Popular Science Monthly/Volume 50/December 1896/Igneous Intrusions and Volcanoes
|IGNEOUS INTRUSIONS AND VOLCANOES.|
PROFESSOR OF GEOLOGY IN THE UNIVERSITY OF MICHIGAN.
MANY geologists have watched the action of volcanoes in eruption, and have gazed into their craters when in a state of mild activity. One of the most striking of the phenomena revealed at such times is that great volumes of steam are given off from the molten lava which rises in the craters. This steam either escapes quietly, as in the case of the Hawaiian volcanoes, or with explosive violence, as in eruptions of the Vesuvian type. It is now conceded by probably all students of volcanoes that the proximate cause of the violent explosions accompanying many volcanic eruptions is the sudden escape of highly heated steam. Most modern theories advanced to account for volcanic phenomena are based on the assumption that steam is the propelling force which causes the lava to rise from deeply seated sources and to be extruded at the surface. Steam contained in the molten lava is thought by Shaler and others to cause the molten rock to rise and overflow, in much the same way that carbonic acid generated in dough causes it to expand, or as the carbonic acid in ale makes it overflow when the cork of a bottle in which it is contained is withdrawn. In these theories, heat is considered as the prime source of energy, and that, given the heat, steam will be generated which will force the lava to the surface.
I do not wish to criticise the theories that have been advanced, or even to attempt to review them, but simply to change the point of view from which volcanoes have commonly been studied, in the hope that the phenomena observed will group themselves in another and perhaps more instructive way. Current theories are based largely on what is seen when one looks down into the throat of a volcano in a state of mild activity; let us supplement such a view by endeavoring to form a conception of the conditions that exist far below the surface.
In many instances volcanoes are known to be situated on lines of fracture in the earth's crust. In all volcanoes it is evident that there is a passageway or conduit, leading from an intensely heated region within the earth, to the surface. These conduits must be several thousand feet in depth. Indeed, it is not unreasonable to assume that they may have a depth of several miles or possibly tens of miles. What one sees, therefore, in looking into a crater of an active volcano is the summit of a column of molten rock, the bottom of which is tens of thousands of feet below.
Judd has compared the mild activity of Stromboli to the boiling of mush in a tall vessel, the heat being applied at the bottom. Steam is generated in the mush, and, rising through it in bubbles, elevates the surface. When the bubbles of steam burst, portions of the viscid material are blown into the air. Such an analogy is certainly sustained by what is seen at the summit of a column of molten lava when we look into the crater of a volcano.
In seeking for information concerning the conditions that exist far below the surface, when a volcano is giving off steam at the top, we may obtain a few facts to guide us by studying the ruins of extinct volcanoes and the nature of igneous intrusions as laid bare by erosion.
Volcanic necks tell something of the conditions that exist within a volcanic mountain. In more deeply eroded volcanic districts one finds dikes and intruded sheets. These are connected, in reference to mode of origin, with Plutonic plugs, laccolites, and what I have termed subtuberant mountains. These various forms taken by intruded rocks and surface extrusions, as I have attempted to show in the article just referred to, belong in a single genetically connected series. A break in the earth's crust which reaches a region of great heat may be injected with plastic rock and form a dike; if the fracture terminates above in a region of horizontally stratified beds, the magma rising through it may expand widely, at the same time lifting a broad cover to a comparatively small height—that is, form an intruded sheet; or be more restricted in its expansion, according to its degree of fusion, depth below the surface, and possibly other causes, and raise a cover of less diameter to a greater height—that is, form a laccolite; or, if a great volume of plastic material is intruded, give origin to a subtuberant mountain. Should the fissure reach the earth's surface, molten material may be forced through it and a volcanic eruption initiated. In brief, variations in one process may lead to the formation of dikes, sheets, laccolites, and other forms of intrusion, and to volcanic eruptions. Possibly all these results might follow from the opening of a single fissure.
In all the instances enumerated the magma may be the same, so also the source of the heat and the origin of the pressure which forces the molten rock into the earth's crust, or causes it to be discharged at the surface.
