THE RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE

VOLCANIC MOUNTAINS OF EJECTED MATERIALS

The mechanics of crater explosions.—If we now turn from the lava volcano to the active cinder cone, we encounter an entire change of scene. In place of the quiet flow and convulsive movements of the molten lava, we here meet with repeated explosions of greater or less violence. If we are to profitably study the manner of the explosions, considering the volcanic vent as a great experimental apparatus, it would be well to select for our purpose a volcano which is in a not too violent mood. The well-known cinder cone of Stromboli in the Eolian group of islands north of Sicily has, with short and unimportant interruptions, remained in a state of light explosive activity since the beginning of the Christian era. Rising as it does some three thousand feet directly out of the Mediterranean, and displaying by day a white steam cap and an intermittent glow by night, its summit can be seen for a distance of a hundred miles at sea and it has justly been called the “Lighthouse of the Mediterranean.” The “flash” interval of this beacon may vary from one to twenty minutes, and it may show, furthermore, considerable variation of intensity.

For the reason that the crater of the mountain is located at one side and at a considerable distance below the actual summit, the opportunity here afforded of looking into the crater is most favorable whenever the direction of the wind is such as to push aside the overhanging steam cloud (Fig. 111). Long ago the Italian vulcanologist Spallanzani undertook to make observations from above the crater, and many others since his day have profited by his example.

Within the crater of the volcano there is seen a lava surface lightly frozen over and traversed by many cracks from which vapor jets are issuing. Here, as in the Kilauea crater, there are open pools of boiling lava. From some of these, lava is seen welling out to overflow the frozen surface; from others, steam is ejected in puffs as though from the stack of a locomotive. Within others lava is seen heaving up and down in violent ebullition, and at intervals a great bubble of steam is ejected with explosive violence, carrying up with it a considerable quantity of the still molten lava, together with its scumlike surface, to fall outside the crater and rattle down the mountain’s slope into the sea. Following this explosion the lava surface in the pool is lowered and the agitation is renewed, to culminate after the further lapse of a few minutes in a second explosion of the same nature. The rise of the lava which precedes the ejection appears at night as a brighter reflection or glow from the overhanging steam cloud—the flash seen by the mariner from his vessel.

What is going on within the crater of Stromboli we may perhaps best illustrate by the boiling of a stiff porridge over a hot fire. Any one who has made corn mush over a hot camp fire is fully aware that in proportion as the mush becomes thicker by the addition of the meal, it is necessary to stir the mass with redoubled vigor if anything is to be retained within the kettle. The thickening of the mush increases its viscosity to such an extent that the steam which is generated within it is unable to make its escape unless aided by openings continually made for it by the stirring spoon. If the stirring motion be stopped for a moment, the steam expands to form great bubbles which soon eject the pasty mass from the kettle.

For the crater of Stromboli this process is illustrated by the series of diagrams in Fig. 112. As the lava rises toward the surface, presumably as a result of convectional currents within the chimney of the volcano, the contained steam is relieved from pressure, so that at some depth below the surface it begins to separate out in minute vesicles or bubbles, which, expanding as they rise, acquire a rapidly accelerating velocity. Soon they flow together with a quite sudden increase of their expansive energy, and now shooting upward with further accelerated velocity, a layer of liquid lava with its cover of scum is raised on the surface of a gigantic bubble and thrown high into the air. Cooled during their flight, the quickly congealed lava masses become the tuff or volcanic ash which is the material of the cinder cone.

Grander volcanic eruptions of cinder cones.—Most cinder and composite cones, in the intervals between their grander eruptions, if not entirely quiescent, lapse into a period, of light activity during which their crater eruptions appear to be in all essential respects like the habitual explosions within the Strombolian crater. This phase of activity is, therefore, described as Strombolian. By contrast, the occasional grander eruptions which have punctuated the history of all larger volcanoes are described in the language of Mercalli as Vulcanian eruptions, from the best studied example.

Just what it is that at intervals brings on the grander Vulcanian outburst within a volcano is not known with certainty; but it is important to note that there is an approach to periodicity in the grander eruptions. It is generally possible to distinguish eruptions of at least two orders of intensity greater than the Strombolian phase; a grander one, the examples of which may be separated by centuries, and one or more orders of relatively moderate intensity which recur at intervals perhaps of decades, their time intervals subdividing the larger periods marked off by the eruptions of the first order.

The eruption of Volcano in 1888.—In the Eolian Islands to the north of Sicily was located the mythical forge of Vulcan. From this locality has come our word “volcano”, and both the island and the mountain bear no other name to-day (Fig. 113). There is in the structure of the island the record of a somewhat complex volcanic history, but the form of the large central cinder cone was, according to Scrope, acquired during the eruption of 1786, at which time the crater is reported to have vomited ash for a period of fifteen days. Passing after this eruption into the solfatara condition, with the exception of a light eruption in 1873, the volcano remained quiet until 1886. So active had been the fumeroles within the crater during the latter part of this period that an extensive plant had been established there for the collection especially of boracic acid. In 1886 occurred a slight eruption, sufficient to clear out the bottom of the crater, though not seriously to disturb the English planter whose vineyards and fig orchards were in the valley or atrio near the point d upon the map (Fig. 113), nearly a mile from the crater rim. On the 3d of August, 1888, came the opening discharge of an eruption, which, while not of the first order of magnitude, was yet the greatest in more than a century of the mountain’s history, and may serve us to illustrate the Vulcanian phase of activity within a cinder cone. During the day, to the accompaniment of explosions of considerable violence, projectiles fell outside the crater rim and rolled down the steep slopes toward the atrio. These explosions were repeated at intervals of from twenty to thirty minutes, each beginning in a great upward rush of steam and ash, accompanied by a low rumbling sound. During the following night the eruptions increased in violence, and the anxious planter remained on watch in his villa a mile from the crater. Falling asleep toward morning, he was rudely awakened by a rain of projectiles falling upon his roof. Hastily snatching up his two children he ran toward the door just as a red hot projectile, some two feet in diameter, descended through the roof, ceiling, and floor of the drawing room, setting fire to the building. A second projectile similar to the first was smashed into fragments at his feet as he was emerging from the house, burning one of the children. Making his escape to Vulcanello at the extremity of the island, the remainder of the night and the following day, until rescue came from Lipari, were spent just beyond the range of the falling masses.

