Chapter 11

How did the surfacemm´come to receive its cover of sandstonesb? From the thickness and coarseness of these sediments draw inferences as to the land mass from which they were derived. Was it risingor subsiding? high or low? Were its streams slow or swift? Was the amount of erosion small or great?Note the strong dip of these sandstonesb. Was the surfacemm´tilted as now when the sandstones were deposited upon it? When was it tilted? Draw a diagram showing the attitude of the rocks after this tilting occurred, and their height relative to sea level.The surfacenn´is remarkably even, although diversified by some low hills which rise into the bedded rocks ofc, and it may be traced for long distances up and down the canyon. Were the layers ofband the surfacemm´always thus cut short bynn´as now? What has made the surfacenn´so even? How does it come to cross the hard crystalline rocks a and the weaker sandstonesbat the same impartial level? How did the sediments ofccome to be laid upon it? Give now the entire history recorded in the section, and in addition that involved in the production of the platformP, shown inFigure 130, and that of the cutting of the canyon. How does the time involved in the cutting of the canyon compare with that required for the production of the surfacesmm´,nn´, andP?

Note the strong dip of these sandstonesb. Was the surfacemm´tilted as now when the sandstones were deposited upon it? When was it tilted? Draw a diagram showing the attitude of the rocks after this tilting occurred, and their height relative to sea level.

The surfacenn´is remarkably even, although diversified by some low hills which rise into the bedded rocks ofc, and it may be traced for long distances up and down the canyon. Were the layers ofband the surfacemm´always thus cut short bynn´as now? What has made the surfacenn´so even? How does it come to cross the hard crystalline rocks a and the weaker sandstonesbat the same impartial level? How did the sediments ofccome to be laid upon it? Give now the entire history recorded in the section, and in addition that involved in the production of the platformP, shown inFigure 130, and that of the cutting of the canyon. How does the time involved in the cutting of the canyon compare with that required for the production of the surfacesmm´,nn´, andP?

Fig. 209.Unconformity between the Cambrian and Pre-Cambrian Rocks, Wisconsina, pre-Cambrian rocks, igneous and metamorphic, greatly deformed;a´, zone of decomposed pre-Cambrian rocks and residual clays on which rest the Cambrian sandstonesb. What unconformity do you find here? What two peneplains do you discover? Which is the older? Which was the more complete? To what stage has the present erosion cycle advanced? Suggest a reason why the valleys in the Cambrian are wider than those in the pre- Cambrian. When did the decay of the pre-Cambrian rocks of zonea´take place, and under what circumstances? Give the entire history recorded in this section, stating the successive cycles of erosion in their order and the causes which brought each cycle to a close

Fig. 209.Unconformity between the Cambrian and Pre-Cambrian Rocks, Wisconsin

a, pre-Cambrian rocks, igneous and metamorphic, greatly deformed;a´, zone of decomposed pre-Cambrian rocks and residual clays on which rest the Cambrian sandstonesb. What unconformity do you find here? What two peneplains do you discover? Which is the older? Which was the more complete? To what stage has the present erosion cycle advanced? Suggest a reason why the valleys in the Cambrian are wider than those in the pre- Cambrian. When did the decay of the pre-Cambrian rocks of zonea´take place, and under what circumstances? Give the entire history recorded in this section, stating the successive cycles of erosion in their order and the causes which brought each cycle to a close

CHAPTER X

EARTHQUAKES

Any sudden movement of the rocks of the crust, as when they tear apart when a fissure is formed or extended, or slip from time to time along a growing fault, produces a jar called an earthquake, which spreads in all directions from the place of disturbance.

The Charleston earthquake.On the evening of August 31, 1886, the city of Charleston, S.C., was shaken by one of the greatest earthquakes which has occurred in the United States. A slight tremor which rattled the windows was followed a few seconds later by a roar, as of subterranean thunder, as the main shock passed beneath the city. Houses swayed to and fro, and their heaving floors overturned furniture and threw persons off their feet as, dizzy and nauseated, they rushed to the doors for safety. In sixty seconds a number of houses were completely wrecked, fourteen thousand chimneys were toppled over, and in all the city scarcely a building was left without serious injury. In the vicinity of Charleston railways were twisted and trains derailed. Fissures opened in the loose superficial deposits, and in places spouted water mingled with sand from shallow underlying aquifers.The point of origin, orfocus, of the earthquake was inferred from subsequent investigations to be a rent in the rocks about twelve miles beneath the surface. From the center of greatest disturbance, which lay above the focus, a few miles northwest of the city, the surface shock traveled outward in every direction, with decreasing effects, at the rate of nearly two hundred miles per minute. It was felt from Boston to Cuba, and from eastern Iowa to the Bermudas, over a circular area whose diameter was a thousand miles.

