The Elements of Geology; Adapted to the Use of Schools and Colleges Part 11

SECTION II.--IGNEOUS CAUSES.

I. _Of the Temperature of the Ma.s.s of the Earth._--Heat has been the most efficient agent in determining and modifying the structure of the earth; and, in order that the explanations of the phenomena referable to this cause may be intelligible, some idea must be formed of the actual present condition of the ma.s.s of the earth with respect to heat.

At any point of the surface there are variations of temperature, depending on external causes. But these variations are found to extend only a little way below the surface,--never more than a hundred feet. At greater depths, it is found that the temperature invariably increases with the depth. Deep mines have always a temperature above the mean annual temperature at the surface. The water obtained by deep boring is always tepid when it comes to the surface. The thermal springs, so abundant in this country and in Europe, are so situated as to justify the impression that their waters come from great depths. To make these general observations of any value, we must determine the law by which the temperature increases. The result of all the observations yet made, in mines and upon wells and springs, is that, below the first hundred feet, the temperature increases by one degree of Fahrenheit's scale for every forty-five feet.

Regarding this law of increment as applicable to all depths, at ten miles below the surface we should have a temperature above that produced by the combustion of wood; and at twenty-five miles, a temperature of three thousand degrees, by which nearly all mineral substances would be reduced to a state of fusion.

The general conclusion of a temperature sufficient to melt the mineral substances of which rocks are composed, at no considerable distance below the surface, is confirmed by the fact that portions of the interior of the earth--at least, at the volcanic centres--are in a melted state. The intimate connection between some volcanoes situated a hundred miles or more apart, so that they are alternately in a state of activity and rest, indicates that these centres are connected,--that subterranean melted lava extends from one to the other, so that when one is active, the elastic force is relieved at the other. These deep-seated lakes of lava must therefore underlie large areas.

We are justified, then, in concluding that the ma.s.s of the earth, with the exception of a comparatively thin superficial layer, has a very high temperature.

By way of accounting for this temperature, it is now generally a.s.sumed that the earth was originally in a state of fusion; that it was a ma.s.s of liquid lava (if, indeed, it had not a temperature sufficient to reduce it to the aeriform state). Starting with this a.s.sumption, there must necessarily be a gradual reduction of temperature by radiation, and a time must arrive when the surface would be crusted over with solidified lava; and this crust would increase in thickness as the cooling advanced, the interior still retaining its heat and liquidity.

The present condition of the crust of the earth, its form, that of an oblate spheroid, with the exact difference of the equatorial and polar diameters which is found to exist, as well as the phenomena of volcanic eruptions, will all admit of explanation on this hypothesis.

It has, however, been rejected by some; and, to account for the heat of the interior of the earth, it is suggested that, if the bases of the earths and alkalies, particularly pota.s.sium, sodium and calcium, exist in their metallic state beneath the surface, the rapid oxidation of them by the access of water would generate heat of sufficient intensity to melt the oxidized materials, and thus account for the phenomena attributable to heat.

Either of these hypotheses may be adopted; but it is not necessary to account at all for the existence of this temperature. The fact is susceptible of proof; and, though we may not be able to frame any hypothesis to account for its existence, we may yet employ the fact in the explanation of other phenomena.

II. _The Action of Internal Heat in producing Volcanoes._

The phenomena of volcanoes and earthquakes are evidently produced by some force operating from below. The effect of heat alone would be to reduce the rock to a liquid state. There is no reason to suppose that it is ever sufficient to reduce them to the aeriform state. The elastic force must therefore depend upon some other substance a.s.sociated with the lava, and this substance is water.

This will be shown by an examination of lavas. At the time of their ejection, they are in a fluid or semi-fluid state; but it is not a complete fusion. Even the most fluid lavas contain particles of minerals in a solid state. The liquidity depends upon the fusion of the more fusible portions, and upon the steam of water at a high temperature, which fills the interstices between the solid particles. The porous character of cooled lavas is produced by the steam which filled the cavities previous to solidification. Steam always escapes from the surface of a lava current while it is cooling, and it is always discharged in immense volumes from the orifice of eruption, in connection with the lava, and especially at the close of an eruption.

