=—Accurate determinations of the temperatures in the center of the lava-columns, where they have been least reduced by contact with the rock-walls, have not yet been made, but it is clear from the whiteness of the lavas that their temperatures are often appreciably above the melting-point. This is also a necessary inference from the length of time they remain fluid, notwithstanding the great surface contact of the column in its miles of ascent, the conversion of contact water into steam, and the expansion and escape of the gases. In cases where determination has been practicable (and they certainly do not represent the maximum temperatures) it has been found that the melting-points of silver, about 960° C., and of copper, about 1060° C., are reached. In connection with overflows, it has been found that brass is decomposed into its component metals, the copper actually crystallizing. Silver has been sublimed, and made to redeposit itself in crystalline form. This implies much more than the bare melting temperatures. Even the fine edges of flints have been fused. It is, therefore, probably safe to assume that the original temperatures of the lavas as they rise to the surface sometimes reach considerably beyond 2000° Fahr. (1093° C.), and may perhaps even attain 3000° Fahr. or more. Even these temperatures must be somewhat below the original subterranean temperatures of the lavas, because some heat must necessarily be lost in rising, partly by contact with the walls of the colder rocks through which they pass, probably for as much as a score of miles at least, and partly from the expansion of the gases within them. If any considerable part of these gases is derived from waters which joined the lava in its upward course in the fracture zone, the energy consumed in raising the water to the high temperatures of the lavas must be subtracted from the original heat, and must be a further source of reduction of temperature. It is important to emphasize this point in view of its bearing upon the origin of the lavas. It has been suggested that lavas may be due to an aqueo-igneous fusion, a kind of fusion which may take place at comparatively moderate temperatures. It seems obvious, however, from the phenomena themselves, that temperatures as high as ordinary dry fusion, and perhaps even higher, are attained. It is clear also that the maintenance of the liquid condition in a constant state of ebullition for a long period of time implies a large surplus of heat above that necessary for liquefaction simply. This is especially true if the ebullition comes from surface-waters penetrating to and becoming absorbed in the lava-column below. This process must tend rapidly to exhaust the heat in the column of lava. If, on the other hand, the gases are derived from the deep interior, and the ebullition at the surface is due to their escape, they may bring up new supplies of heat to counteract the cooling effects of their expansion.

=Depth of source.=—Attempts have been made to ascertain the depth from which lavas rise, by means of the earthquake tremors that accompany eruptions. The estimates have ranged from seven or eight to thirty miles. The mode of estimate is that discussed under earthquakes, and is subject to the corrections there indicated. If these could be perfectly applied, the estimates might probably all fall within ten miles, and not improbably all within six miles of the surface. But in any case the method really tells very little as to the true point of origin of the lava. At most it probably only tells 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, because of its nature, be capable of giving rise to seismic tremors. The behavior of the tremors perhaps forces us to locate the origin of lava movement _at least as low as the bottom of the fracture zone_, but it probably offers no sufficient ground for limiting the lava’s origin to this or any other specific depth.

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.