The point of origin, orfocus, of the earthquake was inferred from subsequent investigations to be a rent in the rocks about twelve miles beneath the surface. From the center of greatest disturbance, which lay above the focus, a few miles northwest of the city, the surface shock traveled outward in every direction, with decreasing effects, at the rate of nearly two hundred miles per minute. It was felt from Boston to Cuba, and from eastern Iowa to the Bermudas, over a circular area whose diameter was a thousand miles.

An earthquake is transmitted from the focusthrough the elastic rocks of the crust, as a wave, or series of waves, of compression and rarefaction, much as a sound wave is transmitted through the elastic medium of the air. Each earth particle vibrates with exceeding swiftness, but over a very short path. The swing of a particle in firm rock seldom exceeds one tenth of an inch in ordinary earthquakes, and when it reaches one half an inch and an inch, the movement becomes dangerous and destructive.

Fig. 210.Block of the Earth’s Crust shaken by an Earthquakex, focus;a,b,c,d, successive spheroidal waves in the crust;a´,b´,c´,d´, successive surface waves produced by the outcropping ofa,b,c, andd

The velocity of earthquake waves, like that of all elastic waves, varies with the temperature and elasticity of the medium. In the deep, hot, elastic rocks they speed faster than in the cold and broken rocks near the surface. The deeper the point of origin and the more violent the initial shock, the faster and farther do the vibrations run.

Great earthquakes, caused by some sudden displacement or some violent rending of the rocks, shake the entire planet. Their waves run through the body of the earth at the rate of about three hundred and fifty miles a minute, and more slowly round its circumference, registering their arrival at opposite sides of the globe on the exceedingly delicate instruments of modern earthquake observatories.

Geological effects.Even great earthquakes seldom produce geological effects of much importance. Landslides may be shaken down from the sides of mountains and hills, and cracks may be opened in the surface deposits of plains; butthe transient shiver, which may overturn cities and destroy thousands of human lives, runs through the crust and leaves it much the same as before.

The India earthquake of 1897.No earthquake of history has produced greater geological effects than that which shook northeastern India in 1897. It laid in ruins a region thrice the size of the state of New York. In places not a masonry building was left standing and hard-wood trees were snapped across. Foothills of the Himalayas were stripped of soil and forests from base to summit by landslides. Streams which before were busily cutting down their rocky beds were now overloaded with waste from the slides. They were compelled to cease eroding their beds while they spread their valleys deep with sand, over which they now flow in broad and shallow channels. The incoherent deposits of the alluvial plains were riddled with rents through which ground water was forced out in such quantities as to flood considerable areas.

Certain other effects often attributed to the earthquake are rather the manifestations of the dislocation to which the earthquake was due. Permanent changes of level were effected. Some hills were found to have been lifted twenty feet, while others were lowered, and resurveys proved that the entire region had been compressed horizontally from north to south. Displacements occurred along several fault lines. One of these, with a throw of twenty-five feet at the surface and a length of twelve miles, crossed a river repeatedly, causing a series of waterfalls and lakes. All these disturbances are best explained by the theory that the shock was due to a slip along a deep and hidden thrust plane, accompanied by other movements of the strata along minor faults connected with it, some of which reached the surface.

Earthquakes attending great displacements.Great earthquakes frequently attend the displacement of large masses of the rocks of the crust. In 1822 the coast of Chile was suddenly raised three or four feet, and the rise was five or six feet a mile inland. In 1835 the same region was again upheaved from two to ten feet. In each instance a destructive earthquake was felt for one thousand miles along the coast.

Perhaps the most violent earthquake which ever visited the United States attended the depression, in 1812, of a region seventy-five miles long and thirty miles wide, near New Madrid, Mo. Much of the area was converted into swamps and some into shallow lakes, while a region twenty miles in diameter was bulged up athwart the channel of the Mississippi. Slight quakes are still felt in this region from time to time, showing that the strains to which the dislocation was due have not yet been fully relieved.

Earthquakes originating beneath the sea.Many earthquakes originate beneath the sea, and in a number of examples they seem to have been accompanied, as soundings indicate, by local subsidences of the ocean bottom. There have been instances where the displacement has been sufficient to set the entire Pacific Ocean pulsating for many hours. In mid ocean the wave thus produced has a height of only a few feet, while it may be two hundred miles in width. On shores near the point of origin destructive waves two or three score feet in height roll in, and on coasts thousands of miles distant the expiring undulations may be still able to record themselves on tidal gauges.

Distribution of earthquakes.Every half hour some considerable area of the earth’s surface is sensibly shaken by an earthquake, but earthquakes are by no means uniformly distributed over the globe. As we might infer from what we know as to their causes, earthquakes are most frequent in regions now undergoing deformation. Such are young rising mountain ranges, fault lines where readjustments recur from time to time, and the slopes of suboceanic depressions whose steepness suggests that subsidence may there be in progress.