When the writer visited the island some months later, the eruption was still so vigorous that the crater could not be reached. The ruined villa, smashed and charred, stood with its walls half buried in ash and lapilli, among which were partly smashed pumiceous lava projectiles. The entire atrio about the mountain lay buried in cinder to the depth of several feet and was strewn with projectiles which varied in size from a man’s fist to several feet in diameter (Fig. 114). The larger of these exhibited the peculiar “bread-crust” surface and had generally been smashed by the force of their fall after the manner of a pumpkin which has been thrown hard against the ground. One of these projectiles fully three feet in diameter was found at the distance of a mile and a half from the crater. Though diminished considerably in intensity, the rhythmic explosions within the crater still recurred at intervals varying from four minutes to half an hour, and were accompanied by a dull roar easily heard at Lipari on a neighboring island six miles away. Simultaneously, a dark cloud of “smoke”, the peculiar “cauliflower cloud” or pino mounted for a couple of miles above the crater (Fig. 115), and the rise was succeeded by a rain of small lava fragments or lapilli outside the crater rim.

There seems to be no good reason to doubt that Vulcanian cinder eruptions of this type differ chiefly in magnitude from the rhythmic explosion within the crater of Stromboli, if we except the elevation of a considerable quantity of accessory and older tuff which is derived from the inner walls of the crater and carried upward into the air together with the pasty cakes of fresh lava derived from the chimney. It is this accessory material which gives to the pino its dark or even black appearance.

The eruption of Taal volcano on January 30, 1911.—The recent eruption of the cinder cone known as Taal volcano is of interest, not only because so fresh in mind, but because two neighboring vents erupted simultaneously with explosions of nearly equal violence (Fig. 116). This Philippine volcano lies near the center of a lake some fifteen miles in diameter and about fifty miles south of the city of Manila. After a period of rest extending over one hundred and fifty years, the symptoms of the coming eruption developed rapidly, and on the morning of January 30 grand explosions of steam and ash occurred simultaneously in the neighboring craters, and the condensed moisture brought down the ash in an avalanche of scalding mud which buried the entire island. Almost the entire population of the island, numbering several hundreds, was literally buried in the blistering mud (Fig. 117); and the gases from the explosions carried to the distant shores of the lake added to this number many hundred victims.

The shocks which accompanied the explosions raised a great wave upon the surface of the lake, which, advancing upon the shores, washed away structures for a distance of nearly a half mile.

The materials and the structure of cinder cones.—Obviously the materials which compose cinder cones are the cooled lava fragments of various degrees of coarseness which have been ejected from the crater. If larger than a finger joint, such fragments are referred to as volcanic projectiles, or, incorrectly, as “volcanic bombs.” Of the larger masses it is often true that the force of expulsion has not been applied opposite the center of mass of the body. Thus it follows that they undergo complex whirling motions during their flight, and being still semiliquid, they develop curious pear-shaped or less regular forms (Fig. 118). When crystals have already separated out in the lava before its rise in the chimney of the volcano, the surrounding fluid lava may be blown to finely divided volcanic dust which floats away upon the wind, thus leaving the crystals intact to descend as a crystal rain about the crater. Such a shower occurred in connection with the eruption of Etna in 1669, and the black augite crystals may to-day be gathered by the handful from the slopes of the Monti Rossi (Fig. 125, p. 125).

The term lapilli, or sometimes rapilli, is applied to the ejected lava fragments when of the average size of a finger joint. This is the material which still partially covers the unexhumed portions of the city of Pompeii. Volcanic sand, ash, and dust are terms applied in order to increasingly fine particles of the ejected lava. The finest material, the volcanic dust, is often carried for hundreds and sometimes even for thousands of miles from the crater in the high-level currents of the atmosphere. Inasmuch as this material is deposited far from the crater and in layers more or less horizontal, such material plays a small rÔle in the formation of the cinder cone. The coarser sands and ash, on the other hand, are the materials from which the cinder cone is largely constructed.

The manner of formation and the structure of cinder cones may be illustrated by use of a simple laboratory apparatus (Fig. 119). Through an opening in a board, first white and then colored sand is sent up in a light current of air or gas supplied from suitable apparatus. The alternating layers of the sand form in the attitudes shown; that is to say, dipping inward or toward the chimney of the volcano at all points within the crater rim, and outward or away from it at all points outside (Fig. 119). If the experiment is carried so far that at its termination sand slides down the crater walls into the chimney below, the inward dipping layers will be truncated, or even removed entirely, as shown in Fig. 119 b.

