Chapter IV: Part 4

In the whole annals of noise there is nothing which can be compared to the records. Lloyd’s agent at Batavia, 94 miles distant, says that on the morning of August 27 the reports and concussions were deafening. At Carimon, Java Island, reports were heard which led to the belief that some vessel offshore was making signals of distress, and boats were accordingly put out to render succor, but no vessel was found, as the reports were from Krakatoa, at a distance of 355 miles. At Macassar, in Celebes, explosions were heard all over the province. Two steamers were sent out to discover the cause, for the authorities did not then know that what they heard came from Krakatoa, 969 miles away. But mere hundreds of miles will not suffice to exemplify the range of this stupendous siren. In St. Lucia Bay, in Borneo, a number of natives, who had been guilty of murder, thought they heard the sounds of vengeance in the approach of an attacking force. They fled from their village, little fancying that what alarmed them really came from Krakatoa, 1,116 miles distant. All over the island of Timor alarming sounds were heard, and so urgent did the situation appear that the government was aroused, and sent off a steamer to ascertain the cause. The sounds had, however, come 1,351 miles, all the way from Krakatoa. In the Victoria Plains of West Australia the inhabitants were startled by the discharge of artillery--an unwonted noise in that peaceful district--but the artillery was at Krakatoa, 1,700 miles distant. The inhabitants of Daly Waters, in South Australia, were rudely awakened at midnight on Sunday, August 26, by an explosion resembling the blasting of a rock, which lasted for a few minutes. The time and other circumstances show that here again was Krakatoa heard, this time at the monstrous distance of 2,023 miles. But there is undoubted testimony that to distances even greater than 2,023 miles the waves of sound conveyed tidings of the mighty convulsion. Diego Garcia, in the Chagos Islands, is 2,267 miles from Krakatoa, but the thunders traversed even this distance, and created the belief that there must be some ship in distress, for which a diligent but necessarily ineffectual search was made. To pass at once to the most remarkable case of all, we have a report from Mr. James Wallis, chief of police in Rodriguez, that “several times during the night of August 26-27, 1883, reports were heard coming from the eastward, like the distant roar of heavy guns. These reports continued at intervals of between three and four hours.” We have thus the astounding fact that almost across the whole wide extent of the Indian Ocean, that is, to a distance of nearly 3,000 miles (2,968), the sound of the throes of Krakatoa was propagated.

I shall content myself with the mention of three facts in illustration of the great sea waves which accompanied the eruption of Krakatoa. Of these, probably the most unusual is the magnitude of the area over which the undulations were perceived. Thus, to mention but a single instance, and that not by any means an extreme one, we find that the tide gauge at Table Bay reveals waves which, notwithstanding that they have traveled 5,100 miles from Krakatoa, have still a range of eighteen inches when they arrive at the southern coast of Africa. The second fact that I mention illustrates the magnitude of the seismic waves by the extraordinary inundations that they produced on the shores of the Straits of Sunda. Captain Wharton shows that the waves, as they deluged the land, must have been fifty feet, or, in one well authenticated case, seventy-two feet high. It was, of course, these vast floods which caused the fearful loss of life. The third illustrative fact concerns the fate of a man-of-war, the _Berouw_. This unhappy vessel was borne from its normal element and left high and dry in Sumatra, a mile and three-quarters inland, and thirty feet above the level of the sea.

During the crisis on August 26-27, the volume of material blown into the air was sufficiently dense to obscure the coasts of Sumatra to such a degree that at 10 A. M. the darkness there is stated to have been more intense than it is even in the blackest of nights. The fire-dust ascended to an elevation which, as we have already mentioned, is estimated to have been as much as seventeen miles. Borne aloft into these higher regions of our atmosphere, the clouds of dust at once became the sport of the winds and the currents which may be found there. If we had not previously known the prevailing tendency of the winds at these elevations and in these latitudes, the journey of the Krakatoa dust would have taught us.

It seems certain that, having attained their lofty elevation, the mighty clouds of dust were seized by easterly winds, and were swept along with a velocity which may not improbably be normal at a height of twenty miles above the earth’s surface.

It appears that this cloud of dust started immediately from Krakatoa for a series of voyages round the world. The highway which it at first pursued may, for our present purpose, be sufficiently defined by the Tropic of Cancer and the Tropic of Capricorn, though it hardly approached these margins at first. Westward the dust of Krakatoa takes its way. In three days it had crossed the Indian Ocean and was rapidly flying over the heart of Equatorial Africa; for another couple of days it was making a transatlantic journey; and then it might be found, for still a couple of days more, over the forests of Brazil ere it commenced the great Pacific voyage which brought it back to the East Indies. The dust of Krakatoa had put a girdle round the earth in thirteen days! The shape of the cloud appears to have been elongated, so that it took two or three days to complete the passage over any stated place.