Earthquakes, often of extreme severity, frequently visit the lofty and young ranges of the Andes, while they are little known in the subdued old mountains of Brazil. The Highlands of Scotland are crossed by a deep and singularly straight depression called the Great Glen, which has been excavated along a very ancient line of dislocation. The earthquakes which occur from time to time in this region, such as the Inverness earthquake in 1891, are referred to slight slips along this fault plane.In Japan, earthquakes are very frequent. More than a thousand are recorded every year, and twenty-nine world-shaking earthquakes occurred in the three years ending with 1901. They originate, for the most part, well down on the eastern flank of the earth fold whose summit is the mountainous crest of the islands, and which plunges steeply beneath the sea to the abyss of the Tuscarora Deep.

In Japan, earthquakes are very frequent. More than a thousand are recorded every year, and twenty-nine world-shaking earthquakes occurred in the three years ending with 1901. They originate, for the most part, well down on the eastern flank of the earth fold whose summit is the mountainous crest of the islands, and which plunges steeply beneath the sea to the abyss of the Tuscarora Deep.

Minor causes of earthquakes.Since any concussion within the crust sets up an earth jar, there are several minor causes of earthquakes, such as volcanic explosions and even the collapse of the roofs of caves. The earthquakes which attend the eruption of volcanoes are local, even in the case of the most violent volcanic paroxysms known. When the top of a volcano has been blown to fragments, the accompanying earth shock has sometimes not been felt more than twenty-five miles away.

Depth of focus.The focus of the Charleston earthquake, estimated at about twelve miles below the surface, was exceptionally deep. Volcanic earthquakes are particularly shallow, and probably no earthquakes known have started at a greater depth than fifteen or twenty miles. This distance is so slight compared with the earth’s radius that we may say that earthquakes are but skin-deep.

Should you expect the velocity of an earthquake to be greater in a peneplain or in a river delta?After an earthquake, piles on which buildings rested were found driven into the ground, and chimneys crushed at base. From what direction did the shock come?Chimneys standing on the south walls of houses toppled over on the roof. Should you infer that the shock in this case came from the north or south?How should you expect a shock from the east to affect pictures hanging on the east and the west walls of a room? how the pictures hanging on the north and the south walls?In parts of the country, as in southwestern Wisconsin, slender erosion pillars, or “monuments,” are common. What inference could you draw as to the occurrence in such regions of severe earthquakes in the recent past?

Should you expect the velocity of an earthquake to be greater in a peneplain or in a river delta?

After an earthquake, piles on which buildings rested were found driven into the ground, and chimneys crushed at base. From what direction did the shock come?

Chimneys standing on the south walls of houses toppled over on the roof. Should you infer that the shock in this case came from the north or south?

How should you expect a shock from the east to affect pictures hanging on the east and the west walls of a room? how the pictures hanging on the north and the south walls?

In parts of the country, as in southwestern Wisconsin, slender erosion pillars, or “monuments,” are common. What inference could you draw as to the occurrence in such regions of severe earthquakes in the recent past?

CHAPTER XI

VOLCANOES

Connected with movements of the earth’s crust which take place so slowly that they can be inferred only from their effects is one of the most rapid and impressive of all geological processes,—the extrusion of molten rock from beneath the surface of the earth, giving rise to all the various phenomena of volcanoes.

In a volcano, molten rock from a region deep below, which we may call its reservoir, ascends through a pipe or fissure to the surface. The materials erupted may be spread over vast areas, or, as is commonly the case, may accumulate about the opening, forming a conical pile known as the volcanic cone. It is to this cone that popular usage refers the wordvolcano; but the cone is simply a conspicuous part of the volcanic mechanism whose still more important parts, the reservoir and the pipe, are hidden from view.

Volcanic eruptions are of two types,—effusiveeruptions, in which molten rock wells up from below and flows forth in streams oflava(a comprehensive term applied to all kinds of rock emitted from volcanoes in a molten state), andexplosiveeruptions, in which the rock is blown out in fragments great and small by the expansive force of steam.

Eruptions of the Effusive Type

The Hawaiian volcanoes.The Hawaiian Islands are all volcanic in origin, and have a linear arrangement characteristic of many volcanic groups in all parts of the world. They are strung along a northwest-southeast line, their volcanoes standing intwo parallel rows as if reared along two adjacent lines of fracture or folding. In the northwestern islands the volcanoes have long been extinct and are worn low by erosion. In the southeastern island, Hawaii, three volcanoes are still active and in process of building. Of these Mauna Loa, the monarch of volcanoes, with a girth of two hundred miles and a height of nearly fourteen thousand feet above sea level, is a lava dome the slope of whose sides does not average more than five degrees. On the summit is an elliptical basin ten miles in circumference and several hundred feet deep. Concentric cracks surround the rim, and from time to time the basin is enlarged as great slices are detached from the vertical walls and engulfed. Such a volcanic basin, formed by the insinking of the top of the cone, is called acaldera.