The profile lines of cinder cones.—The shapes of cinder cones are notably different from those of lava mountains. While the latter are domes, the mountains constructed of cinder are conical and have curves of profile that are concave upward instead of convex (Fig. 120). In the earlier stages of its growth the cinder cone has a crater which in proportion to the height of the mountain is relatively broad (Fig. 99, p. 104).

Speaking broadly, the diameter of the crater is a measure of the violence of the explosions within the chimney. A single series of short and violent explosive eruptions builds a low and broad cinder cone. A long-continued succession of moderately violent explosions, on the other hand, builds a high cone with crater diameter small if compared with the mountain’s altitude, and the profile afforded is a remarkably beautiful sweeping curve (Fig. 121). Toward the summit of such a cone the loose materials of which it is composed are at as steep an angle as they can lie, the so-called angle of repose of the material; whereas lower down the flatter slopes have been determined by the distribution of the cinder during its fall from the air. When one makes the ascent of such a mountain, he encounters continually steeper grades, with the most difficult slope just below the crest.

The composite cone.—The life histories of volcanoes are generally so varied that lava domes and the pure types of cinder cones are less common than volcanoes in which paroxysmal eruptions have alternated with explosions, and where, therefore, the structure of the mountain represents a composite of lava and cinder. Such composite cones possess a skeleton of solid rock upon which have been built up alternate sloping layers of cinder and lava. In most respects such cones stand in an intermediate position between lava domes and cinder cones.

Regarded as a retaining wall for the lava which mounts in the chimney, the cinder cone is obviously the weakest of all. Should lava rise in a cinder cone without an explosion occurring, the cone is at once broken through upon one side by the outwelling of the lava near the base. Thus arises the characteristic breached cone of horseshoe form (Fig. 122).

Quite in contrast with the weak cinder cone is the lava dome with its rock walls and relatively flat slopes. Considered as a retaining wall for lava it is much the strongest type of volcanic mountain, and it is likely that the hydrostatic pressure of the lava within the crater would seldom suffice to rupture the walls, were it not that the molten rock first fuses its way into old stream tunnels buried under the mountain slopes (see ante, p. 112). Composite cones have a strength as retaining walls for lava which is intermediate between that of the other types. Their Vulcanian eruptions of the convulsive type are initiated by the formation of a rent or fissure upon the mountain flanks at elevations well above the base, the opening of the fissure being generally accompanied by a local earthquake of greater or less violence.

From one or more such fissures the lava issues usually with sufficient violence at the place of outflow to build up over it either an enlarged type of driblet cone, referred to as a “mouth”, or bocca[1] (Fig. 123), or one or more cinder cones which from their position upon the flanks of the larger volcano are referred to as parasitic cones (Fig. 124). The lava of Vesuvius more frequently yields bocchi at the place of outflow, whereas the flanks of Etna are pimpled with great numbers of parasitic cinder cones, each the monument to some earlier eruption (Fig. 125).

It is generally the case that a single eruption makes but a relatively small contribution to the bulk of the mountain. From each new cone or bocca there proceeds a stream of lava spread in a relatively narrow stream extending down the slopes (Fig. 126).

The caldera of composite cones.—Because of the varied episodes in the history of composite cones, they lack the regular lines characteristic of the two simpler types. The larger number of the more important composite cones have been built up within an outer crater of relatively large diameter, the Somma cone or caldera, which surrounds them like a gigantic ruff or collar. This caldera is clearly in most cases at least the relic of an earlier explosive crater, after which successive eruptions of lesser violence have built a more sharply conical structure. This can only be interpreted to mean that most larger and long-active volcanoes have been born in the grandest throes of their life history, and that a larger or smaller lateral migration of the vent has been responsible for the partial destruction of the explosion crater. Upon Vesuvius we find the crescent-like rim of Monte Somma; on Etna it is the Val del Bove, etc. It is this caldera of composite cones which gave rise to the theory of the “elevation crater” of von Buch (see ante, p. 95, and Fig. 127).

The eruption of Vesuvius in 1906.—The volcano Vesuvius rises on the shores of the beautiful bay of Naples only about ten miles distant from the city of Naples. The mountain consists of the remnant of an earlier broad-mouthed explosion crater, the Monte Somma, and an inner, more conical elevation, the Monte Vesuvio. Before the eruption of 1906 this central cone was sharply conical and rose to a height of about 4300 feet above the surface of the bay, or above the highest point of the ancient caldera. The base of this inner cone is at an elevation of something less than half that of the entire mass, and is separated from the encircling ring wall of the old crater by the atrio, to which corresponds in height a perceptible shelf or piano upon the slope toward the bay of Naples (Fig. 128).

An active composite cone like that of Vesuvius is for the greater part of the time in the Strombolian condition; that is to say, light crater explosions continue with varying intensity and interval, except when the mountain has been excited to the periodic Vulcanian outbreaks with which its history has been punctuated. The Strombolian explosions have sufficient violence to eject small fragments of hot lava, which, falling about the crater, slowly built up a rather sharp cone. The period of Strombolian activity has, therefore, been called the cone-producing period. Just before each new outbreak of the Vulcanian type, the altitude of the mountain has, therefore, reached a maximum, and since the larger explosive eruptions remove portions of this cone at the same time that they increase the dimensions of the crater, the Vulcanian stage in contrast to the other has been called the crater-producing period. In this period, then, the material ejected during the explosions does not consist solely of fresh lava cakes, but in part of the older dÉbris derived from the crater walls, whence it is avalanched upon the chimney after each larger explosion. The overhanging cloud, which during the Strombolian period has consisted largely of steam and is noticeably white, now assumes a darker tone, the “smoke” which characterizes the Vulcanian eruption.