Returning to the hypothesis that steam is the mainspring of volcanic phenomena, it will be conceded, I think, by most observers that steam contained in a magma can not be called upon to account for its rise in a deeply seated fissure so as to form a dike, or, when the conditions are varied, give origin to laccolites, etc. Not only are the rocks composing such intrusions, the densest of igneous rocks, but they are without steam cavities. Besides the filling of a fissure with plastic or fluid rock, and still more strikingly in the production of other varieties of intrusions, a bulk of matter, measured in some instances by cubic miles, is forced in among the solid rocks of the earth's crust. There is thus a bodily transfer of matter, frequently for long distances, from one place to another deep within the earth's crust and against an enormous pressure. All these facts are adverse to the conception that bodies of liquid or plastic rock are moved by the expansive force of the steam contained in them. The energy expended in producing igneous intrusions is in numerous instances so far in excess of that manifest in any explosive volcanic eruption that has been recorded—not excepting Cosequina, Sumbawa, or Krakatoa, but rather combining them all and more in one—that it becomes of a different order of magnitude, and a different origin is to be suspected.
In the case of subterranean injections, it is evident that the source of the heat which renders the rocks plastic, and the source of the pressure which forces the plastic material into fissures, etc., are distinct and should be separately considered.
The heat manifest in both subterranean and surface igneous phenomena, as is well known, has been variously accounted for, but I do not wish to consider this problem at present. The consensus of opinion, however, seems to be that the heat referred to is mainly and essentially the internal heat of the earth—i. e., the residual heat of a cooling globe. It is conceded also that the matter composing the earth at a depth of a few miles below the surface is so highly heated that it would become plastic or even highly fluid if the pressure under which it exists were removed. The best conception we can frame of the general physical condition of the earth is, that it consists of a more or less spherical mass, which is highly heated and in a potentially plastic condition within, and that inclosing this inner sphere is a comparatively thin shell of solid rocks—the passage from the hot and potentially plastic interior to the cold and rigid outer shell being gradual, one merging with the other by insensible gradations.
The crust of the earth rests on the sphere of plastic material within and exerts a pressure upon it. Contraction of the progressively cooling crust also causes pressure to be exerted on the inner sphere. If the pressure of the crust on the material it incloses were equal at all points, the inner mass, except for the effect of rotation, would be a perfect sphere. Variations in the pressure of the crust at different localities might result from several causes, such as unequal cooling in the crust itself, the transfer of material from the inner sphere into or to the surface of the crust, the shifting of material from one locality to another on the earth's surface, etc. Of these disturbing conditions I am inclined, provisionally at least, to ascribe the greatest potency to the effects of erosion, transportation, and sedimentation on the earth's surface, thus lightening certain areas and loading others.
If we conceive of the earth as a sphere without rotation, it is evident, from our present point of view, that it would remain a sphere only so long as the pressure of the crust on the material within was equal at all points. Local variations in the pressure of the crust would deform the inner sphere and result in a change in he shape of the earth. It is not to the general problems of isostasy, as formulated by Dutton, that I wish to direct attention, however, but rather to the results that might be expected to follow should the crust of the earth be broken.
A fracture in the earth's crust would establish a line of weakness, which, so far as the reaction of the crust on the interior is concerned, would be equivalent to a local relief of pressure. Should a fissure reach the highly heated interior, the rocks in its vicinity would become plastic and be pressed into the opening, and tend to widen it both by pressure and by the fusing of its walls. As the magma from a deep source rose in the fissure, the resistance to be overcome would be less and less, thus insuring greater plasticity, and, if it gained the surface, establishing conditions commonly recognized in volcanic eruptions.
Should the plastic rock fail to reach the surface, but cool in the fracture, a dike would be the result. If the fracture terminated above in a region of horizontal stratified rocks, lateral expansion of the magma rising in it might occur, and intruded sheets, laccolite, etc., be formed.
As to the origin of fractures we have but few facts to guide us. It is well known from the study of faults, dikes and volcanoes, that breaks, at least in the superficial portion of the earth's crust, have been of common occurrence. Whether any of these breaks have extended through what we term the earth's crust is unknown, but the fact that fissures have in numerous instances become filled with fused rocks is evidence that the breaks referred to are sometimes of sufficient depth to reach regions of intense heat, and many facts seem to favor the idea that this highly heated region is the potentially plastic interior of the earth.
The principle that areas which become weighted by the shifting of material on the earth's surface subside, while unloaded areas rise, has been advocated by several American geologists, and is now common property. It has frequently been asked, however, why an area that is being unloaded should cease to rise so long as erosion continues, and becomes stable or even undergoes a reverse movement; and why an area that is having material added to it should cease to sink and be re-elevated.