The geographical position of volcanoes, also, leads to the conclusion that water is essential to their activity. There are five princ.i.p.al lines of volcanic activity. One, commencing at the southern extremity of South America, extends northward along the Andes and Cordilleras to California or Oregon. The second has a north-east and south-west direction, from the Aleutian Islands through the Kurule, j.a.panese, and Philippine islands, till it meets the third line, lying in a nearly east and west direction, embracing Sumatra, Java, and most of the Pacific volcanic islands. A fourth band commences in the Grecian islands, and extends westward so as to include the volcanoes of Italy and the adjacent islands, and the Azores. The fifth band embraces the volcanic islands of the West Indies, crosses Mexico in about the lat.i.tude of the city of Mexico, and extends into the Pacific. There are also some isolated centres of volcanic activity, such as Iceland. These volcanic bands embrace about three hundred volcanoes. It will be seen that they must nearly all be in close proximity to the ocean, or to large seas.

About two-thirds of them are on islands. Moreover, the volcanic vents which are wholly submarine are probably very numerous.

This circ.u.mstance of the position of volcanoes establishes a presumption that they cannot exist at a distance from some large body of water; and, taking it in connection with the constant presence of aqueous vapor in lava, we are justified in the conclusion that _the presence of water is an essential condition of volcanic activity_.

Knowing that heat and water exist at the volcanic centres, it is not difficult to form an idea of _their mode of operation_. The water, diffused through the interstices of the lava, and subjected to a temperature sufficient to melt the lava, would possess an _elastic power_, which, though never computed, we may well suppose capable of overcoming any resistance which the crust of the earth might present.

The repressing force will be the tenacity and weight of the superinc.u.mbent strata. Whenever the elasticity is superior to this repressing force, it will manifest itself in the fracture of the strata, and often in the ejection or lava to the surface.

This fracturing of the strata, produced by an uplifting subterranean force, is believed to be the cause of the noise and the vibratory motion which are the chief phenomena of earthquakes. The elastic force may raise lava to the surface, and thus the fracture would become a volcano.

But the force may expend itself by the discharge of vapor into the fissure, or by merely filling it with lava. In either case, the only evidence of the existence of the volcanic force would be the noise and the wave-like motion experienced at the surface. The cause of the volcano and earthquake is therefore the same, though the phenomena which characterize them are different.

When the strata are is thus fractured, lava may for a time be discharged along the whole line. By the cooling of lava in the fracture, it would become partially reunited. Still, this would be the line of least resistance. It would therefore be again burst through in certain places, which would long continue to be orifices of discharge, and thus the original fracture would determine _a line of volcanic activity_.

The repressing force may become greater at an orifice of eruption than at some other point, either by the great acc.u.mulation of ejected materials around the opening, or by the dormancy of the volcano long enough for the complete solidification of the lava with which the channel was filled. The least resistance may then be far from any previous vent, when a new orifice of discharge will be opened, and _a new volcano make its appearance_. It seems probable, also, that volcanoes may become extinct by the reduction of temperature at the volcanic centre, and that new volcanic centres may be formed; but the cause of this change of temperature is not yet well understood. New volcanoes have broken out in the sea, near Iceland, in several instances; others in the volcanic line east of Asia. Graham Island, situated between Sicily and Africa, was formed by an eruption which broke out in the bed of the sea where the soundings were more than one hundred fathoms. The island was at one time two hundred feet 'above the sea, and three miles in circ.u.mference. It was, however, gradually destroyed by the action of the waves, and now remains a dangerous reef, covered by less than two fathoms water. The volcano of Jorullo, in Mexico, was formed in this way. Previous to the formation of the mountain, the region where it now is was a cultivated table-land. During the year 1759 volcanic action commenced and continued, until, at the expiration of twelve months, a cone had been formed having an elevation of sixteen hundred feet above the adjacent plain.

An orifice of eruption is at first but little elevated above the general surface; but, by the acc.u.mulation of ejected matter, a cone is at length formed around the vent. The upper portion of a cone always consists of these materials, but there may also be in progress a general elevation of that part of the earth's crust, and the cone will partake of that general elevation. The cones of the Andes owe their height, in a great measure, to a general movement of elevation; those of aetna and Vesuvius, in a greater degree, to acc.u.mulation of ejected matter.

In either way, the height may become so great that the force necessary to raise a column of lava to the top would be greater than the sides of the cone, weakened as they always are by fractures in all directions, can sustain. Hence, the highest craters of aetna and South America have long been closed, and the lava escapes through fissures at a lower level, and _lateral cones_ are produced.