On several historical occasions the cone of Vesuvius has been lowered by several hundred feet, the greatest of relatively recent truncations having occurred in 1822 and in 1906. Between Vulcanian eruptions the Strombolian activity is by no means uniform, and so the upward growth of the cone is subject to lesser interruptions and truncations (Fig. 129).

The Vesuvian eruption of 1906 has been selected as a type of the larger Vulcanian eruption of composite cones, because it combined the explosive and paroxysmal elements, and because it has been observed and studied with greater thoroughness than any other. The latest previous eruption of the Vulcanian order had occurred in 1872. Some two years later the period of active cone building began and proceeded with such rapidity that by 1880 the new cone began to appear above the rim of the crater of 1872. From this time on occasional light eruptions interrupted the upbuilding process, and as the repairs were not in all cases completed before a new interruption, a nest of cones, each smaller than the last, arose in series like the outdrawn sections of an old-time spyglass. At one time no less than five concentric craters were to be seen.

For a brief period in the fall of 1904 Vesuvius had been in almost absolute repose, but soon thereafter the Strombolian crater explosions were resumed. On May 25, 1905, a small stream of lava began to issue from a fissure high up upon the central cone, and from this time on the lava continued to flow down to the valley or atrio, separating the inner cone from the caldera remnant of Monte Somma. Seen in the night, this stream of lava appeared from Naples like a red hot wire laid against the mountain’s side (Fig. 130). With gradual augmentation of Strombolian explosions and increase in volume of the flowing lava stream, the same condition continued until the first days of April in 1906. The flowing lava had then overrun the tracks of the mountain railway and accumulated in considerable quantity within the atrio (Fig. 131).

On the morning of April 4, a preliminary stage of the eruption was inaugurated by the opening of a new radial fissure about 500 feet below the summit of the cone (Fig. 132 a), and by early afternoon the cone-destroying stage began with the rise of a dark “cauliflower cloud” or pino to replace the lighter colored steam cloud. The cone was beginning to fall into the crater, and old lava dÉbris was mingled in the ejections with the lava clots blown from the still fluid material within the chimney. From now on short and snappy lightning flashes played about the black cloud, giving out a sharp staccato “tack-a-tack.” The volume and density of the cloud and the intensity of the crater explosions continued to increase until the culmination on April 7. On April 5 at midnight a new lava mouth appeared upon the same fissure which had opened near the summit, but now some 300 feet lower (Fig. 132 b). The lava now welled out in larger volume corresponding to its greater head, and the stream which for ten months had been flowing from the highest outlet upon the cone now ceased to flow. The next morning, April 6, at about 8 o’clock, lava broke out at several points some distance east of the opening b, and evidently upon another fissure transverse to the first (Fig. 132 c). The lava surface within the chimney must still have remained near its old level,—effective draining had not yet begun,—since early upon the following morning a small outflow began nearly at the top of the cone upon the opposite side and at least a thousand feet higher.

The culmination of the eruption came in the evening of April 7, when, to the accompaniment of light earthquakes felt as far as Naples, lava issued for the first time in great volume from a mouth more than halfway down the mountain side (Fig. 132 f), and thus began the drainage of the chimney. At about the same time with loud detonations a huge black cloud rose above the crater in connection with heavy explosions, and a rain of cinder was general in the region about the mountain but especially within the northeast quadrant. Those who were so fortunate as to be in Pompeii had a clear view of the mountain’s summit where red hot masses of lava were thrown far into the air. The direction of these projections was reported to have been not directly upward, but inclined toward the northeast quadrant of the mountain; but since with a northeast surface wind the heaviest deposit of ash and dust should have been upon the southwestern quadrant of the mountain, it is evident that the material was carried upward until it reached the contrary upper currents of the atmosphere, to be by them distributed.

When the heavy curtain of ash, which now for a number of succeeding days overhung all the circum-Vesuvian country, began to lift (Fig. 133), it was seen that the summit of the cone had been truncated an average of some 500 feet (Fig. 134). All the slopes and much of the surrounding country had the aspect of being buried beneath a cocoa-colored snow of a depth to the northeastward of several feet, where it had drifted into all the hollow ways so as almost to efface them (Fig. 135). More than thrice as heavy as water, the weak roof timbers of the houses at the base of the mountain gave way beneath the added load upon them, thus making many victims. Inasmuch, however, as the ash-fall partakes of the same general characters as in eruptions from cinder cones, we may here give our attention especially to the streams of lava which issued upon the opposite flank of the mountain (Fig. 136).

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The main lava stream descended the first steep slopes with the velocity of a mile in twenty-five minutes, about the strolling speed of a pedestrian, but this rate was gradually reduced as the stream advanced farther from the mouth. Taking advantage of each depression of the surface, the black stream advanced slowly but relentlessly toward the cities at the southwest base of the mountain. With a motion not unlike that of a heap of coal falling over itself down a slope, the block lava advances without burning the objects in its path (Fig. 137).

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The beautiful pines are merely charred where snapped off and are carried forward upon the surface of the stream (Fig. 138). When a real obstruction, such as a bridge or a villa, is encountered, the stream is at first halted, but the rear crowding upon the van, unless a passage is found at the side, the lava front rises higher and higher until by its weight the obstruction is forced to give way (Figs. 139 and 140).

