Chapter 12

Fig. 229.A Volcanic Cone, Arizona

Ancient tuffs are known by the fragmental character of their volcanic material, even though they have been altered to firm rock. Some remains of land animals and plants may be found imbedded to tell that the beds were laid in open air; while the remains of marine organisms would prove as surely that the tuffs were deposited in the sea.

In these ways ancient volcanoes have been recognized near Boston, in southeastern Pennsylvania, about Lake Superior, and in other regions of the United States.

The Life History of a Volcano

The invasion of a region by volcanic forces is attended by movements of the crust heralded by earthquakes. A fissure or a pipe is opened and the building of the cone or the spreading of wide lava sheets is begun.

Volcanic cones.The shape of a volcanic cone depends chiefly on the materials erupted. Cones made of fragments may have sides as steep as the angle of repose, which in the case of coarsescoria is sometimes as high as thirty or forty degrees. About the base of the mountain the finer materials erupted are spread in more gentle slopes, and are also washed forward by rains and streams. The normal profile is thus a symmetric cone with a flaring base.

Fig. 230.Sarcoui, a Trachyte Dome, France

Cones built of lava vary in form according to the liquidity of the lava. Domes of gentle slope, as those of Hawaii, for example, are formed of basalt, which flows to long distances before it congeals. When superheated and emitted from many vents, this easily melted lava builds great plateaus, such as that of Iceland. On the other hand, lavas less fusible, or poured out at a lower temperature, stiffen when they have flowed but a short distance, and accumulate in a steep cone. Trachyte has been extruded in a state so viscid that it has formed steepsided domes like that of Sarcoui (Fig. 230).

Most volcanoes are built, like Vesuvius, both of lava flows and of tuffs, and sections show that the structure of the cone consists of outward-dipping, alternating layers of lava, scoria, and ashes.

Fig. 231.Section of VesuviusV, Vesuvius;S, Somma, a mountainous rampart half encircling Vesuvius, and like it built of outward-dipping sheets of tuff and lava;a, crystalline rocks;b, marine strata;c, tuffs containing seashells. Which is the older mountain, Vesuvius or Somma? Of what is Somma a remnant? Draw a diagram showing its original outline. Suggest what processes may have brought it to its present form. What record do you find of the earliest volcanic activity? What do you infer as to the beginnings of the volcano?

Fig. 231.Section of Vesuvius

V, Vesuvius;S, Somma, a mountainous rampart half encircling Vesuvius, and like it built of outward-dipping sheets of tuff and lava;a, crystalline rocks;b, marine strata;c, tuffs containing seashells. Which is the older mountain, Vesuvius or Somma? Of what is Somma a remnant? Draw a diagram showing its original outline. Suggest what processes may have brought it to its present form. What record do you find of the earliest volcanic activity? What do you infer as to the beginnings of the volcano?

From time to time the cone is rent by the violence of explosions and by the weight of the column of lava in the pipe. The fissures are filled with lava and some discharge on the sides of the mountain, building parasitic cones, while all form dikes, which strengthen the pile with ribs of hard rock and make it more difficult to rend.

Fig. 232.Crater Lake, OregonHow wide and deep is the basin which holds the lake? The mountain walls which enclose it are made of outward-dipping sheets of lava. Draw a diagram restoring the volcano of which they are the remnant. No volcanic fragments of the same nature as the materials of which the volcano is built are found about the region. What theory of the destruction of the cone does this fact favor?W´, Wizard Island, is a cinder cone. When was it built?

Fig. 232.Crater Lake, Oregon

How wide and deep is the basin which holds the lake? The mountain walls which enclose it are made of outward-dipping sheets of lava. Draw a diagram restoring the volcano of which they are the remnant. No volcanic fragments of the same nature as the materials of which the volcano is built are found about the region. What theory of the destruction of the cone does this fact favor?W´, Wizard Island, is a cinder cone. When was it built?

Great catastrophes are recorded in the shape of some volcanoes which consist of a circular rim perhaps miles in diameter, inclosing a vast crater or a caldera within which small cones may rise. We may infer that at some time the top of the mountain has been blown off, or has collapsed and been engulfed because some reservoir beneath had been emptied by long-continued eruptions (Fig. 232).