It remains to give some brief account of the optical phenomena due to the presence of dust, unusual both in quantity and in character, in the upper atmosphere. Beautiful pictures show the twilight and after-glow effects as seen by Mr. W. Ascroft on the bank of the Thames a little west of London, on the evening of November 26, 1883. Analogous phenomena were seen almost universally during November and December in the same year. Who is there that does not remember the wondrous loveliness of the twilights and the after-glows during that remarkable winter! These appearances at sunrise and sunset are only the more generally recognized of a whole system of strange optical phenomena. One of the most striking indications of the presence of the dust-stream in its first voyage round the earth was given by the strange blue hue it imparted to the sun. The dust-stream was also visible in its rapid voyages as a lofty haze or extensive cloud of cirro-stratus. Then, too, strange halos were often seen, there were occasional blue or green moons, and the sun was sometimes glorified by a corona that had its origin in our atmosphere. Everywhere in the world there were remarkable features in the sky that winter: from Tierra del Fuego to Lake Superior; from China to the Gulf of Guinea; from Panama to Australia. Wherever on land there were inhabitants with sufficient intelligence to note the unusual, wherever on the sea there were mariners who kept a careful log, from all such observers we learn that in the autumn and winter months following the great eruption of Krakatoa, there were extraordinary manifestations witnessed in the heavens.

VOLCANOES
--SIR ARCHIBALD GEIKIE

The term volcanic action (volcanism or volcanicity) embraces all the phenomena connected with the expulsion of heated materials from the interior of the earth to the surface. Among these phenomena, some possess an evanescent character, while others leave permanent proofs of their existence. It is naturally to the latter that the geologist gives chief attention, for it is by their means that he can trace former phases of volcanic activity in regions where, for many ages, there have been no volcanic eruptions. In the operations of existing volcanoes, he can observe only superficial manifestations of volcanic action. But examining the rocks of the earth’s crust, he discovers that amid the many terrestrial revolutions which geology reveals, the very roots of former volcanoes have been laid bare, displaying subterranean phases of volcanism which could not be studied in any modern volcano. Hence an acquaintance only with active volcanoes will not afford a complete knowledge of volcanic action. It must be supplemented and enlarged by an investigation of the traces of ancient volcanoes preserved in the crust of the earth.

The word “volcano” is applied to a conical hill or mountain (composed mainly or wholly of erupted materials), from the summit and often also from the sides of which hot vapors issue, and ashes and streams of molten rock are intermittently expelled. The term “volcanic” designates all the phenomena essentially connected with one of these channels of communication between the surface and the heated interior of the globe. Yet there is good reason to believe that the active volcanoes of the present day do not afford by any means a complete type of volcanic action. The first effort in the formation of a new volcano is to establish a fissure in the earth’s crust. A volcano is only one vent or group of vents established along the line of such a fissure. But in many parts of the earth, alike in the Old World and the New, there have been periods in the earth’s history when the crust was rent into innumerable fissures over areas thousands of square miles in extent, and when the molten rock, instead of issuing, as it does at a modern volcano, in narrow streams from a central elevated cone, welled out from numerous small vents along the rents, and flooded enormous tracts of country without forming any mountain or conspicuous volcanic cone in the usual sense of these terms. Of these “fissure-eruptions,” apart from central volcanic cones, no examples appear to have occurred within the times of human history, except in Iceland, where vast lava-floods issued from a fissure in 1783. They can best be studied from the remains of former convulsions.

The materials erupted from volcanic vents may be classed as (1) gases and vapors, (2) water, (3) lava, (4) fragmentary substances.

Gases and vapors exist dissolved in the molten magma within the earth’s crust. They play an important part in volcanic activity, showing themselves in the earliest stages of a volcano’s history, and continuing to appear for centuries after all other subterranean action has ceased. By much the most abundant of them all is water-gas, which, ultimately escaping as steam, has been estimated to form 999-1000ths of the whole cloud that hangs over an active volcano. In great eruptions, steam rises in prodigious quantities, and is rapidly condensed into a heavy rainfall. M. Fouqué calculated that, during 100 days, one of the parasitic cones on Etna had ejected vapor enough to form, if condensed, 2,100,000 cubic metres (462,000,000 gallons) of water. But even from volcanoes which, like the Solfatara of Naples, have been dormant for centuries, steam sometimes still rises without intermission and in considerable volume. Jets of vapor rush out from clefts in the sides and bottom of a crater with a noise like that made by the steam blown off by a locomotive. The number of these funnels or “fumaroles” is often so large, and the amount of vapor so abundant, that only now and then, when the wind blows the dense cloud aside, can a momentary glimpse be had of a part of the bottom of the crater; while at the same time the rush and roar of the escaping steam remind one of the din of some vast factory. Aqueous vapor rises likewise from rents on the outside of the volcanic cone. It issues so copiously from some flowing lavas that the stream of rock may be almost concealed from view by the cloud; and it continues to escape from fissures of the lava, far below the point of exit, for a long time after the rock has solidified and come to rest.

Abundant discharges of water accompany some volcanic explosions. Three sources of this water may be assigned: (1) from the melting of snow by a rapid accession of temperature previous to or during an eruption; this takes place from time to time on Etna, in Iceland, and among the snowy ranges of the Andes, where the cone of Cotopaxi is said to have been entirely divested of its snow in a single night by the heating of the mountain; (2) from the condensation of the vast clouds of steam which are discharged during an eruption; this undoubtedly is the chief source of the destructive torrents so frequently observed to form part of the phenomena of a great volcanic explosion; and (3) from the disruption of reservoirs of water filling subterranean cavities, or of lakes occupying crater-basins; this has several times been observed among the South American volcanoes, where immense quantities of dead fish, which inhabited the water, have been swept down with the escaping torrents. The volcano of Agua in Guatemala received its name from the disruption of a crater-lake at its summit by an earthquake in 1540, whereby a vast and destructive debacle of water was discharged down the slopes of the mountain. In the beginning of the year 1817, an eruption took place at the large crater of Idjèn, one of the volcanoes of Java, whereby a steaming lake of hot acid water was discharged with frightful destruction down the slopes of the mountain. After the explosion, the basin filled again with water, but its temperature was no longer high.