The sequence of events within the chimney.—The thorough study of this Vesuvian eruption has placed us in a position to infer with some confidence in our conclusions the sequence of events within the chimney and crater of the volcano, both before and during the eruption. Anticipating some conclusions derived from the observed dissection of volcanoes, which will be discussed below, it may be stated that what might be termed the core of the composite cone—the chimney—is a more or less cylindrical plug of cooled lava which during the active period of the vent has an interior bore of probably variable caliber. This plug in its lower section appears in solid black in all the diagrams of Fig. 141. During the cone-building period (Fig. 141 a and b) the plug is obviously built upward along with the cone, for lava often flows out at a level a few hundred feet only below the crater rim. By what process this chimney building goes on is not well understood, though some light is thrown upon it by the post-eruption stage of Mont PelÉ in 1902-1903 (see below).

Both the older and newer sections of this plug or chimney are furnished some support against the outward pressure of the contained lava by the surrounding wall of tuff; and they are, therefore, in a condition not unlike that of the inner barrel of a great gun over which sleeves of metal have been shrunk so as to give support against bursting pressures. On the other hand, when not sustaining the hydrostatic pressure of the liquid lava within, the chimney would tend to be crushed in by the pressure of the surrounding tuff. Its strength to withstand bursting pressures is dependent not alone upon the thickness of its rock walls, but also upon its internal diameter or caliber. A steam cylinder of given thickness of wall, as is well known, can resist bursting pressures in proportion as its internal diameter is small. So in the volcanic chimney, any tendency to remelt from within the chimney walls must weaken them in a twofold ratio.

We are yet without accurate temperature observations upon the lava in volcanic chimneys, but it seems almost certain that these temperatures rise as the Vulcanian stage is approaching, and such elevation of temperature must be followed by a greater or less re-fusion of the chimney walls. The sequence of events during the late Vesuvian eruption is, then, naturally explained by progressive re-fusion and consequent weakening of the chimney walls, thus permitting a radial fissure to open near the top and gradually extend downwards. Thus at first small and high outlets were opened insufficient to drain the chimney, but later, on April 7, after this fissure had been much extended and a new and larger one had opened at a lower level, the draining began and the surface of lava commenced rapidly to sink.

When the rapid sinking of the lava surface occurred, the lower lava layers were almost immediately relieved of pressure, thus causing a sudden expansion of the contained steam and resulting in grand crater explosions. The partially refused and fissured upper chimney, now unable to withstand the inward pressure of the surrounding tuff walls, since outward pressures no longer existed, crushed in and contributed its materials and those of the surrounding tuff to the fragments of fresh lava rising in volume in the grand explosions (Fig. 141 c). In outline, then, these seem to be the conditions which are indicated by the sequence of observed events in connection with the late Vesuvian outbreak.

The spine of PelÉ.—The disastrous eruption of Mont PelÉ upon Martinique in the year 1902 is of importance in connection with the interesting problem of the upward growth of volcanic chimneys during the cone-building period of a volcano. After the conclusion of this great Vulcanian eruption, a spine of lava grew upward from the chimney of the main crater until it had reached an elevation of more then a thousand feet above its base, a figure of the same order of magnitude as the probable height of the upper section of the Vesuvian chimney previous to the eruption of 1906. The PelÉ spine (Fig. 142) did not grow at a uniform rate, but was subject to smaller or larger truncations, but for a period of 18 days the upward growth was at the rate of about 41 feet per day. Later, the mass split upon a vertical plane revealing a concave inner surface, and was somewhat rapidly reduced in altitude to 600 feet (Fig. 143), only to rise again to its full height of about 1000 feet some three months later.

While apparently unique as an observed phenomenon, and not free from uncertainty as to its interpretation, the growth of this obelisk has at least shown us that a mass of rock can push its way up above the chimney of an active volcano even when there are no walls of tuff about it to sustain its outward pressures.

The aftermath of mud flows.—When the late Vulcanian explosions of Vesuvius had come to an end, all slopes of the mountain, but especially the higher ones, were buried in thick deposits of the cocoa-colored ash, included in which were larger and smaller projectiles. As this material is extremely porous, it greedily sucks up the water which falls during the first succeeding rains. When nearly saturated, it begins to descend the slopes of the mountain and soon develops a velocity quite in contrast with that of the slow-moving lava. The upper slopes are thus denuded, while the fields and even the houses about the base are invaded by these torrents of mud (lava d’acqua). Inasmuch as these mud flows are the inevitable aftermath of all grander explosive eruptions, the Italian government has of late spent large sums of money in the construction of dikes intended to arrest their progress in the future. It was streams of this sort that buried the city of Herculaneum after the explosive eruption of 79 A.D.

After the mud flows have occurred, the Vesuvian cone, like all similar volcanic cones under the same conditions, is found with deep radial corrugations (Fig. 144), such as were long ago described as “barrancoes” and supposed to support the “elevation crater” theory of volcano formation.

The dissection of volcanoes.—To the uninitiated it might appear a hopeless undertaking to attempt to learn by observation the internal structure of a volcano, and especially of a complex volcano of the composite type. The earliest successful attempt appears to have been made by Count Caspar von Sternberg in order to prove the correctness of the theory of his friend, the poet Goethe. Goethe had claimed that a little hill in the vicinity of Eger, on the borders of Bohemia, was an extinct volcano, though the foremost geologist of the time the famous Werner, had promulgated the doctrine that this hill, in common with others of similar aspect, originated in the combustion of a bed of coal. The elevation in question, which is known as the KammerbÜhl, consists mainly of cinder, and Goethe had maintained that if a tunnel were to be driven horizontally into the mountain from one of its slopes, a core or plug of lava would be encountered beneath the summit. The excavations, which were completed in 1837, fully verified the poet’s view, for a lava plug was found to occupy the center of the mass and to connect with a small lava stream upon the side of the hill (Fig. 145).