In reference to the first of these questions the nature of igneous intrusions seems to furnish an answer. If the rise of a region of denudation is due to an injection of plastic material beneath it, in response to the resulting relief of pressure, the reservoir of highly heated rock forced from below into the cooler rocks above or a protuberance on the surface of the inner sphere will cool, and consequently become more and more rigid. When the intruded rock becomes solid, the portion of the earth's crust lightened by erosion will be increased in weight by material added from below. For the reason that the intruded magma tends to fuse the rocks with which it comes in contact, it will be welded to them as cooling progresses, and when solidification occurs the crust may have greater strength than before the intrusion. Each of these changes would tend to check the upward movement. Dikes and other forms of injections thus tend to strengthen the rocks into which they are forced in much the same way that fractures in the earth's crust are healed and the strata strengthened by the deposition of quartz and other minerals in them so as to form veins. In the case of subterranean injections the cooling of the reservoir of molten rock will be accompanied by contraction, which would cause a subsidence of the surface.
Stating the ideas that I have just attempted to convey more briefly, we should expect that the rise of a denuded area from the effect of internal pressure causing an influx of plastic material beneath it would be checked by the cooling and hardening of the injected material, and a reverse movement or subsidence of the surface initiated as the injected material contracted on cooling.
I am well aware that in these suggestions we are dealing with a single portion of a complex machine and that other results than those considered may follow. That the sole cause of the rise of an eroded area is not the injection of a molten magma beneath is apparent from the fact that such an injection would cause a rise in temperature in the rocks above, and thus by expanding them, increase their upward tendency. The subsequent cooling of these heated and possibly metamorphosed rocks would also tend to renewed subsidence of the surface. In other words, subterranean intrusions are accompanied by an abnormal rise of the isogeotherms and their loss of heat by a return to normal conditions.
In progressively loaded areas, as has been pointed out by Reade, Shaler, and others, there is a blanketing of the earth's heat and a rise of the isogeotherms; the accompanying expansion of the rocks tends to check subsidence and limit accumulation. The processes of erosion, transportation, and sedimentation in special areas are thus limited by conditions within the earth. Erosion favors elevation until the plastic material transferred to the region beneath the lightened area cools and hardens. A decrease in elevation due to contraction then ensues, and is accompanied by a decrease in erosion, which comes to an end when the land is reduced to base level. Loading favors sedimentation by causing subsidence until the thickened sediments become heated and by reason of their expansion elevate the surface to or above base level. There is a mutual interaction beneath the earth's crust also, since, if only one region of erosion and one of sedimentation are considered, the checking of elevation in a region of erosion by the cooling and hardening of injected magma beneath will give greater resistance to the flow of plastic material from beneath the region of sedimentation.
Deep erosion of subtuberant mountains should reveal a central area of igneous rock surrounded by a belt of metamorphosed rocks which on its outer border, in case the injection occurred in ordinary sedimentary strata, should pass into unaltered sand-stone, shale, etc. In such an instance a radial section should reveal a gradation from igneous rock through metamorphosed rocks to unaltered sedimentary beds. The breadth of the central core of igneous rock would vary with the size of the intrusion, and, down to a certain limit at least, with the depth of the plane of erosion. One or more generations of dikes might occur in either the central area or in metamorphosed or sedimentary rocks surrounding it. Great intrusions if deeply eroded would thus present the conditions sometimes cited as examples of "regional metamorphism." Some of the features observed in the crystalline region of Canada seem to illustrate the surface features that would be found if a great subtuberant uplift should be pared away by erosion.
The fact that a large majority of volcanoes are situated near the sea has led to the supposition that sea water gaining access to highly heated rocks is the chief if not the essential cause of volcanic eruptions. The hypothesis, however, that the sea is the source of the water which, converted into steam, takes such a conspicuous part in volcanic eruption is open to several objections.
In almost all land areas the rocks below the surface are saturated with water, the source of which is mainly rain. Excepting that the pressure of the sea on its floor tends to force water into the rocks beneath, there does not seem any good reason for concluding that the earth's crust where covered by the sea is more highly charged with water than the portions beneath land areas.