The cone-building stage may be said to continue until eruptions of lava and fragmental materials cease altogether. Sooner or later the volcanic forces shift or die away, and no further eruptions add to the pile or replace its losses by erosion during periods of repose. Gases however are still emitted, and, as sulphur vapors are conspicuous among them, such vents are calledsolfataras. Mount Hood, in Oregon, is an example of a volcano sunk to this stage. From a steaming rift on its side there rise sulphurous fumes which, half a mile down the wind, will tarnish a silver coin.

Fig. 233.Old Faithful Geyser in Eruption, Yellowstone National Park

Geysers and hot springs.The hot springs of volcanic regions are among the last vestiges of volcanic heat. Periodically eruptive boiling springs are termed geysers. In each of the geyser regions of the earth—the Yellowstone National Park, Iceland, and New Zealand—the ground water of the locality is supposed to be heated by ancient lavas that, because of the poor conductivity of the rock, still remain hot beneath the surface.

Old Faithful, one of the many geysers of the Yellowstone National Park, plays a fountain of boiling water a hundred feet in air; while clouds of vapor from the escaping steam ascend to several times that height. The eruptions take place at intervals of from seventy to ninety minutes. In repose the geyser is a quiet pool, occupying a craterlikedepression in a conical mound some twelve feet high. The conduit of the spring is too irregular to be sounded. The mound is composed of porous silica deposited by the waters of the geyser.

Geysers erupt at intervals instead of continuously boiling, because their long, narrow, and often tortuous conduits do not permit a free circulation of the water. After an eruption the tube is refilled and the water again gradually becomes heated. Deep in the tube where it is in contact with hot lavas the water sooner or later reaches the boiling point, and bursting into steam shoots the water above it high in air.

Fig. 234.Terrace and Cones of Siliceous Sinter deposited by Geysers, Yellowstone National Park

Carbonated springs.After all the other signs of life have gone, the ancient volcano may emit carbon dioxide as its dying breath. The springs of the region may long be charged with carbon dioxide, or carbonated, and where they rise through limestone may be expected to deposit large quantities of travertine. We should remember, however, that many carbonated springs, and many hot springs, are wholly independent of volcanoes.

Fig. 235.Mount Shasta, California

Fig. 236.Mount Hood, Oregon

The destruction of the cone.As soon as the volcanic cone ceases to grow by eruptions the agents of erosion begin to wear it down, and the length of time that has elapsed since the period of active growth may be roughly measured by the degree to which the cone has been dissected. We infer that Mount Shasta, whose conical shape is still preserved despite the gullies one thousand feet deep which trench its sides (Fig. 235), is younger than Mount Hood, which erosive agencies have carved to apyramidal form (Fig. 236). The pile of materials accumulated about a volcanic vent, no matter how vast in bulk, is at last swept entirely away. The cone of the volcano, active or extinct, is not old as the earth counts time; volcanoes are short- lived geological phenomena.

Fig. 237.Crandall Volcano

Fig. 238.Fossil Tree Trunks, Yellowstone National Park

Crandall volcano.This name is given to a dissected ancient volcano in the Yellowstone National Park, which once, it is estimated, reared its head thousands of feet above the surrounding country and greatly exceeded in bulk either Mount Shasta or Mount Etna. Not a line of the original mountain remains; all has been swept away by erosion except some four thousand feet of the base of the pile. This basal wreck now appears as a rugged region about thirty miles in diameter, trenched by deep valleys and cut into sharp peaks and precipitous ridges. In the center of the area is found the nucleus (N, Fig. 237),—a mass of coarsely crystalline rock that congealed deep in the old volcanic pipe. From it there radiate in all directions, like the spokes of a wheel, long dikes whose rock grows rapidly finer of grain as it leaves the vicinity ofthe once heated core. The remainder of the base of the ancient mountain is made of rudely bedded tuffs and volcanic breccia, with occasional flows of lava, some of the fragments of the breccia measuring as much as twenty feet in diameter. On the sides of canyons the breccia is carved by rain erosion to fantastic pinnacles. At different levels in the midst of these beds of tuff and lava are many old forest grounds. The stumps and trunks of the trees, now turned to stone, still in many cases stand upright where once they grew on the slopes of the mountain as it was building (Fig. 238). The great size and age of some of these trees indicate, the lapse of time between the eruption whose lavas or tuffs weathered to the soil on which they grew and the subsequent eruption which buried them beneath showers of stones and ashes.Near the edge of the area lies Death Gulch, in which carbon dioxide is given off in such quantities that in quiet weather it accumulates in a heavy layer along the ground and suffocates the animals which may enter it.