The term lava is applied generally to all the molten rocks of volcanoes. The use of the word in this broad sense is of great convenience in geological descriptions, by directing attention to the leading character of the rocks as molten products of volcanic action, and obviating the confusion and errors which are apt to arise from an ill-defined or incorrect lithological terminology.

While still flowing or not yet cooled, lavas differ from each other in the extent to which they are impregnated with gases and vapors. Some appear to be saturated, others contain a much smaller gaseous impregnation; and hence arise important distinctions in their behavior. After solidification, lavas present some noticeable characters, then easily ascertainable. (1) Their average specific gravity may be taken as ranging between 2.37 and 3.22. (2) The heavier varieties contain much magnetic or titaniferous iron, with augite and olivine, their composition being basic, and their proportion of silica averaging about 45 to 55 per cent. (3) Lavas differ much in structure and texture. (4) Lavas vary greatly in color and general external aspect. The heavy basic kinds are usually dark gray, or almost black, though, on exposure to the weather, they acquire a brown tint from the oxidation and hydration of their iron. Their surface is commonly rough and ragged, until it has been sufficiently decomposed by the atmosphere to crumble into soil which, under favorable circumstances, supports a luxuriant vegetation. The less dense lavas, such as phonolites and trachytes, are frequently paler in color, sometimes yellow or buff, and decompose into light soils; but the obsidians present rugged black sheets of rock, roughened with ridges and heaps of gray froth-like pumice. Some of the most brilliant surfaces of color in any rock-scenery on the globe are to be found among volcanic rocks. The walls of active craters glow with endless hues of red and yellow. The Grand Cañon of the Yellowstone River has been dug out of the most marvelously tinted lavas and tuffs.

Volcanic action may be either constant or periodic. Stromboli, in the Mediterranean, so far as we know, has been uninterruptedly emitting hot stones and steam, from a basin of molten lava, since the earliest period of history. Among the Moluccas, the volcano Sioa, and in the Friendly Islands, that of Tofua, have never ceased to be in eruption since their first discovery. The lofty cone of Sangay, among the Andes of Quito, is always giving off hot vapors; Cotopaxi, too, is ever constantly active. But, though examples of unceasing action may thus be cited from widely different quarters of the globe, they are nevertheless exceptional. The general rule is that a volcano breaks out from time to time with varying vigor, and after longer or shorter intervals of quiescence.

It is usual to class volcanoes as _active_, _dormant_, and _extinct_. This arrangement, however, often presents considerable difficulty in its application. An active volcano can not of course be mistaken, for even when not in eruption, it shows by its discharge of steam and hot vapors that it might break out into activity at any moment. But in many cases it is impossible to decide whether a volcano should be called extinct or only dormant. The volcanoes of Silurian age in Wales, of Carboniferous age in Ireland, of Permian age in the Harz, of Miocene age in the Hebrides, of younger Tertiary age in the Western States and Territories of North America, are certainly all extinct. But the older Tertiary volcanoes of Iceland are still represented there by Skaptar-Jökull, Hecla, and their neighbors. Somma, in the First Century of the Christian era, would have been naturally regarded as an extinct volcano. Its fires had never been known to have been kindled; its vast crater was a wilderness of wild vines and brushwood, haunted, no doubt, by wolf and wild boar. Yet in a few days, during the autumn of the year 79, the half of the crater walls was blown out by a terrific series of explosions, the present Vesuvius was then formed within the limits of the earlier crater, and since that time volcanic action has been intermittently exhibited up to the present day. Some of the intervals of quietude, however, have been so considerable that the mountain might then again have been claimed as an extinct volcano. Thus, in the 131 years between 1500 and 1631, so completely had eruptions ceased that the crater had once more become choked with copse-wood. A few pools and springs of very salt and hot water remained as memorials of the former condition of the mountain. But this period of quiescence closed with the eruption of 1631--the most powerful of all the known explosions of Vesuvius, except the great one of 79.

In short, no essential distinction can be drawn between dormant and extinct volcanoes. Volcanic action is apt to show itself again and again, even at vast intervals, within the same regions and over the same sites. The dormant or waning condition of a volcano, when only steam and various gases and sublimates are given off, is sometimes called the Solfatara phase, from the well-known dormant crater of that name near Naples.

The interval between two eruptions of an active volcano shows a gradual augmentation of energy. The crater, emptied by the last discharge, has its floor slowly upraised by the expansive force of the lava-column underneath. Vapors rise in constant outflow, accompanied sometimes by discharges of dust or stones. Through rents in the crater-floor red-hot lava may be seen only a few feet down. Where the lava is maintained at or above its fusion-point and possesses great liquidity, it may form boiling lakes, as in the great crater of Kilauea, where acres of seething lava may be watched throwing up fountains of molten rock, surging against the walls and re-fusing large masses that fall into the burning flood. The lava-column inside the pipe of a volcano is all this time gradually rising, until some weak part of the wall allows it to escape, or until the pressure of the accumulated vapors becomes great enough to burst through the hardened crust of the crater-floor and give rise to the phenomena of an eruption.