[Ill.u.s.tration: Fig. 79.]

The form which the materials have, when ejected from volcanoes, depends mainly upon the degree of liquidity of the lavas at the volcanic foci.

If the liquidity is very perfect, the aqueous vapor will readily rise through the lava. The steam thus separated will drive before it whatever rocks, or previous lavas, may obstruct it. In their progress they would be reduced to sand and powder, and ejected as _volcanic cinders_. (Fig.

79.) If the lava possess considerable viscidity, the aqueous vapor will separate with more difficulty, and the lava and vapor will ascend the channel together. Large bubbles of vapor will, however, collect with more or less of frequency; and, as they rise through the lava, will drive forward a portion of it, and cause the overflow to take place by pulsations. As the bubbles reach the surface, their bursting causes the loud reports, which are compared to the discharge of heavy artillery.

With each explosion some of the lava will be projected violently into the air, and, cooling, will fall to the surface as scoriae,--or, if the lava be highly vitreous, it will be drawn out into fibres, and descend as volcanic gla.s.s.

III. _Geological Phenomena referable to Volcanic Action._

Volcanic agency has probably never been less than it is now, and we ought therefore to find its effects very general and important.

1. The most obvious of these effects are the _fractures_ with which the crust of the earth is everywhere intersected. The uplifting force upon which all volcanic phenomena depend would necessarily fracture the crust, and the wave-like motion resulting from the fracture would cause numerous secondary fractures, having a parallel direction. They are often of such extent, during earthquakes, as to endanger life. During the great earthquake at Lisbon, in 1755, a fracture opened of sufficient width to swallow up the quay, and several thousands of persons who had fled there for safety. The chasm remained permanently open to the depth of six hundred feet. The earthquakes with which the valley of the Mississippi was visited in 1811 so often fissured the surface, that the inhabitants protected themselves by clinging to the trunks of trees, which they felled transversely to the direction of the fissures.

[Ill.u.s.tration: Fig. 80.]

The first fracture which is produced by the upheaving force will open upwards, and scarcely reach down to the seat of the force. But there will be other parallel fractures, dependent upon the first, and opening downward. Thus, the primary fracture at _a_ (Fig. 80) will be at once followed by the fracture _b_, opening toward the lava, which will be injected into it, and which, on cooling, will form a _dike_. Their formation is mostly concealed from observation, but not always. During the eruption of aetna, in 1669, numerous fissures opened, one of which was six feet wide and twelve miles in length; and the light emitted from it indicated that it was filled with lava to near the surface. The process was as perfectly seen as from the nature of the case it could be.

2. The conversion of the lower sedimentary strata into _metamorphic rocks_ has been effected by volcanic heat. The material of which dikes consist has been injected in a highly-heated state; and, by observing the effect which they have had upon the adjacent rocks, we may judge of the effect which subterranean heat must have upon the lower mechanical strata. Wherever the dikes are of considerable thickness, they have converted the adjacent shales into primary slate, the sandstones into quartz rock, and the dark and friable limestones into granular marble, and destroyed the organic impressions. In the southern extremity of Norway there is a district in which granite protrudes in a large ma.s.s through fossiliferous strata. These strata are invariably _altered_ to a distance of from fifty to four hundred yards from the granite. The shales have become flinty, and resemble jasper; and near the granite they contain hornblende. The siliceous matter of the shales has become quartz rock, which sometimes contains hornblende and mica, and therefore const.i.tutes a kind of granite. The limestone, which at points remote from the injected rock is an earthy, blue, coralline limestone, has become a white, granular marble, near the granite, and the corals are obliterated. The altered shales and limestones in many places contain garnets, ores of iron, lead, &c. The annexed (Fig. 81) is a plan of this granite and altered rock.

[Ill.u.s.tration: Fig. 81.]