Another argument for the presence of sea water in volcanoes is that after eruption the country about a volcano is sometimes whitened for many miles with salt, and also that some of the vapors arising from volcanic vents are such as might be expected to occur if the substances contained in sea water were sufficiently heated. It is to be remembered, however, that large bodies of salt derived in some instances from the evaporation of sea water occur among stratified rocks, and also that many sedimentary deposits are saturated with saline water. It thus becomes evident that communication between the conduit of a volcano and the sea is not the only means by which saline water can come in contact with molten rocks.
It is well known that volcanoes as a rule are located near the borders of continents, or on the floor of the sea. This fact is more in harmony, however, with the idea proposed by Dana, that the margins of continents are determined by the location of weak belts in the earth's crust, along which maximum movement takes place, than that the presence of surface water bodies is essential to the existence of volcanoes. In support of this conclusion it may be pointed out that volcanoes of recent date occur in the Great Basin, hundreds of miles distant from the Pacific. The Great Basin is a region of faults, and as much a belt of weakness in the earth's crust as if it had chanced to be situated near the sea.
Owing to the increase of pressure with depth, it is evident that cavities in rocks in which any considerable bodies of water can be stored must become less and less frequent as the distance below the surface increases. As has been shown by Van Hise, at a depth in excess of about thirty thousand feet what may be termed appreciable cavities can not exist. Rocks under pressure become compact, so that deeply seated rocks must be less porous than similar material near the surface. These considerations lead to the conclusion that water-charged portions of the earth's crust are superficial. Hence the water given off by volcanoes in the form of steam, and probably also the gases produced by the dissociation of the elements composing water and the vaporization of the various salts it contains, must reach volcanic conduits in their upper portions. These considerations add strength to the conclusion advanced on a previous page, that the primary force which causes lava to rise in the conduit of a volcano is not steam pressure.
How molten lava becomes charged with water can only be conjectured. It is well known that many liquids, especially when highly heated and under heavy pressure, will absorb gases. In a similar way we may conceive that liquid or plastic rock, on coming in contact with water, will absorb the steam produced.
When molten lava rising in the conduit of a volcano passes through water-charged rocks and nears the surface, pressure is relieved and the occluded steam escapes. This escape is either quiet or explosive, dependent on the nature of the magma in which the steam is dissolved. If the magma is highly fluid, as in the case of many basic lavas when extruded, the steam escapes quietly; but if the magma is viscous, as is the usual condition of acid lavas when erupted, violent explosions are apt to occur. The quantity of steam absorbed also influences the fusibility of a magma. Apparently the larger the quantity of occluded steam, the more liquid the molten rock becomes. Greater freedom may thus be afforded for the passage of a magma in the upper portion of the conduit through which it rises than obtains at lower levels. Something of the intermittent character of volcanic eruptions may depend on this cause. Probably, also, the quantity of water present in a magma has an influence on the nature of the minerals formed as it cools. For this reason one would expect differences to appear in the mineralogical composition of rocks formed from magma that have cooled near the surface, and those that failed to reach the water-charged portion of the earth's crust.
The intimate connection between subterranean injections and volcanoes leads to the suggestion that the domes above intruded magmas may become fractured and give origin to volcanoes which would be supplied by local reservoirs. Something like "craters of elevation" may be thus formed.
If two or more cisterns of molten rock should be formed in the earth's crust near each other, or at different levels near the same radius of the earth, and fractures formed above them which would admit of the escape of their material to the surface, the striking phenomena of two adjacent volcanoes erupting independently of each other might result. Such an occurrence is rendered more probable by the fact that reservoirs beneath subtuberant mountains are supplied through fissures from below, which might become closed, thus isolating bodies of injected material in the earth's crust. Even if the feeding fissures were not closed, the large cisterns of fused rock to which they lead might discharge some of their material without immediately affecting the plastic central mass of the earth, which in these suggestions is considered the primary source from which injections are derived.
It has been thought by some who have speculated on the condition of the earth's interior that isolated reservoirs of fused rock exist in the generally cool earth's crust, due to unequal cooling another origin for such lakes of lava may be postulated if we consider them as injected magmas not yet cooled.