Kluge has sought to trace a connection between the years of maximum and minimum sun-spots and those of greatest and feeblest volcanic activity, and has constructed lists to show that years which have been specially characterized by terrestrial eruptions have coincided with those marked by few sun-spots and diminished magnetic disturbance. Such a connection can not be regarded as having yet been satisfactorily established. Again, the same author has called attention to the frequency and vigor of volcanic explosions at or near the time of the August meteoric shower. But in this case, likewise, the cited examples can hardly yet be looked upon as more than coincidences.

At many volcanic vents the eruptive energy manifests itself with more or less regularity. At Stromboli, which is constantly in an active state, the explosions occur at intervals varying from three or four to ten minutes and upward. A similar rhythmical movement has been often observed during the eruptions at other vents which are not constantly active. Volcano, for example, during its eruption of September, 1873, displayed a succession of explosions which followed each other at intervals of from twenty to thirty minutes. At Etna and Vesuvius a similar rhythmical series of convulsive efforts has often been observed during the course of an eruption. Among the volcanoes of the Andes a periodic discharge of steam has been observed; Mr. Whymper noticed outrushes of steam to proceed at intervals of from twenty to thirty minutes from the summit of Sangai, while during his inspection of the great crater of Cotopaxi, this volcano was seen to blow off steam at intervals of about half an hour. At the eruption of the Japanese volcano, Oshima, in 1877, Mr. Milne observed that the explosions occurred nearly every two seconds, with occasional pauses of 15 or 20 seconds. Kilauea, in Hawaii, seems to show a regular system of grand eruptive periods. Dana has pointed out that outbreaks of lava have taken place from that volcano at intervals of from eight to nine years, this being the time required to fill the crater up to the point of outbreak, or to a depth of 400 or 500 feet.

The approach of an eruption is not always indicated by any premonitory symptoms, for many tremendous explosions are recorded to have taken place in different parts of the world without perceptible warning. Much in this respect would appear to depend upon the condition of liquidity of the lava, and the amount of resistance offered by it to the passage of the escaping vapors through its mass. In Hawaii, where the lavas are remarkably liquid, vast outpourings of them have taken place quietly without earthquakes during the present century. But even there the great eruption of 1868 was accompanied by violent earthquakes.

The eruptions of Vesuvius are often preceded by failure or diminution of wells and springs. But more frequent indications of an approaching outburst are conveyed by sympathetic movements of the ground. Subterranean rumblings and groanings are heard; slight tremors succeed, increasing in frequency and violence till they become distinct earthquake shocks. The vapors from the crater grow more abundant as the lava-column in the pipe or funnel of the volcano ascends, forced upward and kept in perpetual agitation by the passage of elastic vapors through its mass. After a long previous interval of quiescence, there may be much solidified lava toward the top of the funnel, which will restrain the ascent of the still molten portion underneath. A vast pressure is thus exercised on the sides of the cone, which, if too weak to resist, will open in one or more rents, and the liquid lava will issue from the outer slope of the mountain; or the energies of the volcano will be directed toward clearing the obstruction in the chief throat, until with tremendous explosions, and the rise of a vast cloud of dust and fragments, the bottom and sides of the crater are finally blown out, and the top of the cone disappears. The lava may now escape from the lowest part of the lip of the crater, while, at the same time, immense numbers of red-hot bombs, scoriæ, and stones are shot up into the air. The lava at first rushes down like one or more rivers of melted iron, but, as it cools, its rate of motion lessens. Clouds of steam rise from its surface, as well as from the central crater. Indeed, every successive paroxysmal convulsion of the mountain is marked, even at a distance, by the rise of huge ball-like wreaths or clouds of steam, mixed with dust and stones, forming a column which towers sometimes a couple of miles or more above the summit of the cone. By degrees these eructations diminish in frequency and intensity. The lava ceases to issue, the showers of stones and dust decrease, and after a time, which may vary from hours to days or months, even in the _régime_ of the same mountain, the volcano becomes once more tranquil.

The convulsions which culminate in the formation of a volcano usually split open the terrestrial crust by a more or less nearly rectilinear fissure, or by a system of fissures. In the subsequent progress of the mountain, the ground at and around the focus of action is liable to be again and again rent open by other fissures. These tend to diverge from the focus; but around the vent where the rocks have been most exposed to concussion, the fissures sometimes intersect each other in all directions. In the great eruption of Etna, in the year 1669, a series of six parallel fissures opened on the side of the mountain. One of these, with a width of two yards, ran for a distance of 12 miles, in a somewhat winding course, to within a mile of the top of the cone.

In the deeper portions of a volcanic vent the convulsive efforts of the lava-column to force its way upward must often produce lateral as well as vertical rifts, and into these the molten material will rush, exerting as it goes an enormous upward pressure on the mass of rock overlying it. At a modern volcano these subterranean manifestations can not be seen, but among the volcanoes of Tertiary and older times they have been revealed by the progress of denudation.

Though lava very commonly issues from the lateral fissures on a volcanic cone, it may sometimes approach the surface in them without actually flowing out. The great fissure on Etna in 1669, for example, was visible even from a distance, by the long line of vivid light which rose from the incandescent lava within. Again, it frequently happens that minor volcanic cones are thrown up on the line of a fissure, either from the congelation of the lava round the point of emission, or from the accumulation of ejected scoriæ round the fissure-vent. One of the most remarkable examples of this kind is that of the Laki fissure in Iceland, the whole length of which (12 miles) bristles with small cones and craters almost touching each other.