It is not, however, to such expensive projects that reference is here made, but rather to processes which are continually going on in nature, and on a far grander scale. The most important dissecting agent for our purpose is running water, which is continually paring down the earth’s surface and disclosing its buried structures. How much more convincing than any results of artificial excavation, as evidence of the internal structure of a volcano, is the monument represented in Fig. 146, since here the lava plug stands in relief like a gigantic thumb still surrounded by a remnant of cinder deposits. Such exposed chimneys of former volcanoes are found in many regions, and have become known as volcanic necks, pipes, or plugs.

Not infrequently the beds of tuff composing the flanks of the volcano, upon dissection by the same process, bring to light walls of cooled lava standing in relief (Fig. 147)—the filling of the fissure which gave outlet to the flanks of the mountain at the time of the eruption. Study of exposed dikes formed in connection with recent eruptions of Vesuvius has shown that in many instances they are still hollow, the lava having drained from them before complete consolidation.

Another agent which is effective in uncovering the buried structures of volcanoes is the action of waves on shores. Always a relatively vigorous erosive agency, the softer structures of volcanic cones are removed with especial facility by this agent. On the shores of the island of Volcano, the little cone of Vulcanello has been nearly half carried away by the waves, so as to reveal with especial perfection the structure of the cinder beds as well as the internal rock skeleton of the mass. Here the characteristic dips of lava streams, intercalated as they now are between tuff deposits and the lava which consolidated in fissures, are both revealed.

In mid-Atlantic a quite perfect crater, the St. Paul’s Rocks, has been cut nearly in half so as to produce a natural harbor (Fig. 148).

In still other instances we may thank the volcano itself for opening up the interior of the mountain for our inspection. The eruption in 1888 of the Japanese volcano of Bandai-san, by removing a considerable part of the ancient cone, has afforded us a section completely through the mountain. The summit and one side of the small Bandai was carried completely away, and there was substituted a yawning crater eccentric to the former mountain and having its highest wall no less than 1500 feet in height (Fig. 149). In two hours from the first warning of the explosion the catastrophe was complete and the eruption over.

The eruption of Krakatoa in 1883, probably the grandest observed volcanic explosion in historic times, left a volcanic cone divided almost in half and open to inspection (Fig. 150). Rakata, Danan, and Perbuatan had before constituted a line of cones built up round individual craters subsequent to the partial destruction of an earlier caldera, portions of which were still existent in the islands Verlaten and Lang. By the eruption of 1883 all the exposed parts and considerable submerged portions of the two smaller cones were entirely destroyed, and the larger one, known as Rakata, was divided just outside the plug so as to leave a precipitous wall rising directly from the sea and showing lava streams in alternation with somewhat thicker tuff layers, the whole knit together by numerous lava dikes.

In order to carry our dissecting process down to levels below the base of the volcanic mountain, it is usually necessary to inspect the results of erosion by running water. Here the plug or chimney, instead of being surrounded by tuff, is inclosed by the country rock of the region, which is commonly a sedimentary formation. Such exposed lower sections of volcanic chimneys are numerous along the northwestern shores of the British Isles. Where aligned upon a dislocation or noteworthy fissure in the rocks, the group of plugs has been referred to as a scar or cicatrice (Fig. 151). Associated with the plugs of the cicatrice are not infrequently dikes, or, it may be, sheets of lava extended between layers of sediment and known as sills.

If we are able to continue the dissection process to still greater depths, we encounter at last igneous rock having a texture known as granitic and indicating that the process of consolidation was not only exceedingly slow but also uninterrupted. This rock is found in masses of larger dimensions, and though generally of more or less irregular form, no one dimension is of a different order of magnitude from the others. Such masses are commonly described as bosses, or, if especially large, as batholites (Fig. 152). Wherever the rock beds appear as though they had been forced up by the upward pressure of the igneous mass, the latter takes the form of a mushroom and has been described as a laccolite (Figs. 479-481, pp. 441-442). Evidence seems, however, to accumulate that in the greater number of cases the molten rock has fused its way upward, in part assimilating and in part inclosing the rock which it encountered. This process of upward fusion has been likened to the progress of a red hot iron burning its way through a board.

The formation of lava reservoirs.—The discarding of the earlier notion that the earth has a liquid interior makes it proper in discussing the subject of volcanoes to at least touch upon the origin of the molten rock material. As already pointed out, such reservoirs as exist must be local and temporary, or it would be difficult to see how the existing condition of earth rigidity could be maintained. From the rate at which rock temperatures rise, at increasing depths below the surface, it is clear that all rocks would be melted at very moderate depths only, if they were not kept in a solid state by the prodigious loads which they sustain. Any relief from this load should at once result in fusion of the rock.

Now the restriction of active volcanoes to those zones of the earth’s surface within which mountains are rising, and where in consequence earthquakes are felt, has furnished us at least a clew to the origin of the lava. Regarded as a structure capable of sustaining a load, the competency of an arch is something quite remarkable, so that the arching up of strong rock formations into anticlines within the upper layers of the zone of flow, or of combined fracture and flow, would be sufficient to remove the load from relatively weak underlying beds, which in consequence would be fused and form local reservoirs of lava (Figs. 152 and 153).