Fig. 211.Mauna Loa

Fig. 212.Caldera of Kilauea

On the flanks of Mauna Loa, four thousand feet above sea level, lies the caldera of Kilauea, an independent volcano whose dome has been joined to the larger mountain by the gradual growth of the two. Ineach caldera the floor, which to the eye is a plain of black lava, is the congealed surface of a column of molten rock. At times of an eruption lakes of boiling lava appear which may be compared to air holes in a frozen river. Great waves surge up, lifting tons of the fiery liquid a score of feet in air, to fall back with a mighty plunge and roar, and occasionally the lava rises several hundred feet in fountains of dazzling brightness. The lava lakes may flood the floor of the basin, but in historic times have never been known to fill it and overflow the rim. Instead, the heavy column of lava breaks way through the sides of the mountain and discharges in streams which flow down the mountain slopes for a distance sometimes of as much as thirty-five miles. With the drawing off of the lava the column in the duct of the volcano lowers, and the floor of the caldera wholly or in part subsides. A black and steaming abyss marks the place of the lava lakes (Fig. 213). After a time the lava rises in the duct, the floor is floated higher, and the boiling lakes reappear.

Fig. 213.A Portion of the Caldera of Kilauea after a Collapse following an Eruption

The eruptions of the Hawaiian volcanoes are thus of the effusive type. The column of lava rises, breaks through the side of the mountain, and discharges in lava streams. There are no explosions, and usually no earthquakes, or very slight ones, accompany the eruptions. The lava in the calderas boils because of escaping steam, but the vapor emitted is comparatively little, and seldom hangs above the summits in heavy clouds. We see here in its simplest form the most impressive and important fact in all volcanic action, molten rock has been driven upward to the surface from some deep-lying source.

Fig. 214.Pahoehoe Lava, Hawaii

Lava flows.As lava issues from the side of a volcano or overflows from the summit, it flows away in a glowing stream resembling molten iron drawn white-hot from an iron furnace. The surface of the stream soon cools and blackens, and the hard crust of nonconducting rock may grow thick and firm enough to form a tunnel, within which the fluid lava may flow far before it loses its heat to any marked degree. Such tunnels may at last be left as caves by the draining away of the lava, and are sometimes several miles in length.

Pahoehoe and aa.When the crust of highly fluid lava remains unbroken after its first freezing, it presents a smooth, hummocky, and ropy surface known by the Hawaiian termpahoehoe(Fig. 214). On theother hand, the crust of a viscid flow may be broken and splintered as it is dragged along by the slowly moving mass beneath. The stream then appears as a field of stones clanking and grinding on, with here and there from some chink a dull red glow or a wisp of steam. It sets to a surface calledaa, of broken, sharp-edged blocks, which is often both difficult and dangerous to traverse (Fig. 215).

Fig. 215.Lava Flow of theAaType; Cinder Cones in the Distance, Arizona

Fissure eruptions.Some of the largest and most important outflows of lava have not been connected with volcanic cones, but have been discharged from fissures, flooding the country far and wide with molten rock. Sheet after sheet of molten rock has been successively outpoured, and there have been built up, layer upon layer, plateaus of lava thousands of feet in thickness and many thousands of square miles in area.

Iceland.This island plateau has been rent from time to time by fissures from which floods of lava have outpoured. In some instances the lava discharges along the whole length of the fissure, but more often only at certain points upon it. The Laki fissure, twenty miles long, was in eruption in 1783 for seven months. The inundation offluid rock which poured from it is the largest of historic record, reaching a distance of forty-seven miles and covering two hundred and twenty square miles to an average depth of a hundred feet. At the present time the fissure is traced by a line of several hundred insignificant mounds of fragmental materials which mark where the lava issued (Fig. 216).