Near the edge of the area lies Death Gulch, in which carbon dioxide is given off in such quantities that in quiet weather it accumulates in a heavy layer along the ground and suffocates the animals which may enter it.

CHAPTER XII

UNDERGROUND STRUCTURES OF IGNEOUS ORIGIN

It is because long-continued erosion lays bare the innermost anatomy of an extinct volcano, and even sweeps away the entire pile with much of the underlying strata, thus leaving the very roots of the volcano open to view, that we are able to study underground volcanic structures. With these we include, for convenience, intrusions of molten rock which have been driven upward into the crust, but which may not have succeeded in breaking way to the surface and establishing a volcano. All these structures are built of rock forced when in a fluid or pasty state into some cavity which it has found or made, and we may classify them therefore, according to the shape of the molds in which the molten rock has congealed, as (1) dikes, (2) volcanic necks, (3) intrusive sheets, and (4) intrusive masses.

Dikes.The sheet of once molten rock with which a fissure has been filled is known as a dike. Dikes are formed when volcanic cones are rent by explosions or by the weight of the lava column in the duct, and on the dissection of the pile they appear as radiating vertical ribs cutting across the layers of lava and tuff of which the cone is built. In regions undergoing deformation rocks lying deep below the ground are often broken and the fissures are filled with molten rock from beneath, which finds no outlet to the surface. Such dikes are common in areas of the most ancient rocks, which have been brought to light by long erosion.

In exceptional cases dikes may reach the length of fifty or one hundred miles. They vary in width from a fraction of a foot to even as much as three hundred feet.

Fig. 239.Dikes, Spanish Peaks, Colorado

Dikes are commonly more fine of grain on the sides than in the center, and may have a glassy and crackled surface where they meet the inclosing rock. Can you account for this on any principle which you have learned?

Fig. 240.A Dissected Volcanic ConeN, volcanic neck;l,l, lava-topped table mountains;t,t, beds of tuff;d,d, dikes; dotted lines indicate the initial profile

Volcanic necks.The pipe of a volcano rises from far below the base of the cone,—from the deep reservoir from which its eruptions are supplied. When the volcano has become extinct this great tube remains filled with hardened lava. It forms a cylindrical core of solid rock, except for some distance below the ancient crater, where it may contain a mass of fragments which had fallen back into the chimney after being hurled into the air.

Fig. 241.Mount Johnson, a Volcanic Neck near Montreal

As the mountain is worn down, this central column known as thevolcanic neckis left standing as a conical hill (Fig. 240). Even when every other trace of the volcano has been swept away, erosion will not have passed below this great stalk on which the volcano was borne as a fiery flower whose site it remains to mark. In volcanic regions of deep denudation volcanic necks rise solitary and abrupt from the surrounding country as dome-shaped hills. They are marked features inthe landscape in parts of Scotland and in the St. Lawrence valley about Montreal (Fig. 241).

Fig. 242.The Palisades of the Hudson, New Jersey

Intrusive sheets.Sheets of igneous rocks are sometimes found interleaved with sedimentary strata, especially in regions where the rocks have been deformed and have suffered from volcanic action. In some instances such a sheet is seen to becontemporaneous(p. 248). In other instances the sheet must beintrusive. The overlying stratum, as well as that beneath, has been affected by the heat of the once molten rock. We infer that the igneous rock when in a molten state was forced between the strata, much as a card may be pushed between the leaves of a closed book. The liquid wedged its way between the layers, lifting those above to make room for itself. The source of the intrusive sheet may often be traced to some dike (known therefore as thefeeding dike), or to some mass of igneous rock.