It has been further quite generally observed that lines of volcanoes, in so far as they betray any relation in position to neighboring mountain ranges, tend to appear upon the rear or flatter limb of unsymmetrical arches, or where local tension would favor the opening of channels toward the surface. Moreover, wherever recent block movements of surface portions of the earth’s shell have been disclosed in the neighborhood of volcanoes, the latter appear to be connected with downthrown blocks, as though the lava had, so to speak, been squeezed out from beneath the depressed block or blocks.

We must not, however, forget that the igneous rocks are greatly restricted in the range of their chemical composition. No igneous rock type is known which could be formed by the fusion of any of the carbonate rocks such as limestone or dolomite, or of the more siliceous rocks, such as sandstone or quartzite. There remains only the argillaceous class of sediments, the shales and slates, and so soon as we examine the composition of these rocks we are struck by the remarkable resemblance to that of the class of igneous rocks. For purposes of comparison there is given below the composite or average constitution of igneous rocks in parallel column, with the average attained by combining the analyses of 56 slates and shales, the latter recalculated with water excluded:

	Average Igneous Rock						Average Shale
	(Clark)			(Washington)
SiO2	61.25			61.69			63.34
Al2O3	15.81			15.94			15.56
Fe2O3	2.70	}	6.31	1.88	}	4.53	4.41	}	7.89
FeO	3.61			2.65			3.48
MgO	4.47			4.90			3.54
CaO	5.03			5.02			3.33
Na2O	3.64			4.09			1.29
K2O	2.87			3.35			3.52
TiO2	.62			.48			.53
	100.00			100.00			100.00

This close resemblance is probably of deep significance, for the reason that shales and slates are structurally the weakest of all rocks and for the further reason that they rather generally directly underlie the carbonate rocks, which are by contrast the strongest (see ante, p. 37). For these reasons shales and slates are the only rocks which are likely to be fused by relief from load through the formation of anticlinal arches within the earth’s zone of flow. If this view is well founded, lavas and other igneous rocks are in large part fused argillaceous sediments formed in connection with the process of folding, or are refused rocks of igneous origin and similar composition.

Character profiles.—The character profiles of features connected in their origin with volcanoes are particularly easy to recognize, and in a few cases in which they might be confused with others of a different origin, an examination of the materials of the features should lead to a definitive judgment.

The lava plains which result from massive outflows of basalt might perhaps strictly be regarded as lack of feature, so great may be their continuous extent. Wherever definite vents exist, a broad flat dome is the usual result of the extravasation of a basaltic lava. The puys of France and many of the Kuppen of Germany, being formed from less fluid lava, have afforded profiles with relatively small radius of curvature.

In its youthful stage, the cinder cone usually presents a broad summit sag and relatively short side slopes, whereas the cone of later stages is apt to present long sweeping and upwardly concave curves with both the gradient and the radius of curvature increasing rapidly toward the summit. In contrast, too, with the earlier stage, the crest is relatively small. A marked reduction in the high symmetry of such profiles is noted wherever a breaching by lava outflow has occurred (Fig. 154).

With the composite cone, complexity and corresponding lack of symmetry is introduced, especially in the partially ruined caldera, and by the more or less accidental distribution of parasitic cones, as well as by migrations of the central cone. Peculiarly similar acuminated profiles result from spatter-cone formation, from the formation of a superchimney spine, and by the uncovering of the chimney through denudational processes—the volcanic neck.

Another important feature resulting from denudation is the Mesa or table mountain with its protecting basalt cap above softer rocks. Its profile most resembles that of table mountains due to differential erosion of alternately strong and weak horizontally bedded rocks, such as compose the upper portion of the section in the Grand CaÑon of the Colorado. Here, however, in place of a single unusually strong top layer there are found several strong layers in alternation with weaker ones so as to produce additional steps in the profile.

Reading References to Chapters IX and X

General works:—

Paulett Scrope. The Geology of the Extinct Volcanoes of Central France. John Murray, London, 1858, pp. 258. (An epoch-making work of early date which, like the following reference, may be studied to advantage to-day.)

Sir Charles Lyell. Principles of Geology, vol. 1, Chapters xxiii-xxv.

Melchior Neumayr. Erdgeschichte, vol. 1, Allgemeine Geologie, revised edition by v. Uhlig, 1897, pp. 133-277 (a storehouse of valuable information clearly presented).

J. D. Dana. Characteristics of Volcanoes, with Contributions of Facts and Principles from the Hawaiian Islands. Dodd, Mead, and Company, New York, 1890, pp. 397.

Tempest Anderson. Volcanic Studies in Many Lands, being reproductions of photographs by the author with explanatory notes. John Murray, London, 1903, pp. 200, pls. 105.

T. G. Bonney. Volcanoes, their Structure and Significance. John Murray, London, 1899, pp. 331.

I. C. Russell. Volcanoes of North America. Macmillan, New York, 1897, pp. 346.

ElisÉe RÉclus. Les volcans de la terre, Belgian Society of Astronomy, Meteorology, and Physics of the Globe, 1906-1910 (a valuable descriptive geographical and bibliographical work of reference).

G. Mercalli. I vulcani attivi della terre. Hoepli, Milan, 1907, pp. 421. (A most valuable work, beautifully illustrated, but in the Italian language.)

Arrangement of volcanic vents:—

Th. Thoroddsen. Die Bruchlinien und ihre Beziehungen zu den Vulkanen, Pet. Mitt., vol. 51, 1905, pp. 1-5, pl. 5.

R. D. M. Verbeek. Various volumes and atlases of maps covering the Dutch East Indies and fully cited in the following reference (p. 21).