Fig. 216.Small Cinder Cones marking an Eruptive Fissure, Iceland

The distance to which the fissure eruptions of Iceland flow on slopes extremely gentle is noteworthy. One such stream is ninety miles in length, and another seventy miles long has a slope of little more than one half a degree.Where lava is emitted at one point and flows to a less distance there is gradually built up a dome of the shape of an inverted saucer with an immense base but comparatively low. Manylava domeshave been discovered in Iceland, although from their exceedingly gentle slopes, often but two or three degrees, they long escaped the notice of explorers.The entire plateau of Iceland, a region as large as Ohio, is composed of volcanic products,—for the most part of successive sheets of lava whose total thickness falls little short of two miles. The lava sheets exposed to view were outpoured in open air and not beneath the sea; for peat bogs and old forest grounds are interbedded with them, and the fossil plants of these vegetable deposits prove that the plateau haslong been building and is very ancient. On the steep sea cliffs of the island, where its structure is exhibited, the sheets of lava are seen to be cut with manydikes,—fissures which have been filled by molten rock,—and there is little doubt that it was through these fissures that the lava outwelled in successive flows which spread far and wide over the country and gradually reared the enormous pile of the plateau.

Where lava is emitted at one point and flows to a less distance there is gradually built up a dome of the shape of an inverted saucer with an immense base but comparatively low. Manylava domeshave been discovered in Iceland, although from their exceedingly gentle slopes, often but two or three degrees, they long escaped the notice of explorers.

The entire plateau of Iceland, a region as large as Ohio, is composed of volcanic products,—for the most part of successive sheets of lava whose total thickness falls little short of two miles. The lava sheets exposed to view were outpoured in open air and not beneath the sea; for peat bogs and old forest grounds are interbedded with them, and the fossil plants of these vegetable deposits prove that the plateau haslong been building and is very ancient. On the steep sea cliffs of the island, where its structure is exhibited, the sheets of lava are seen to be cut with manydikes,—fissures which have been filled by molten rock,—and there is little doubt that it was through these fissures that the lava outwelled in successive flows which spread far and wide over the country and gradually reared the enormous pile of the plateau.

Fig. 217.Diagram illustrating the Structure of a Lava Plateau such as Icelandlf, lava flows;d, dikes

Eruptions of the Explosive Type

In the majority of volcanoes the lava which rises in the pipe is at least in part blown into fragments with violent explosions and shot into the air together with vast quantities of water vapor and various gases. The finer particles into—which the lava is exploded are calledvolcanic dustorvolcanic ashes, and are often carried long distances by the wind before they settle to the earth. The coarser fragments fall about the vent and there accumulate in a steep, conical, volcanic mountain. As successive explosions keep open the throat of the pipe, there remains on the summit a cup-shaped depression called thecrater.

Stromboli.To study the nature of these explosions we may visit Stromboli, a low volcano built chiefly of fragmental materials, which rises from the sea off the north coast of Sicily and is in constant though moderate action.Over the summit hangs a cloud of vapor which strikingly resembles the column of smoke puffed from the smokestack of a locomotive, in that it consists of globular masses, each the product of a distinct explosion. At night the cloud of vapor is lighted with a red glow at intervals of a few minutes, like the glow on the trail of smoke behind the locomotive when from time to time the fire box is opened. Because of this intermittent light flashing thousands of feet above the sea, Stromboli has been given the name of the Lighthouse of the Mediterranean.Looking down into the crater of the volcano, one sees a viscid lava slowly seething. The agitation gradually increases. A great bubble forms. It bursts with an explosion which causes the walls of the crater to quiver with a miniature earthquake, and an outrush of steam carries the fragments of the bubble aloft for a thousand feet to fall into the crater or on the mountain side about it. With the explosion the cooled and darkened crust of the lava is removed, and the light of the incandescent liquid beneath is reflected from the cloud of vapor which overhangs the cone.

Over the summit hangs a cloud of vapor which strikingly resembles the column of smoke puffed from the smokestack of a locomotive, in that it consists of globular masses, each the product of a distinct explosion. At night the cloud of vapor is lighted with a red glow at intervals of a few minutes, like the glow on the trail of smoke behind the locomotive when from time to time the fire box is opened. Because of this intermittent light flashing thousands of feet above the sea, Stromboli has been given the name of the Lighthouse of the Mediterranean.

Looking down into the crater of the volcano, one sees a viscid lava slowly seething. The agitation gradually increases. A great bubble forms. It bursts with an explosion which causes the walls of the crater to quiver with a miniature earthquake, and an outrush of steam carries the fragments of the bubble aloft for a thousand feet to fall into the crater or on the mountain side about it. With the explosion the cooled and darkened crust of the lava is removed, and the light of the incandescent liquid beneath is reflected from the cloud of vapor which overhangs the cone.

At Stromboli we learn the lesson that the explosive force in volcanoes is that of steam. The lava in the pipe is permeated with it much as is a thick boiling porridge. The steam in boiling porridge is unable to escape freely and gathers into bubbles which in breaking spurt out drops of the pasty substance; in the same way the explosion of great bubbles of steam in the viscid lava shoots clots and fragments of it into the air.