Intrusive sheets may extend a score and more of miles, and, like the longest surface flows, the most extensive sheets consist of the more fusible and fluid lavas,—those of the basic class of which basalt is an example. Intrusive sheets are usually harder than the strata in which they lie and are therefore often left in relief after long denudation of the region (Fig. 315).

Fig. 243.Diagram of the Palisades of the Hudsoni, intrusive sheet;s, sandstone;d, feeding dike;HR, Hudson River

On the west bank of the Hudson there extends from New York Bay north for thirty miles a bold cliff several hundred feet high,— thePalisades of the Hudson. It is the outcropping edge of a sheet of ancient igneous rock, which rests on stratified sandstones and is overlain by strata of the same series. Sandstones and lava sheet together dip gently to the west and the latter disappears from view two miles back from the river.It is an interesting question whether the Palisades sheet iscontemporaneousorintrusive. Was it outpoured on the sandstones beneath it when they formed the floor of the sea, and covered forthwith by the sediments of the strata above, or was it intruded among these beds at a later date?

It is an interesting question whether the Palisades sheet iscontemporaneousorintrusive. Was it outpoured on the sandstones beneath it when they formed the floor of the sea, and covered forthwith by the sediments of the strata above, or was it intruded among these beds at a later date?

Fig. 244.Section of Electric Peak. E. and Gray Peak, G,Yellowstone National ParkIntrusive sheets and masses of igneous rock are drawn in black

The latter is the case: for the overlying stratum is intensely baked along the zone of contact. At the west edge of the sheet is found the dike in which the lava rose to force its way far and wide between the strata.Electric Peak, one of the prominent mountains of the Yellowstone National Park, is carved out of a mass of strata into which many sheets of molten rock have been intruded. The western summit consists of such a sheet several hundred feet thick. Studying the section ofFigure 244, what inference do you draw as to the source of these intrusive sheets?

Electric Peak, one of the prominent mountains of the Yellowstone National Park, is carved out of a mass of strata into which many sheets of molten rock have been intruded. The western summit consists of such a sheet several hundred feet thick. Studying the section ofFigure 244, what inference do you draw as to the source of these intrusive sheets?

Intrusive Masses

Fig. 245.Stone Mountain, Georgia, a Granite Boss

Bosses.This name is generally applied to huge irregular masses of coarsely crystalline igneous rock lying in the midst of other formations. Bosses vary greatly in size and may reach scores of miles in extent. Seldom are there any evidences found that bosses ever had connection with the surface. On the other hand, it is often proved that they have been driven, or have melted their way, upward into the formations in which they lie; for they give off dikes and intrusive sheets, and have profoundly altered the rocks about them by their heat.

Fig. 246.Map of Granite Bosses near Baltimore (areas horizontally Lined)

The texture of the rock of bosses proves that consolidation proceeded slowly and at great depths, and it is only because of vast denudation that they are now exposed to view. Bosses are commonly harder than the rocks about them, and stand up, therefore, as rounded hills and mountainous ridges long after the surrounding country has worn to a low plain (Fig. 245).

Figure 246exhibits a few small bosses of granite near Baltimore as examples ofnumerous areas of igneous rock within the Piedmont Belt which represent bodies of molten rock which solidified deep below the surface.

TheSpanish Peaksof southeastern Colorado were formed by the upthrust of immense masses of igneous rock, bulging and breaking the overlying strata. On one side of the mountains the throw of the fault is nearly a mile, and fragments of deep-lying beds were dragged upward by the rising masses. The adjacent rocks were altered by heat to a distance of several thousand feet. No evidence appears that the molten rock ever reached the surface, and if volcanic eruptions ever took place either in lava flows or fragmental materials, all traces of them have been effaced. The rock of the intrusive masses is coarsely crystalline, and no doubt solidified slowly under the pressure of vast thicknesses of overlying rock, now mostly removed by erosion.A magnificent system of dikes radiates from the Peaks to a distance of fifteen miles, some now being left by long erosion as walls a hundred feet in height (Fig. 239). Intrusive sheets fed by the dikes penetrate the surrounding strata, and their edges are cut by canyons as much as twenty-five miles from the mountain. In these strata are valuable beds of lignite, an imperfect coal, which the heat of dikes and sheets has changed to coke.