Apart from the appearance of visible fissures, volcanic energy may be, as it were, concentrated on a given point, which will usually be the weakest in the structure of that part of the terrestrial crust, and from which the solid rock, shattered into pieces, is hurled into the air by the enormous expansive energy of the volcanic vapors. The history of the cone of Vesuvius brings before us a long series of such explosions, beginning with that of A. D. 79, and coming down to the present day. Even now, in spite of all the lava and ashes poured out during the last eighteen centuries, it is easy to see how stupendous must have been that earliest explosion by which the southern half of the ancient crater was blown out. At every successive important eruption, a similar but minor operation takes place within the present cone. The hardened cake of lava forming the floor is burst open, and with it there usually disappears much of the upper part of the cone, and sometimes, as in 1872, a large segment of the crater-wall. The islands of Santorin bring before us evidence of a prehistoric catastrophe of a similar nature, by which a large volcanic cone was blown up. The existing outer islands are a chain of fragments of the periphery of the cone, the centre of which is now occupied by the sea. In the year 1538 a new volcano, Monte Nuovo, was formed in twenty-four hours on the margin of the Bay of Naples. An opening was drilled by successive explosions, and such quantities of stones, scoriæ, and ashes were thrown out from it as to form a hill that rose 440 English feet above the sea-level, and was more than a mile and a half in circumference.

A communication having been opened, either by fissuring or explosion, between the heated interior and the surface, fragmentary materials are commonly ejected from it, consisting at first mainly of the rocks through which the orifice has been opened, afterward of volcanic substances. In a great eruption, vast numbers of red-hot stones are shot up into the air, and fall back partly into the crater and partly on the outer slopes of the cone. According to Sir W. Hamilton, cinders were thrown by Vesuvius, during the eruption of 1779, to a height of 10,000 feet. Instances are known where large stones, ejected obliquely, have described huge parabolic curves in the air, and fallen at a great distance. Stones eight pounds in weight occur among the ashes which buried Pompeii. The volcano of Antuco in Chili is said to send stones flying to a distance of thirty-six miles, Cotopaxi is reported to have hurled a 200-ton block nine miles, and the Japanese volcano, Asama, is said to have ejected many blocks of stone measuring from 40 to more than 100 feet in diameter.

But in many great eruptions, besides a constant shower of stones and scoriæ, a vast column of exceedingly fine dust rises out of the crater, sometimes to a height of several miles, and then spreads outward like a sheet of cloud. The remarkable fineness of this dust may be understood from the fact that during great volcanic explosions no boxes, watches, or close-fitting joints have been found to be able to exclude it. Mr. Whymper collected some dust that fell sixty-five miles away from Cotopaxi, and which was so fine that from 4,000 to 25,000 particles were required to weigh a grain. So dense is the dust-cloud as to obscure the sun, and for days together the darkness of night may reign for miles around the volcano. The eruption of Cotopaxi, on 26th June, 1877, began by an explosion that sent up a column of fine ashes to a prodigious height into the air, where it rapidly spread out and formed so dense a canopy as to throw the region below it into total darkness. So quickly did it diffuse itself, that in an hour and a half a previously bright morning became at Quito, thirty-three miles distant, a dim twilight, which in the afternoon passed into such darkness that the hand placed before the eye could not be seen. At Guayaquil, on the coast, 150 miles distant, the shower of ashes continued till the 1st of July. Dr. Wolf collected the ashes daily, and estimated that at that place there fell 315 kilogrammes on every square kilometre during the first thirty hours, and on the 30th of June, 209 kilogrammes in twelve hours.

One of the most stupendous outpourings of volcanic ashes on record took place, after a quiescence of twenty-six years, from the volcano Coseguina, in Nicaragua, during the early part of the year 1835. On that occasion, utter darkness prevailed over a circle of thirty-five miles radius, the ashes falling so thickly that, even eight leagues from the mountain, they covered the ground to a depth of about ten feet. It was estimated that the rain of dust and sand fell over an area at least 270 geographical miles in diameter. Some of the finer materials, thrown so high as to come within the influence of an upper air-current, were borne away eastward, and fell, four days afterward, at Kingston, in Jamaica--a distance of 700 miles. During the great eruption of Sumbawa, in 1815, the dust and stones fell over an area of nearly one million square miles, and were estimated by Zollinger to amount to fully fifty cubic miles of material, and by Junghuhn to be equal to one hundred and eighty-five mountains like Vesuvius. Toward the end of the Eighteenth Century, during a time of great disturbance among the Japanese volcanoes, one of them, Sakurajima, threw out so much pumiceous material that it was possible to walk a distance of twenty-three miles upon the floating débris in the sea.

The varying degree of liquidity or viscosity of the lava probably modifies the force of explosions, owing to the different amounts of resistance offered to the upward passage of the absorbed gases and vapors. Thus explosions and accompanying scoriæ are abundant at Vesuvius, where the lavas are comparatively viscid; they are almost unknown at Kilauea, where the lava is remarkably liquid.