Krakatoa.The most violent eruption of history, that of Krakatoa, a small volcanic island in the strait between Sumatra and Java, occurred in the last week of August, 1883. Continuous explosions shot a column of steam and ashes. seventeen miles in air. A black cloud, beneath which was midnight darkness and from which fell a rain of ashes and stones, overspread the surrounding region to a distance of one hundred and fifty miles. Launched on the currents of the upper air, the dust was swiftly carried westward to long distances. Three days after the eruption it fell on the deck of a ship sixteen hundred miles away, and in thirteen days the finest impalpable powder from the volcano had floated round the globe. For many months the dust hung over Europe and America as a faint lofty haze illuminated at sunrise and sunset with brilliant crimson. In countries nearer the eruption, as in India and Africa, the haze for some time was so thick that it colored sun and moon with blue, green, and copper-red tints and encircled them with coronas.At a distance of even a thousand miles the detonations of the eruption sounded like the booming of heavy guns a few miles away. In one direction they were audible for a distance as great as that from San Francisco to Cleveland. The entire atmosphere was thrown into undulations under which all barometers rose and fell as the air waves thrice encircled the earth. The shock of the explosions raised sea waves which swept round the adjacent shores at a height of more than fifty feet, and which were perceptible halfway around the globe.At the close of the eruption it was found that half the mountain had been blown away, and that where the central part of the island had been the sea was a thousand feet deep.

At a distance of even a thousand miles the detonations of the eruption sounded like the booming of heavy guns a few miles away. In one direction they were audible for a distance as great as that from San Francisco to Cleveland. The entire atmosphere was thrown into undulations under which all barometers rose and fell as the air waves thrice encircled the earth. The shock of the explosions raised sea waves which swept round the adjacent shores at a height of more than fifty feet, and which were perceptible halfway around the globe.

At the close of the eruption it was found that half the mountain had been blown away, and that where the central part of the island had been the sea was a thousand feet deep.

Fig. 218.Ruins of St. Pierre, Martinique; Mt. Pelée in the Distance

Martinique and St. Vincent.In 1902 two dormant volcanoes of the West Indies, Mt. Pelee in Martinique and Soufriére in St. Vincent, broke into eruption simultaneously. No lava was emitted, but there were blown into the air great quantities of ashes, which mantled the adjacent parts of the islands with a pall as of gray snow. In early stages of the eruption lakes which occupied old craters were discharged and swept down the ash-covered mountain valleys in torrents of boiling mud.On several occasions there was shot from the crater of each volcano a thick and heavy cloud of incandescent ashes and steam, which rushed down the mountain side like an avalanche, red with glowing stones and scintillating with lightning flashes. Forests and buildings in its path were leveled as by a tornado, wood was charred and set on fire by the incandescent fragments, all vegetation was destroyed, and to breathe thesteam and hot, suffocating dust of the cloud was death to every living creature. On the morning of the 8th of May, 1902, the first of these peculiar avalanches from Mt. Pelee fell on the city of St. Pierre and instantly destroyed the lives of its thirty thousand inhabitants.

On several occasions there was shot from the crater of each volcano a thick and heavy cloud of incandescent ashes and steam, which rushed down the mountain side like an avalanche, red with glowing stones and scintillating with lightning flashes. Forests and buildings in its path were leveled as by a tornado, wood was charred and set on fire by the incandescent fragments, all vegetation was destroyed, and to breathe thesteam and hot, suffocating dust of the cloud was death to every living creature. On the morning of the 8th of May, 1902, the first of these peculiar avalanches from Mt. Pelee fell on the city of St. Pierre and instantly destroyed the lives of its thirty thousand inhabitants.

Fig. 219.An Eruption of Vesuvius, 1872The huge column of dust and steam rises to a height of about four miles above the sea. Drifting down the wind, the vapor condenses into copious rains. Such often produce destructive torrents of mud as they sweep down the ash-covered mountain side, and during the historic eruption of Vesuvius inA.D.69 the city of Herculaneum was thus buried. Lava flows are marked by the overhanging clouds of aqueous vapor condensed from the steam which the molten rock gives off.

Fig. 219.An Eruption of Vesuvius, 1872

The huge column of dust and steam rises to a height of about four miles above the sea. Drifting down the wind, the vapor condenses into copious rains. Such often produce destructive torrents of mud as they sweep down the ash-covered mountain side, and during the historic eruption of Vesuvius inA.D.69 the city of Herculaneum was thus buried. Lava flows are marked by the overhanging clouds of aqueous vapor condensed from the steam which the molten rock gives off.

The eruptions of many volcanoes partake of both the effusive and the explosive types: the molten rock in the pipe is in partblown into the air with explosions of steam, and in part is discharged in streams of lava over the lip of the crater and from fissures in the sides of the cone. Such are the eruptions of Vesuvius, one of which is illustrated inFigure 219.