William H. Hobbs. The Evolution and the Outlook of Seismic Geology, Proc. Am. Phil. Soc., vol. 48, 1909, pp. 17-27.

Birth of volcanoes:—

F. Omori. The Usu-san Eruption and Earthquake and Elevation Phenomena, Bull. Earthq. Inv. Com., Japan, vol. 5, No. 1, 1911, pp. 1-37, pls. 1-13.

Fissure eruptions:—

Th. Thoroddsen. Island, IV, Vulkane, Pet. Mitt., ErgÄnzungsh. 153, 1906, pp. 108-111.

A. Geikie. Text-book of Geology, 4th ed., pp. 342-346.

Lava domes of Hawaii:—

J. D. Dana. Characteristics of Volcanoes (as above).

C. H. Hitchcock. Hawaii and Its Volcanoes. Honolulu, 1909, pp. 314.

Eruption of Matavanu volcano in 1906:—

Karl Sapper. Der Matavanu-Ausbruch auf Savaii, 1905-1906, Zeit. d. Gesell. f. Erdk. z. Berlin, vol. 19, 1906, pp. 686-709, 4 pls.

H. J. Jensen. The Geology of Samoa, and the Eruptions in Savaii, Proc. Linn. Soc., New South Wales, vol. 31, 1906, pp. 641-672, pls. 54-64.

Tempest Anderson. The Volcano of Matavanu in Savaii, Quart. Jour. Geol. Soc., London, vol. 66, 1910, pp. 621-639, pls. 45-52.

Eruption of Volcano in 1888:—

H. J. Johnston-Lavis. The South Italian Volcanoes. Naples, 1891, pp. 342, pls. 16.

Eruption of Taal volcano in 1911:—

W. E. Pratt. The Eruption of Taal Volcano, January 30, 1911, Phil. Jour. Sci., vol. 6, No. 2, Sec. A, 1911, pp. 63-86, pls. 1-14.

F. H. Noble. Taal Volcano, album of views of 1911 eruption, Manila, 1911, pp. 1-48.

The volcano of Etna:—

G. vom Rath. Der Aetna. Bonn, 1872, pp. 1-33. (A beautiful piece of descriptive writing from both the geological and scenic standpoints.)

Sartorius von Waltershausen. Der Aetna. Leipzig, 1880, 2 quarto vols., pp. 371 and 548.

The eruption of Vesuvius in 1906:—

H. J. Johnston-Lavis. Geological Map of Monte Somma and Vesuvius, with a short and concise account, etc. Geo. Philip & Son, London, 1891.

H. J. Johnston-Lavis. The Eruption of Vesuvius in April, 1906, Trans. Roy. Dublin Soc., vol. 9, 1909, Pt. VIII, pp. 139-200, pls. 3-23 (the most authoritative work upon the subject).

T. A. Jaggar, Jr. The Volcano Vesuvius in 1906, Tech. Quart., vol. 19, 1906, pp. 105-115.

W. Prinz. L’Éruption du Vesuv d’avril, 1906, Ciel et Terre, 27e AnnÉe, 1906, pp. 1-49.

Frank A. Perret. Notes on the Electrical Phenomena of the Vesuvian Eruption, April, 1906, Sci. Bull., Brooklyn Inst. Arts and Sci., vol. 1, No. 11, pp. 307-312; Vesuvius, Characteristics and Phenomena of the Present Repose Period, Am. Jour. Sci., vol. 28, 1909, pp. 413-430.

William H. Hobbs. The Grand Eruption of Vesuvius in 1906, Jour. Geol., vol. 14, 1906, pp. 636-655.

The spine of PelÉe:—

E. O. Hovey. The New Cone of Mont PelÉe and the Gorge of the RiviÈre Blanche, Martinique, Am. Jour. Sci., vol. 16, 1903, pp. 269-281, pls. 11-14.

A. Heilprin. The Tower of PelÉe. Philadelphia, 1904, pp. 62, pls. 22.

A. Lacroix. La montagne PelÉe et ses Éruptions, Acad. des Sciences, Paris, 1904, Chapter iii.

Karl Sapper. In den Vulkangebieten Mittelamerikas und Westindiens, Stuttgart, 1905, pp. 172-178.

A. C. Lane. Absorbed Gases of Vulcanism, Science, N.S., vol. 18, 1903, p. 760.

G. K. Gilbert. The Mechanism of the Mont PelÉe Spine, ibid., vol. 19, 1904, pp. 927-928.

I. C. Russell. PelÉe Obelisk once More, ibid., vol. 21, 1905, pp. 924-931.

The dissection of volcanoes:—

J. W. Judd. Volcanoes, Chapter v.

S. Sekya and Y. Kikuchi. The Eruption of Bandai-San, Trans. Seis. Soc., Japan, vol. 13, Pt. 2, 1890, pp. 140-222, pls. 1-9.

R. D. M. Verbeek. Krakatau. Batavia, 1885, pp. 557, pls. 25.

Royal Society. The Eruption of Krakatoa and Subsequent Phenomena. London, 1888, pp. 494.

G. K. Gilbert. Report on the Geology of the Henry Mountains, U.S. Geogr. and Geol. Surv., Rocky Mt. Region, Washington, 1877, pp. 22-60.

Sir A. Geikie. Ancient Volcanoes of Great Britain, vol. 2 especially.

D. W. Johnson. Volcanic Necks of the Mount Taylor Region, New Mexico, Bull. Geol. Soc. Am., vol. 18, 1907, pp. 303-324, pls. 25-30.

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