In tranquil conditions of a volcano, the steam, whether collecting into larger or smaller vesicles, works its way upward through the substance of the molten lava, and as the elasticity of this compressed vapor overcomes the pressure of the overlying lava, it escapes at the surface, and there the lava is thus kept in ebullition. But this comparatively quiet operation, which may be watched within the craters of many active volcanoes, does not produce clouds of fine dust. The collision or friction of millions of stones ascending and descending in the dark column above the crater must doubtless cause much dust and sand. But the explosive action of steam is probably also an immediate cause of much trituration. The aqueous vapor or water-gas which is so largely dissolved in many lavas must exist within the lava-column, under an enormous pressure, at a temperature far above its critical point, even at a white heat, and therefore possibly in a state of dissociation. The sudden ascent of lava so constituted relieves the pressure rapidly without sensibly affecting the temperature of the mass. Consequently, the white-hot gases or vapors at length explode, and reduce the molten mass to the finest powder, like water shot out of a gun.

As every shower of dust and sand adds to the height of the ground on which it falls, thick volcanic accumulations may be formed far beyond the base of the mountain. The volcano of Sangay, in Ecuador, for instance, has buried the country around it to a depth of 4,000 feet under its ashes. In such loose deposits are entombed trees and other kinds of vegetation, together with the bodies of animals, as well as the works of man. In some cases, where the layer of volcanic dust is thin, it may merely add to the height of the soil, without sensibly interfering with the vegetation. But it has been observed at Santorin that though this is true in dry weather, the fall of rain with the dust at once acts detrimentally. On the 3d of June, 1866, the vines were there withered up, as if they had been burned, along the track of the smoke cloud. By the gradual accumulation of volcanic ashes, new geological formations arise which, in their component materials, not only bear witness to the volcanic eruptions that produced them, but preserve a record of the land-surfaces over which they spread. In the third place, besides the distance to which the fragments may be hurled by volcanic explosions, or to which they may be diffused by the ordinary aerial movements, we have to take into account the vast spaces across which the finer dust is sometimes borne by upper air-currents. In the instance already cited, ashes from Coseguina fell 700 miles away, having been carried all that long distance by a high counter-current of air, moving apparently at the rate of about seven miles an hour in an opposite direction to that of the wind which blew at the surface. By the Sumbawa eruption, also referred to above, the sea west of Sumatra was covered with a layer of ashes two feet thick. On several occasions ashes from the Icelandic volcanoes have fallen so thickly between the Orkney and Shetland Islands, that vessels passing there have had the unwonted deposit shoveled off their decks in the morning. In the year 1783, during the memorable eruption of Skaptar-Jökull, so vast an amount of fine dust was ejected that the atmosphere over Iceland continued loaded with it for months afterward. It fell in such quantities over parts of Caithness--a distance of 600 miles--as to destroy the crops; that year is still spoken of by the inhabitants as the year of “the ashie.” Traces of the same deposit have been observed in Norway, and even as far as Holland. Hence it is evident that volcanic accumulations may take place in regions many hundreds of miles distant from any active volcano. A single thin layer of volcanic detritus in a group of sedimentary strata would not thus of itself prove the existence of contemporaneous volcanic action in its neighborhood.

At its exit from the side of a volcano, lava glows with a white heat, and flows with a motion which has been compared to that of honey or of melted iron. It soon becomes red, and like a coal fallen from a hot fireplace rapidly grows dull as it moves along, until it assumes a black, cindery aspect. At the same time the surface congeals, and soon becomes solid enough to support a heavy block of stone. The aspect of the stream varies with the composition and fluidity of the lava, form of the ground, angle of slope, and rapidity of flow. Viscous lavas, like those of Vesuvius, break up along the surface into rough brown or black cinder-like slags and irregular ragged cakes, bristling with jagged points, which, in their onward motion, grind and grate against each other with a harsh, metallic sound, sometimes rising into rugged mounds or becoming seamed with rents and gashes, at the bottom of which the red-hot glowing lava may be seen. In lavas possessing somewhat greater fluidity, the surface presents froth-like, curving lines, as in the scum of a slowly flowing river, or is arranged in curious ropy folds, as the layers have successively flowed over each other and congealed. A large area which has been flooded with lava is perhaps the most hideous and appalling scene of desolation anywhere to be found on the surface of the globe.

A lava-stream usually spreads out as it descends from its point of escape, and moves more slowly. Its sides look like huge embankments, or like some of the long mounds of “clinkers” in a great manufacturing district. The advancing end is often much steeper, creeping onward like a great wall or rampart, down the face of which the rough blocks of hardened lava are ever rattling.

In a lofty volcano, lava occasionally rises to the lip of the crater and flows out there; but more frequently it escapes from some fissure or orifice in a weak part of the cone. In minor volcanoes, on the other hand, where the explosions are less violent, and where the thickness of the cone in proportion to the diameter of the funnel is often greater, the lava very commonly rises into the crater. Should the crater-walls be too weak to resist the pressure of the molten mass, they give way, and the lava rushes out from the breach. This is seen to have happened in several of the puys of Auvergne. But if the crater be massive enough to withstand the pressure, the lava may at last flow out from the lowest part of the rim.

As soon as the molten rock reaches the surface, the superheated water-vapor or gas dissolved within its mass escapes copiously, and hangs as a dense white cloud over the moving current. The lava-streams of Vesuvius sometimes appear with as dense a steam-cloud at their lower ends as that which escapes at the same time from the main crater. Even after the molten mass has flowed several miles, steam continues to rise abundantly both from its end and from numerous points along its surface, and continues to do so for many weeks, months, or it may be for several years.