One of the most instructive examples of metamorphic action in this country is found in the White Mountains of New Hamps.h.i.+re. These mountains have, till recently, been thought to consist princ.i.p.ally of granite; but it is now ascertained that this supposed granite is an altered rock of the silurian period. It is represented as "intersected by veins of felspathic granite; and the general ma.s.s is itself in many parts converted into a near approximation to a binary granite, composed of distinctly developed quartz and white felspar, with a few spa.r.s.ely scattered specks of mica. In its weathered surfaces it wears a close resemblance to some fine-grained granites; but, upon inspecting a fresh fracture with a magnifier, we instantly perceive many rounded grains of quartzose sand, and the felspar is imperfectly formed, though the mica has more nearly reached the condition which it has in granite. In some of the coa.r.s.e varieties of this white rock, small rounded pebbles of quartz are to be seen, giving unequivocal evidence, even to the naked eye, of its being an altered sandstone. We feel no hesitation in deciding it to have been a silico-argillaceous white sandstone, now almost granitized by extensive metamorphic action."

Similar ill.u.s.trations, on a small scale, may be seen in every country where the strata have been cut through by intrusive dikes. Sir James Hall has shown the same by actual experiment. He exposed pulverized chalk to heat sufficient to melt it, and under sufficient pressure to prevent the escape of the carbonic acid. After cooling, the chalk was found to have taken the form of crystallized limestone. But instances enough have been given to show what changes should be looked for wherever the sedimentary rocks have been exposed to a high temperature.

The lower strata must have been exposed, for long periods of time, to such a temperature. We do not know at what depth below the surface of the earth the rocks become liquid; but above the line of actual fusion there must be a ma.s.s of rock not melted, yet scarcely retaining the solid form. For a great thickness, perhaps for several miles, it would be in a more or less yielding state. As there is not actual fusion, the stratification is not destroyed, but such a degree of mobility among the particles exists, that some degree of crystallization takes place, and the elastic forces below easily bend, throw into folds, compress, and in every way contort these strata. At the same time, any organic matters which they may contain are decomposed, and the impressions of them are obliterated. And such is the condition in which the metamorphic strata are actually found.

3. _Denudation_ is, in a great measure, dependent on volcanic action. It results from the billowy motion peculiar to the earthquake. This is not simply a violent horizontal motion, but an equally violent vertical one.

It is a series of waves,--a succession of alternate elevations and depressions of the solid crust. The height of these waves can only be judged of by their effects; but it is difficult to account for some of these effects, without supposing the waves to have been several yards in height, and their velocity, in the few instances in which the time has been accurately determined, was twenty miles a minute.

That such earthquake waves actually exist there can be no doubt. During the earthquake in Calabria, in 1783, the flagstones in many of the towns were lifted from their places and thrown down inverted, and trees bent so that their tops touched the ground. During the great earthquake in Chili, in 1835, the walls of houses, which were parallel to the line of oscillation, were thrown down, while those that were at right angles to it, though greatly fractured, were often left standing. Wherever careful observations have been made, during and after severe earthquakes, a.n.a.logous facts have been noticed. Persons are generally affected with sea-sickness. The sea is violently agitated. It often retires to an unusual distance, and then returns upon the sh.o.r.e with most destructive waves. Incredible, therefore, as it may seem, that the solid crust of the earth should be thrown into such wave-like undulations, the fact is well established.

With a velocity of twenty miles an hour, the successive waves may be some miles apart, and yet be sufficient to account for all the phenomena. It is evident, therefore, that the curvature of the wave will be very slight, and yet enough to break into fragments all the rocks thus curved. During the earthquake in Chili, before referred to, "the ground was fissured, in many parts, in north and south lines. Some of the fissures near the cliffs were a yard wide. Many enormous ma.s.ses had fallen on the beach. The effect of the vibrations on the hard primary slates was still more curious. The superficial parts of some narrow ridges were as completely s.h.i.+vered as if they had been blasted by gunpowder." Similar phenomena seem everywhere to be exhibited by earthquakes.

It may be presumed that almost all parts of the earth have, at different periods, been subject to these earthquake waves. Accordingly, we find that the crust of the earth is nowhere in an entire state, but is divided by irregular lines into comparatively small fragments. By this means, the deep fissures produced by fractures opening upwards would be filled with fragments of rock shattered from the uplifted edges. In this way the boulder ma.s.ses were originally loosened from their parent beds, and exposed to the action of ice, or any other transporting agencies. In the same way the rocky bed of the ocean is, to a considerable depth, reduced to a disintegrated ma.s.s. In this condition it will be rapidly removed by marine currents, more or less broken, worn and comminuted, by the movement, and deposited elsewhere. The materials have thus been furnished for a very large proportion of the sedimentary rocks, and especially of those which are composed of distinct fragments of other rocks. By this means, also, wherever the rock formations come to the surface, they are so broken that limestone, sandstone or granite, suitable for architectural purposes, is seldom found, except at considerable depths. This fragmentary condition of the surface rock is such as exposes it to be acted upon readily by any powerfully abrading causes, or to be more rapidly disintegrated by atmospheric and aqueous causes.