A mental picture of the probable occurrences that give origin to subterranean intrusions and volcanoes, and account for many observed phenomena in this connection, may be sketched in outline as follows:
The earth is hot and potentially plastic within, and cold and rigid at the surface. Unequal cooling and the shifting of material on the surface are disturbing conditions that tend to change the shape of the plastic interior, and to crumple and break the crust. If a fissure forms in the lower surface of the crust, the potentially plastic material beneath will become plastic on account of the removal of resistance to pressure, and be forced into the break, and a dike be formed. Under certain conditions the plastic material rising in a fissure may expand between layers of stratified rock so as to form laccolites, subtuberant mountains, etc.
If a break extends entirely through the crust, molten material forced into it may reach the surface. As the molten lava rises in such a break, it passes through rocks that are more and more highly water-charged, the water is vaporized, or perhaps its elements are dissociated, and the vapors and gases formed are absorbed by the fluid rock. As the lava comes to the surface the steam and gases absorbed under great pressure escape and furnish some of the most striking phenomena of volcanic eruptions. Loss of heat as a magma nears the surface also favors the escape of occluded gases.
In this view of the nature of volcanoes it is evident that an arrest of pressure on the reservoirs from which they draw their lavas would stop their action. If a fissure extends through the earth's crust to the potentially plastic interior, it is difficult to see how an outflow of molten material would be checked unless the conduit should become closed. Under the vast pressure that exists at a depth of several miles it is impossible to comprehend how fissures can exist, but the plastic material beneath is under pressure of a similar order of magnitude, tending to force it out through any opening that may be formed. A near balance between the pressure tending to close a fissure and the pressure on the magma below tending to maintain a communication with the surface may therefore be conceived to exist; when the balance is in favor of extrusion volcanic eruptions follow, and when the reverse is the case the conduits become closed. The evidence furnished by dikes and other intrusions, as well as by volcanoes, points to the conclusion that the magmas supplied to them are derived from deeply seated sources, but the fact that the material forming intrusions and the products of volcanoes differ widely among themselves has been cited as evidence that they could not have been derived from a common reservoir. This objection is based on the assumption that the highly heated material forming the earth's interior is homogeneous. It has been argued that, if the material within the surface shell was not homogeneous—excepting so far as density increases with pressure or is influenced by the increase of heat with depth—an adjustment would be established by the flow of matter from one locality to another. In reply, it may be said, however, that this conclusion is inconsistent with the idea of a solid but potentially plastic inner sphere. From the point of view assumed in this essay, it appears that what may be termed a local flow of the matter comprising the earth's interior would not occur unless there was a local relief or a local increase of pressure. The idea that the earth as a whole is a rigid body is in harmony with the conclusions of eminent physicists and astronomers, while the assumption of local plasticity due to local relief of pressure is consistent with the observed movements of elevation and depression familiar to geologists.
Returning to the consideration of the passage of the conduit of a volcano through the water-charged portion of the earth's crust, some of the phenomena displayed by dormant volcanoes may perhaps be explained. If the lava in the conduit of a volcano cools and hardens below the water-charged zone, the life of the volcano to which the conduit leads may be considered as ended. If the cooling takes place at the surface or within the water-charged layer, steam will continue to be generated below the obstruction, and, if means for its gradual escape are not furnished, will ultimately lead to an explosion which may blow away a volcanic mountain. In such an occurrence the main explosion would probably be preceded by a breaking of the rocks and possibly subterranean explosions which would bring temporary relief of pressure. The behavior of many dormant volcanoes and the earthquakes that frequently accompany a renewal of their activity might thus be explained.
When igneous intrusions enter the water-charged portion of the earth's crust but do not reach the surface, steam is also generated, and may assist such intruded magmas in opening passages for themselves and in elevating the domes that are raised above them. When an intruded magma' meets a large body of subterranean water a violent explosion must result, which, when near the surface, would blow away the dome above, leaving a depression of the type of Coon Butte, Arizona, or Lonar Lake, India.
The suggestions offered in this paper are in harmony with the conclusion that many of the phenomena accompanying volcanic eruptions are due to the escape of steam occluded in molten lava, but are opposed to the hypothesis that the rise of lavas from deeply seated sources is due to the same cause. The source of the heat and the source of the pressure manifest when a magma rises through a volcanic conduit are considered to be distinct, and in the main of different origin.
- On the Nature of Igneous Intrusions. In the Journal of Geology, Chicago, vol. iv, 1896, pp. 177-194.