Should the point of escape of a lava-stream lie well down on the cone, far below the summit of the lava-column in the funnel, the molten rock, on its first escape, driven by hydrostatic pressure, will sometimes spout up high into the air--a fountain of molten rock. This was observed in 1794 on Vesuvius, and in 1832 on Etna. In the eruption of 1852 at Mauna Loa, an unbroken fountain of lava, from 200 to 700 feet in height and 1,000 feet broad, burst out at the base of the cone. Similar “geysers” of molten rock have subsequently been noticed in the same region. Thus in March and April, 1868, four fiery fountains, throwing lava to heights varying from 500 to 1,000 feet, continued to play for several weeks. According to Mr. Coan, such outbursts take place from the bottom of a column of lava 3,000 feet high. The volcano of Mauna Loa strikingly illustrates another feature of volcanic dynamics in the position and outflow of lava. It bears upon its flanks at a distance of 20 miles, but 10,000 feet lower, the huge crater Kilauea. As Dana has pointed out, these orifices form part of one mountain, yet the column of lava stands 10,000 feet higher in one conduit than in the other. On a far smaller scale the same independence occurs among the several pipes of some of the geysers in the Yellowstone region of North America.

The rate of movement is regulated by the fluidity of the lava, by its volume, and by the form and inclination of the ground. Hence, as a rule, a lava-stream moves faster at first than afterward, because it has not had time to stiffen, and its slope of descent is usually steeper than further down the mountain. One of the most fluid and swiftly flowing lava-streams ever observed on Vesuvius was that erupted on 12th August, 1805. It is said to have rushed down a space of 3 Italian (3⅔ English) miles in the first four minutes, but to have widened out and moved more slowly as it descended, yet finally to have reached Torre del Greco in three hours. A lava erupted by Mauna Loa in 1852 went as fast as an ordinary stage-coach, or fifteen miles in two hours; but some of the lavas from that mountain have in parts of their course moved with double that rapidity.

In some cases, lava escaping from craters or fissures comes to rest before reaching the base of the slopes, like the obsidian current which has congealed on the side of the little volcanic island of Volcano. In other instances, the molten rock not only reaches the plains, but flows for many miles away from the point of eruption. Sartorius von Waltershausen computed the lava emitted by Etna in 1865 at 92 millions of cubic metres, that of 1852 at 420 millions, that of 1669 at 980 millions, and that of a prehistoric lava-stream near Randazzo at more than 1,000 millions. The most stupendous outpouring of lava on record was that which took place in Iceland in the year 1783. Successive streams issued from a fissure about 12 miles long, filling up river gorges which were sometimes 600 feet deep and 200 feet broad, and advancing into the alluvial plains in lakes of molten rock 12 to 15 miles wide and 100 feet deep. Two currents of lava which, filling up the valley of the Skapta, escaped in nearly opposite directions, extended for 45 and 50 miles respectively, their usual thickness being 100 feet. Bischof estimated that the total amount of lava poured forth during this single eruption “surpassed in magnitude the bulk of Mont Blanc.”

The varying degrees of liquidity are manifested in a characteristic way on the surface of lava. Thus, in the great lava-pools of Hawaii, the rock exhibits a remarkable liquidity, throwing up fountains of molten rock to a height of 300 feet or more. During its ebullition in the crater-pools, jets and driblets a quarter of an inch in diameter are tossed up, and, falling back on one another, make “a column of hardened tears of lava,” one of which was found to have attained a height of 40 feet, while in other places the jets thrown up and blown aside by the wind give rise to long threads of glass which lie thickly together like mown grass, and are known by the natives under the name of “Pele’s Hair,” after one of their divinities. Yet, although the ebullition is caused by the uprise and escape of highly heated vapors, there is no cloud over the boiling lake itself, heavy white vapor only escaping at different points along the edge.

EARTHQUAKES
--WILLIAM HUGHES

It appears, from the accurate records of such phenomena which have been kept within recent periods, that earthquakes are of much more frequent occurrence than is commonly supposed. Upward of three thousand earthquakes are recorded as having occurred within the first half of the Nineteenth Century--an average of more than one for every week throughout the entire period. But not more than one in forty is of considerable importance, by far the greater number consisting of such slight shocks as are occasionally experienced in Great Britain and other countries favored with a like immunity in this regard. An important earthquake, however, in some part of the world or other, appears, from the above average, to occur once in every eight months. In Europe alone, where a more complete record of such occurrences is obtainable than in other parts of the world, as many as 320 distinct earthquakes are recorded to have occurred within a period of ten years (1833-42)--an average of thirty-two annually, and of one such shock for every ten days throughout the period.

[The _geographical area_ within which shocks of earthquakes
are experienced is a widely spread one, and does not appear to
undergo any material change (if, indeed, any change whatever)
as to its limits. At any rate, the regions in which violent
earthquakes are recorded to have occurred in former times are
those in which such disturbances are of most frequent recurrence
at the present day. One of the most striking evidences in favor
of the supposition that the volcanic eruption is due to the same
deeply seated cause which produces the shock of the earthquake,
is afforded by the fact, that all the volcanoes which have been
in eruption within the modern period of geology are found within
regions liable to earthquakes, and, for the most part, to violent
shocks.]