A magnificent system of dikes radiates from the Peaks to a distance of fifteen miles, some now being left by long erosion as walls a hundred feet in height (Fig. 239). Intrusive sheets fed by the dikes penetrate the surrounding strata, and their edges are cut by canyons as much as twenty-five miles from the mountain. In these strata are valuable beds of lignite, an imperfect coal, which the heat of dikes and sheets has changed to coke.

Fig. 247.Section of a Laccolith

Laccoliths.The laccolith (Greek laccos, cistern; lithos, stone) is a variety of intrusive masses in which molten rock has spread between the strata, and, lifting the strata above it to a dome- shaped form, has collected beneath them in a lens-shaped body with a flat base.

TheHenry Mountains, a small group of detached peaks in southern Utah, rise from a plateau of horizontal rocks. Some of the peaks are carved wholly in separate domelike uplifts of the strata of the plateau. In others, as Mount Hillers, the largest of the group, there is exposed on the summit a core of igneous rock from which the sedimentary rocks of the flanks dip steeply outward in all directions. Instill others erosion has stripped off the covering strata and has laid bare the core to its base; and its shape is here seen to be that of a plano-convex lens or a baker’s bun, its flat base resting on the undisturbed bedded rocks beneath. The structure of Mount Hillers is shown inFigure 248. The nucleus of igneous rock is four miles in diameter and more than a mile in depth.

Fig. 248.Section of Mount Hillers

Regional intrusions.These vast bodies of igneous rock, which may reach hundreds of miles in diameter, differ little from bosses except in their immense bulk. Like bosses, regional intrusions give off dikes and sheets and greatly change the rocks about them by their heat. They are now exposed to view only because of the profound denudation which has removed the upheaved dome of rocks beneath which they slowly cooled. Such intrusions are accompanied —whether as cause or as effect is still hardly known—by deformations, and their masses of igneous rock are thus found as the core of many great mountain ranges. The granitic masses of which the Bitter Root Mountains and the Sierra Nevadas have been largely carved are each more than three hundred miles in length. Immense regional intrusions, the cores of once lofty mountain ranges, are found upon the Laurentian peneplain.

Physiographic effects of intrusive masses.We have already seen examples of the topographic effects of intrusive masses in Mount Hillers, the Spanish Peaks, and in the great mountain ranges mentioned in the paragraph on regional intrusions, although in the latter instances these effects are entangled with the effects of other processes. Masses of igneous rock cannot be intruded within the crust without an accompanying deformation on a scale corresponding to the bulk of the intruded mass. The overlying strata are arched into hills ormountains, or, if the molten material is of great extent, the strata may conceivably be floated upward to the height of a plateau. We may suppose that the transference of molten matter from one region to another may be among the causes of slow subsidences and elevations. Intrusions give rise to fissures, dikes, and intrusive sheets, and these dislocations cannot fail to produce earthquakes. Where intrusive masses open communication with the surface, volcanoes are established or fissure eruptions occur such as those of Iceland.

The Intrusive Rocks

The igneous rocks are divided into two general classes,—thevolcanicoreruptiverocks, which have been outpoured in open air or on the floor of the sea, and theintrusiverocks, which have been intruded within the rocks of the crust and have solidified below the surface. The two classes are alike in chemical composition and may be divided into acidic and basic groups. In texture the intrusive rocks differ from the volcanic rocks because of the different conditions under which they have solidified. They cooled far more slowly beneath the cover of the rocks into which they were pressed than is permitted to lava flows in open air. Their constituent minerals had ample opportunity to sort themselves and crystallize from the fluid mixture, and none of that mixture was left to congeal as a glassy paste.