Submarine eruptions.The many volcanic islands of the ocean and the coral islands resting on submerged volcanic peaks prove that eruptions have often taken place upon the ocean floor and have there built up enormous piles of volcanic fragments and lava. The Hawaiian volcanoes rise from a depth of eighteen thousand feet of water and lift their heads to about thirty thousand feet above the ocean bed. Christmas Island (seep. 194), built wholly beneath the ocean, is a coral-capped volcanic peak, whose total height, as measured from the bottom of the sea, is more than fifteen thousand feet. Deep-sea soundings have revealed the presence of numerous peaks which fail to reach sea level and which no doubt are submarine volcanoes. A number of volcanoes on the land were submarine in their early stages, as, for example, the vast pile of Etna, the celebrated Sicilian volcano, which rests on stratified volcanic fragments containing marine shells now uplifted from the sea.

Submarine outflows of lava and deposits of volcanic fragments become covered with sediments during the long intervals between eruptions. Such volcanic deposits are said to becontemporaneous, because they are formed during the same period as the strata among which they are imbedded. Contemporaneous lava sheets may be expected to bake the surface of the stratum on which they rest, while the sediments deposited upon them are unaltered by their heat. They are among the most permanent records of volcanic action, far outlasting the greatest volcanic mountains built in open air.

From upraised submarine volcanoes, such as Christmas Island, it is learned that lava flows which are poured out upon the bottom of the sea do not differ materially either in composition or texture from those of the land.

Volcanic Products

Vast amounts of steam are, as we have seen, emitted from volcanoes, and comparatively small quantities of other vapors, such as various acid and sulphurous gases. The rocks erupted from volcanoes differ widely in chemical composition and in texture.

Fig. 220.Cellular Lava

Acidic and basic lavas.Two classes of volcanic rocks may be distinguished,—those containing a large proportion of silica (silicic acid, SiO2) and therefore calledacidic, and those containing less silica and a larger proportion of the bases (lime, magnesia, soda, etc.) and therefore calledbasic. The acidic lavas, of whichrhyoliteandthrachyteare examples, are comparatively light in color and weight, and are difficult to melt. The basic lavas, of whichbasaltis a type, are dark and heavy and melt at a lower temperature.

Scoria and pumice.The texture of volcanic rocks depends in part on the degree to which they were distended by the steam which permeated them when in a molten state. They harden into compact rock where the steam cannot expand. Where the steam is released from pressure, as on the surface of a lava stream, it forms bubbles (steam blebs) of various sizes, which give the hardened rock a cellular structure (Fig. 220), In this way are formed the rough slags and clinkers calledscoria, which are found on the surface of flows and which are also thrown out as clots of lava in explosive eruptions.

On the surface of the seething lava in the throat of the volcano there gathers a rock foam, which, when hurled into the air, is cooled and falls aspumice,—a spongy gray rock so light that it floats on water.

Fig. 221.Amygdules in Lava

Amygdules.The steam blebs of lava flows are often drawn out from a spherical to an elliptical form resembling that of analmond, and after the rock has cooled these cavities are gradually filled with minerals deposited from solution by underground water. From their shape such casts are called amygdules (Greek,amygdalon, an almond). Amygdules are commonly composed of silica. Lavas contain both silica and the alkalies, potash and soda, and after dissolving the alkalies, percolating water is able to take silica also into solution. Mostagatesare banded amygdules in which the silica has been laid in varicolored, concentric layers (Fig. 222).

Fig. 222.Polished Section of an Agate

Glassy and stony lavas.Volcanic rocks differ in texture according also to the rate at which they have solidified. When rapidly cooled, as on the surface of a lava flow, molten rock chills to a glass, because the minerals of which it is composed have not had time to separate themselves from the fused mixture and form crystals. Under slow cooling, as in the interior of the flow, it becomes a stony mass composed of crystals set in a glassy paste. In thin slices of volcanic glass one may see under the microscope the beginnings of crystal growth in filaments and needles and feathery forms, which are the rudiments of the crystals of various minerals.

Spherulites, which also mark the first changes of glassy lavas toward a stony condition, are little balls within the rock, varying from microscopic size to several inches in diameter, and made up of radiating fibers.Perlitic structure, common among glassy lavas, consists of microscopic curving and interlacing cracks, due to contraction.

Perlitic structure, common among glassy lavas, consists of microscopic curving and interlacing cracks, due to contraction.

Fig. 223.Microsection showing the Beginnings of Crystal Growth in Glassy Lava

Fig. 224.Perlitic Structure and Spherulites,a,a

Flow linesare exhibited by volcanic rocks both to the naked eye and under the microscope. Steam blebs, together with crystals and their embryonic forms, are left arranged in lines and streaks by the currents of the flowing lava as it stiffened into rock.