Regarding the earthquake and the volcanic eruption as the manifestation, under different conditions, of the earth’s internal fires, we readily mark out upon the globe the great regions of geographical distribution in the case of such phenomena. The most widely extended of these coincides with the circuit of the Pacific Ocean. Along the entire western coast of the New World, from Tierra del Fuego to the peninsula of Alaska and the neighborhood of the Aleutian Islands, shocks of earthquakes are known to occur; and, within a large portion of the space, vents of active eruption are found. The subterranean igneous force is, indeed, much more powerfully displayed in the southern than in the northern half of the American continent, and the active volcanoes that occur within the limits referred to are nearly all found amid the cordilleras of the Andes, or upon the plateaus of the Mexican isthmus. One of the Mexican volcanoes--Jorullo--is especially deserving of notice, from the circumstance of its having first risen above the surrounding plain by the accumulation of volcanic matter during an eruption in the year 1759.

The Aleutian Islands connect the volcanic region of the eastern Pacific with that which extends along its western shores. In the latter case, however, it is upon the peninsular regions, or in the chains of islands that adjoin the mainland, that the igneous force is displayed. Kamtchatka, the Kurile Islands, Yesso, the Japanese group, and the entire region of the Malay Archipelago, exhibit the presence of igneous force below the ground. Seven active volcanoes occur in Kamtchatka. The Japanese Archipelago is said to contain at least twenty-seven active volcanoes, eight of them upon Yesso and the adjacent islets. Between Japan and the Loo-choo group is Sulphur Island, an insular volcano, from which smoke is constantly emitted.

The Philippine Islands, in which earthquakes are of frequent occurrence, prolong the volcanic chain to the southward. Thence it is traced, at intervals, along the northern shores of New Guinea, and through the prolonged chains of the Solomon Islands, and the New Hebrides, to the North Island of New Zealand. Slight shocks of earthquake have also been experienced within the southern and eastwardly portions of the Australian mainland.

The numerous volcanoes of the Malay Archipelago, the whole area of which is liable to frequent earthquake shocks, often of the most destructive violence, belong to the eastern portion of this region, and display the agency of subterranean heat on the grandest scale. The island of Java alone contains forty-three active volcanoes, ranging in a linear direction throughout its length. The volcanic chain of Java is prolonged to the eastward through the Lesser Sunda Islands (Sumbawa, etc.), in which direction it is united with that which borders the Pacific waters. There are active volcanoes on an island in the Gulf of Siam, besides the well-known crater of Barren Island, in the Bay of Bengal. The region adjoining the last-named body of water, together with the whole of northern India, is of frequent liability to earthquakes, some of them (as that of Cutch, in 1819) of the most destructive violence. The volcanic island of Mayotta (Comoro group), the active Piton of Réunion or Bourbon Island, and the hot springs and extinct craters of St. Paul and Amsterdam Islands, in a high southern latitude of the Indian Ocean, constitute points which indicate, at distant intervals, the continuity of the volcanic chain.

The southwestern portion of Asia, the southern shores of Europe, and the northwestwardly portion of the African mainland, fall within this region on the one side, as the islands of the West Indies do upon the other. The entire breadth of the Atlantic Ocean, as well as the circuit of the Mediterranean, is thus included within its limits. To the northward, the numerous volcanoes of Iceland, and the more distant cone of Jan Mayen Island, lying within the Arctic circle, must be regarded as within its area; together with, in an opposite direction, the still-burning peak of the Cameroon Mountains, adjoining the upper extremity of the Gulf of Guinea. The volcanic peaks found within the widely detached groups of the Azores and the Cape Verde Islands, with Tenerife, in the Canary group, are among its outlying members.

Throughout the wide region thus indicated, earthquakes are of frequent occurrence. There are fewer active vents of eruption than in the case of the Pacific circuit. But the cones of Etna and Vesuvius, with the island of Santorin, in the Mediterranean, and the numerous volcanoes of Iceland, attest the destructive violence of the subterranean fires. Western Asia, from the Caspian to the shores of the Archipelago (including Armenia, Syria, and the Lesser Asia), Greece, southern Italy, the Spanish peninsula, and the region of Mount Atlas, in Northwestern Africa, are all liable to the frequent repetition of such convulsions. The only portion of the Mediterranean coasts exempt from such disturbing phenomena is on its southern shores, embracing that part of the North African coast which stretches from the Lesser Syrtis to the valley of the Nile. We have no record of the experience of any shocks of earthquake in Egypt. Had it been otherwise, perhaps the pyramids of that land of wonders might have proved less enduring monuments of the past.

The movement imparted to the ground during an earthquake may be either horizontal or vertical. In the former case, the phenomenon consists in an undulating, wave-like movement; in the latter, in an upheaval or subsidence of land. The vertical shock affects most the relative levels of adjacent objects, and produces the most striking permanent changes in the natural aspect of the region in which it is experienced. But the undulatory movement is attended by more serious consequences to man, since it at once shakes the foundations of the strongest edifices, and may overthrow in the space of a few seconds the accumulated labors of prior ages. Whole tracts of land, with their cities or villages, may be elevated or depressed with comparatively little injury to life; but nothing can withstand the force of a motion which rocks the solid strata of the earth itself. The most solidly constructed buildings are not proof against the earthquake any more than the weakest. Indeed, it has in many instances been observed that those erections which displayed the strongest masonry have suffered more from the effects of an earthquake than buildings of slighter structure. The cracking of walls, the falling-in of roofs, and the crash of tumbling houses on every side, burying their inmates beneath the ruins, are among the characteristics of the earthquake in its most violent and frightful form.