They consolidated also under pressure. They are never scoriaceous, for the steam with which they were charged was not allowed to expand and distend them with steam blebs. In the rocks of the larger intrusive masses one may see with a powerful microscope exceedingly minute cavities, to be counted by many millions to the cubic inch, in which the gaseous water which the mass contained was held imprisoned under the immense pressure of the overlying rocks.

Naturally these characteristics are best developed in the intrusives which cooled most slowly, i.e. in the deepest-seatedand largest masses; while in those which cooled more rapidly, as in dikes and sheets, we find gradations approaching the texture of surface flows.

Varieties of the intrusive rocks.We will now describe a few of the varieties of rocks of deep-seated intrusions. All are even grained, consisting of a mass of crystalline grains formed during one continuous stage of solidification, and no porphyritic crystals appear as in lavas.

Granite, as we have learned already, is composed of three minerals,—quartz, feldspar, and mica. According to the color of the feldspar the rock may be red, or pink, or gray. Hornblende—a black or dark green mineral, an iron-magnesian silicate, about as hard as feldspar—is sometimes found as a fourth constituent, and the rock is then known ashornblendic granite. Granite is an acidic rock corresponding to rhyolite in chemical composition. We may believe that the same molten mass which supplies this acidic lava in surface flows solidifies as granite deep below ground in the volcanic reservoir.

Syenite, composed of feldspar and mica, has consolidated from a less siliceous mixture than has granite.

Diorite, still less siliceous, is composed of hornblende and feldspar,—the latter mineral being of different variety from the feldspar of granite and syenite.

Gabbro, a typical basic rock, corresponds to basalt in chemical composition. It is a dark, heavy, coarsely crystalline aggregate of feldspar andaugite(a dark mineral allied to hornblende). It often containsmagnetite(the magnetic black oxide of iron) andolivine(a greenish magnesian silicate).

In the northern states all these types, and many others also of the vast number of varieties of intrusive rocks, can be found among the rocks of the drift brought from the areas of igneous rock in Canada and the states of our northern border.

Fig. 249.Ground Plan of Dikes in Granite. (Scale 80 feet to the inch)What is the relative age of the dikesaa,bb, andcc?

Fig. 250.AandB. Mountains of coarsely Crystalline Igneousi, surrounded by Sedimentary Stratasands´Copy each diagram and complete it, so as to show whether the mass of igneous rock is a volcanic neck, a boss, or a laccolith

Fig. 250.AandB. Mountains of coarsely Crystalline Igneousi, surrounded by Sedimentary Stratasands´

Copy each diagram and complete it, so as to show whether the mass of igneous rock is a volcanic neck, a boss, or a laccolith

Summary.The records of geology prove that since the earliest of their annals tremendous forces have been active in the earth. In all the past, under pressures inconceivably great, molten rock has been driven upward into the rocks of the crust. It has squeezed into fissures forming dikes; it has burrowed among the strata as intrusive sheets; it has melted the rocks away or lifted the overlying strata, filling the chambers which it has made with intrusive masses. During all geological ages molten rock has found way to the surface, and volcanoes have darkened the sky with clouds of ashes and poured streams of glowing lava down theirsides. The older strata,—the strata which have been most deeply buried,—and especially those which have suffered most from folding and from fracture, show the largest amount of igneous intrusions. The molten rock which has been driven from the earth’s interior to within the crust or to the surface during geologic time must be reckoned in millions of cubic miles.

Fig. 251.

1, limestone; 2, tuff; 3, 5, 7, shale with marine shells; 4, 6, lava, dotted portions scoriaceous. Give the history recorded in this section

Fig. 252.

a, sedimentary strata with intrusive sheets;b, sedimentary strata;c, lava flow;d, dike. Give the succession of events recorded in this section

Fig. 253.

Which of the lava sheets of this section are contemporaneous anti which intrusive,—A, whose upper surface is overlain with a conglomerate of rolled lava pebbles;B, the cracks and seams of whose upper surface are filled with the material of the overlying sandstone;C, which breaks across the strata in which it is imbedded;D, which includes fragments of both the underlying and overlying strata and penetrates their crevices and seams?