4. We have already a.s.sumed that one princ.i.p.al division of rocks--the unstratified--is of igneous origin. We have the proof of actual observation, that lavas, and the accompanying _tufas_ and _grits_, are volcanic products. The peculiarities of these products, in situation, structure, and form, and in the imbedded minerals, are so great, that whenever we find these peculiarities in the rocks of a country not now volcanic, we still regard these rocks as of volcanic origin. We thus have lavas, as well as stratified rocks, of different ages. There has probably been no time in the earth's history when they have not been forming.

The _trappean rocks_ are also of igneous origin. It is evident, from their occurring in the form of dikes, that they have been in a melted state. As they rest upon rocks of a sedimentary origin, they must have been thrown up by volcanic forces. Yet they differ from ordinary lavas.

They are not vesicular in their structure, are more crystalline, and there is in no case evidence that they have flowed from craters. If we regard them as the lavas of submarine volcanoes, we shall have conditions which will account for all their peculiarities. At a certain depth the pressure of the water would be sufficient to prevent the formation and escape of vapor, and therefore the lavas thus ejected would not be vesicular. As the rapid cooling of lavas depends, in a great degree, upon the escape of watery vapor, submarine lavas would cool slowly, in consequence of the pressure. The liquidity depending in part upon the retention of the heat, and in part upon the retention of the aqueous vapor, they would consequently remain in a liquid state much longer than the lavas of sub-aerial volcanoes. They would therefore take a more highly crystalline form. All the loose materials thrown out during the eruption would be removed by oceanic currents, and hence no cone would be built up around the orifice of eruption. We may therefore regard the trappean rocks as the lavas of submarine volcanoes. The present volcanoes of this kind are necessarily producing the same kind of rocks, though there will be no other proof that they exist, except the existence of the volcano, till the bed of the sea becomes dry land.

The _granitic rocks_ are also the product of igneous causes. Granite is the most abundant of these crystalline rocks; and the others, such as crystalline limestone, are so intimately a.s.sociated with granite that they must have had the same origin. Granite is everywhere found to send off dikes into the overlying rocks, and must therefore have been in a state of fusion; that is, it must have existed as lava beneath the surface. It is obvious that fluid lava always exists in great quant.i.ty beneath areas of energetic volcanic activity.

Portions of this lava must in succession take the solid form. Wherever the surface is elevated along a line of fracture, the lava which is acc.u.mulated beneath rises above the level of the general reservoir of lava, and will therefore part with its heat more rapidly. On cooling, it becomes the granitic nucleus of the mountain. We ought also to suppose that, by the extremely slow process of the transmission of heat to the surface, the crust of the earth is everywhere increasing in thickness; that is, the upper portion of the great lava ma.s.s is solidifying.

Sir James Hall has shown, by experiment, that earthy substances, reduced to a state of fusion, become more highly crystalline as they are allowed to cool more slowly, and are subjected to greater pressure. It is difficult to conceive of these conditions existing in a higher degree than they do in the cooling ma.s.ses of lava below the stratified rocks.

These lavas must therefore take the highly crystalline form which the granitic rocks are found to have.

All the igneous rocks have therefore existed as subterranean lavas. The volcanic rocks have become vitreous, the granitic are crystalline, and the trappean are intermediate in structure, coinciding with the circ.u.mstances of pressure and rate of cooling under which they have severally been formed.

5. _The Elevation of Mountains_ is another result of volcanic action.

The height of mountains depends, in part, upon general elevation. Yet there is a different action, upon which the existence of the mountain, as such, depends. Whenever igneous action becomes intense under any portion of the earth's surface, and the elastic force greater than the repressive, the solid crust will be broken and raised up, and along this line of fracture the lava will rise above its general level elsewhere.

This lava, thus lifted out of the general ma.s.s, in time solidifies, and forms the nucleus of a mountain. At successive periods the elevating force is renewed, and adds somewhat to the mountain ma.s.s before supplied. In this way the mountain is ultimately formed.