Fig. 225.Flow Lines in Lava

Porphyritic structure.Rocks whose ground mass has scattered through it large conspicuous crystals(Fig. 226) are said to beporphyritic, and it is especially among volcanic rocks that this structure occurs. The ground mass of porphyries either may be glassy or may consist in part of a felt of minute crystals; in either case it represents the consolidation of the rock after its outpouring upon the surface. On the other hand, the large crystals of porphyry have slowly formed deep below the ground at an earlier date.

Fig. 226.Porphyritic Structure

Columnar structure.Just as wet starch contracts on drying to prismatic forms, so lava often contracts on cooling to a mass of close-set, prismatic, and commonly six-sided columns, which stand at right angles to the cooling surface. The upper portion of a flow, on rapid cooling from the surface exposed to the air, may contract to a confused mass of small and irregular prisms; while the remainder forms large and beautifully regular columns, which have grown upward by slow cooling from beneath (Fig. 227).

Fragmental Materials

Rocks weighing many tons are often thrown from a volcano at the beginning of an outburst by the breaking up of the solidified floor of the crater; and during the progress of an eruption large blocks may be torn from the throat of the volcano by the outrush of steam. But the most important fragmental materials are those derived from the lava itself. As lava rises in the pipe, the steam which permeates it is released from pressure and explodes, hurling the lava into the air in fragments of all sizes,—large pieces of scoria,lapilli(fragments the size of a pea or walnut), volcanic “sand” and volcanic “ashes.” The latter resemble in appearance the ashes of wood or coal, but they are not in any sense, like them, a residue after combustion.

Fig. 227.Columnar Structure in Basaltic Lava, Scotland

Volcanic ashes are produced in several ways: lava rising in the volcanic duct is exploded into fine dust by the steam which permeates it; glassy lava, hurled into the air and cooled suddenly, is brought into a state of high strain and tension, and, like Prince Rupert’s drops, flies to pieces at the least provocation. The clash of rising and falling projectiles also produces some dust, a fair sample of which may be made by grating together two pieces of pumice.

Beds of volcanic ash occur widely among recent deposits in the western United States. In Nebraska ash beds are found in twenty counties, and are often as white as powdered pumice. The beds grow thicker and coarser toward the southwestern part of the state, where their thickness sometimes reaches fifty feet. In what direction would you look for the now extinct volcano whose explosive eruptions are thus recorded?

Tuff.This is a convenient term designating any rock composed of volcanic fragments. Coarse tuffs of angular fragments are calledvolcanic breccia, and when the fragments have been rounded and sorted by water the rock is termed avolcanic conglomerate. Even when deposited in the open air, as on the slopes of a volcano, tuffs may be rudely bedded and their fragments more or less rounded, and unless marine shells or the remains of land plants and animals are found as fossils in them, there is often considerable difficulty in telling whether they were laid in water or in air. In either case they soon become consolidated. Chemical deposits from percolating waters fill the interstices, and the bed of loose fragments is cemented to hard rock.

The materials of which tuffs are composed are easily recognized as volcanic in their origin. The fragments are more orless cellular, according to the degree to which they were distended with steam when in a molten state, and even in the finest dust one may see the glass or the crystals of lava from which it was derived. Tuffs often containvolcanic bombs,—balls of lava which took shape while whirling in the air, and solidified before falling to the ground.

Fig. 228.Volcanic Bombs, Cinder Cone, California

Ancient volcanic rocks.It is in these materials and structures which we have described that volcanoes leave some of their most enduring records. Even the volcanic rocks of the earliest geological ages, uplifted after long burial beneath the sea and exposed to view by deep erosion, are recognized and their history read despite the many changes which they may have undergone. A sheet of ancient lava may be distinguished by its composition from the sediments among which it is imbedded. The direction of its flow lines may be noted. The cellular and slaggy surface where the pasty lava was distended by escaping steam is recognized by the amygdules which now fill the ancient steam blebs. In a pile of successive sheets of lava each flow may be distinguished and its thickness measured; for the surface of each sheet is glassy and scoriaceous, while beneath its upper portions the lava of each flow is more dense and stony. The length of time which elapsed before a sheet was buried beneath the materials of succeeding eruptions may be told by the amount of weathering which it had undergone, the depth of ancient soil—now baked to solid rock—upon it, and the erosion which it had suffered in the interval.

If the flow occurred from some submarine volcano, we may recognize the fact by the sea-laid sediments which cover it, filling the cracks and crevices of its upper surface and containing pieces of lava washed from it in their basal layers.

Long-buried glassy lavas devitrify, or pass to a stony condition, under the unceasing action of underground waters; but their flow lines and perlitic and spherulitic structures remain to tell of their original state.

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