Fig. 254.Mato Tepee, Wyoming

This magnificent tower of igneous rock three hundred feet in height has been called by some a volcanic neck. Is the direction of the columns that which would obtain in the cylindrical pipe of a volcano? The tower is probably the remnant of a small laccolith, an outlying member of a group of laccoliths situated not far distant

The Interior Condition of the Earth and Causes of Vulcanism and Deformation

The problems of volcanoes and of deformation are so closely connected with that of the earth’s interior that we may consider them together. Few of these problems are solved, and we may only state some known facts and the probable conclusions which may be drawn as inferences from them.

The interior of the earth is hot.Volcanoes prove that in many parts of the earth there exist within reach of the surface regions of such intense heat that the rock is in a molten condition. Deep wells and mines show everywhere an increase in temperature below the surface shell affected by the heat of summer and the cold of winter,—a shell in temperate latitudes sixty or seventy feet thick. Thus in a boring more than a mile deep at Schladebach, Germany, the earth grows warmer at the rate of 1° F. for every sixty-seven feet as we descend. Taking the average rate of increase at one degree for every sixty feet of descent, and assuming that this rate, observed at the moderate distances open to observation, continues to at least thirty-five miles, the temperature at that depth must be more than three thousand degrees,—a temperature at which all ordinary rocks would melt at the earth’s surface. The rate of increase in temperature probably lessens as we go downward, and it may not be appreciable below a few hundred miles. But there is no reason to doubt thatthe interior of the earth is intensely hot. Below a depth of one or two score miles we may imagine the rocks everywhere glowing with heat.

Although the heat of the interior is great enough to melt all rocks at atmospheric pressure, it does not follow that the interior is fluid. Pressure raises the fusing point of rocks, and the weight of the crust may keep the interior in what may be called a solid state, although so hot as to be a liquid or a gas were the pressure to be removed.

The interior of the earth is dense and heavy.The earth behaves as a globe more rigid than glass under the strains to which it is subjected by the attractions of the sun and moon and other heavenly bodies. The jar of world-shaking earthquakes passes through the earth’s interior with nearly twice the velocity with which it would traverse solid steel, and since the speed of elastic waves depends on the density and elasticity of the medium, it follows that the globe is as a whole more denseand rigid than steel.The interior of the earth is extremely dense and rigid.

The common rocks of the crust are about two and a half times heavier than water, while the earth as a whole weighs five and six-tenths times as much as a globe of water of the same size.The interior is therefore much more heavy than the crust.This may be caused in part by compression of the interior under the enormous weight of the crust, and in part also by an assortment of material, the heavier substances, such as the heavy metals, having gravitated towards the center.

Between the crust, which is solid because it is cool, and the interior, which is hot enough to melt were it not for the pressure which keeps it dense and rigid, there may be an intermediate zone in which heat and pressure are so evenly balanced that here rock liquefies whenever and wherever the pressure upon it may be relieved by movements of the crust. It is perhaps from such a subcrustal layer that the lava of volcanoes is supplied.

The causes of volcanic action.It is now generally believed that theheatof volcanoes is that of the earth’s interior. Other causes, such as friction and crushing in the making of mountains and the chemical reactions between oxidizing agents of the crust and the unoxidized interior, have been suggested, but to most geologists they seem inadequate.

There is much difference of opinion as to theforcewhich causes molten rock to rise to the surface in the ducts of volcanoes. Steam is so evidently concerned in explosive eruptions that many believe that lava is driven upward by the expansive force of the steam with which it is charged, much as a viscid liquid rises and boils over in a test tube or kettle.

But in quiet eruptions, and still more in the irruption of intrusive sheets and masses, there is little if any evidence that steam is the driving force. It is therefore believed by many geologists that it ispressure due to crustal movements and internal stresseswhich squeezes molten rock from below into fissures and ducts in the crust. It is held by some that where considerable water is supplied to the rising column of lava, as from the ground water of the surrounding region, and where the lava is viscid so that steam does not readily escape, the eruption is of the explosive type; when these conditions do not obtain, the lava outwells quietly, as in the Hawaiian volcanoes. It is held by others not only that volcanoes are due to the outflow of the earth’s deep-seated heat, but also that the steam and other emitted gases are for the most part native to the earth’s interior and never have had place in the circulation of atmospheric and ground waters.

Volcanic action and deformation.Volcanoes do not occur on wide plains or among ancient mountains. On the other hand, where movements of the earth’s crust are in progress in the uplift of high plateaus, and still more in mountain making, molten rock may reach the surface, or may be driven upward toward it forming great intrusive masses. Thus extensive lava flows accompanied the upheaval of the block mountains of western North America and the uplift of the Colorado plateau. A line of recent volcanoes may be traced along the system of rift valleys which extends from the Jordan and Dead Sea through eastern Africa to Lake Nyassa. The volcanoes of the Andes show how conspicuous volcanic action may be in young rising ranges. Folded mountains often show a core of igneous rock, which by long erosion has come to form the axis and the highest peaks of the range, as if the molten rock had been squeezed up under the rising upfolds. As we decipher the records of the rocks in historical geology we shall see more fully how, in all the past, volcanic action has characterized the periods of great crustal movements, and how it has been absent when and where the earth’s crust has remained comparatively at rest.

The causes of deformation.As the earth’s interior, or nucleus, is highly heated it must be constantly though slowly losing itsheat by conduction through the crust and into space; and since the nucleus is cooling it must also be contracting. The nucleus has contracted also because of the extrusion of molten matter, the loss of constituent gases given off in volcanic eruptions, and (still more important) the compression and consolidation of its material under gravity. As the nucleus contracts, it tends to draw away from the cooled and solid crust, and the latter settles, adapting itself to the shrinking nucleus much as the skin of a withering apple wrinkles down upon the shrunken fruit. The unsupported weight of the spherical crust develops enormous tangential pressures, similar to the stresses of an arch or dome, and when these lateral thrusts accumulate beyond the power of resistance the solid rock is warped and folded and broken.

Since the planet attained its present mass it has thus been lessening in volume. Notwithstanding local and relative upheavals the earth’s surface on the whole has drawn nearer and nearer to the center. The portions of the lithosphere which have been carried down the farthest have received the waters of the oceans, while those portions which have been carried down the least have emerged as continents.

Although it serves our convenience to refer the movements of the crust to the sea level as datum plane, it is understood that this level is by no means fixed. Changes in the ocean basins increase or reduce their capacity and thus lower or raise the level of the sea. But since these basins are connected, the effect of any change upon the water level is so distributed that it is far less noticeable than a corresponding change would be upon the land.

CHAPTER XIII

METAMORPHISM AND MINERAL VEINS

Under the action of internal agencies rocks of all kinds may be rendered harder, more firmly cemented, and more crystalline. These processes are known asmetamorphism, and the rocks affected, whether originally sedimentary or igneous, are calledmetamorphic rocks. We may contrast with metamorphism the action of external agencies in weathering, which render rocks less coherent by dissolving their soluble parts and breaking down their crystalline grains.

Contact metamorphism.Rocks beneath a lava flow or in contact with igneous intrusions are found to be metamorphosed to various degrees by the heat of the cooling mass. The adjacent strata may be changed only in color, hardness, and texture. Thus, next to a dike, bituminous coal may be baked to coke or anthracite, and chalk and limestone to crystalline marble. Sandstone may be converted into quartzite, and shale intoargillite, a compact, massive clay rock. New minerals may also be developed. In sedimentary rocks there may be produced crystals of mica and ofgarnet(a mineral as hard as quartz, commonly occurring in red, twelve-sided crystals). Where the changes are most profound, rocks may be wholly made over in structure and mineral composition.

In contact metamorphism, thin sheets of molten rock produce less effect than thicker ones. The strongest heat effects are naturally caused by bosses and regional intrusions, and the zone of change about them may be several miles in width. In these changes heated waters and vapors from the masses of igneous rocks undoubtedly play a very important part.

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