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VOLCANO
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VOLCANO
, an opening in the See also:earth's crust, through which heated See also:matter is brought, permanently or temporarily, from the interior of the earth to the See also:surface, where it usually forms a See also: At first they are usually confined to the volcano and its immediate neighbourhood, but may subsequently extend to a considerable distance, though probably never developing into earthquakes of the first magnitude . The sudden opening of a subterranean crack, by rupture of a See also:rock under See also:strain, or the rapid inje4tion of See also:lava into such a fissure, will tend to produce a See also:jar at the surface . For at least sixteen years before the first recorded eruption of See also:Vesuvius in A.U . 79 earthquakes had been frequent in the See also:Campania and had wrought havoc in the cities of See also:Herculaneum and See also:Pompeii . Again, the formation of See also:Monte Nuovo, near See also:Pozzuoli, in 1538, was heralded by local earthquakes beginning several years in advance of the eruption . So too in See also:recent years many volcanic outbursts have been preceded by a See also:succession of earthquakes; but as volcanoes are frequently situated in areas of marked seismic activity, the shocks antecedent to an eruption may not, unless exceptionally violent, receive much See also:attention from local observers . It commonly happens that a volcanic outburst is announced by subterranean roaring and rumbling, often compared to See also:thunder or the See also:discharge of See also:artillery underground . Other precursory symptoms may be afforded by neighbouring springs, which not unusually flow with diminished See also:volume, or even fail altogether . Possibly fissures open underground and drain off the See also:water from the springs and See also:wells in the immediate locality . Occasionally, however, an increased flow has been recorded . In some cases thermal springs make their See also:appearance, whilst the temperature of any existing warm springs may be increased, and perhaps See also:carbon dioxide be evolved . A disturbed state of the See also:atmosphere is by no means a See also:constant forerunner of an eruption, some of the greatest outbursts having occurred in a period of atmospheric stability: indeed the See also:air is often See also:felt to be See also:close and still . Immediately before a renewed outburst in an old volcano, the See also:floor of the crater is generally upheaved to a greater or less extent, whilst the discharge of vapour from any fumaroles is increased . Where a crater has been occupied by water, forming a crater-See also:lake, the water on the approach of an eruption becomes warm, evolves visible vapour, and may even See also:boil . In the See also:case of cones which are capped with See also:snow, the internal See also:heat of the rising lava usually causes a rapid melting of the snow-cap, resulting perhaps in a disastrous See also:deluge . It seems probable that by attention to the premonitory symptoms a careful local observer might in many cases foretell an eruption . It generally happens that a great eruption is preceded by a preliminary phase of feeble activity . Thus, the gigantic See also:catastrophe at Krakatoa on the 27th of See also:August 1883, so far from having been a sudden outburst, was the See also:culmination of a state of excitement, sometimes moderate and sometimes violent, which had been in progress for several months . Emission of Vapour.—Of all volcanic phenomena the most constant is the emission of vapour . It is one of the earliest features of an eruption; it persists during the paroxysms, attaining often to prodigious volume; and it lingers as the last relic of an outburst, so that long after the ejection of ashes and lava has ceased an occasional puff of vapour may be the only memento of the disturbance . By far the greatest proportion of the vapour is See also:steam, which sometimes occurs almost to the exclusion of other gaseous products . Such, at least, is the usual and probably correct view, though it is opposed by A . Brun, who regards the volcanic vapours as chiefly composed of chlorides with steam in only subordinate amount . In the case of a mild eruption, like that occurring normally at Stromboli, the vapours may be discharged in periodical puffs, marking the See also:explosion of bubbles rising more or less rhythmically from the seething lava in the volcanic cauldron .
S
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See also:Wise observed at the volcano of Sangay, in See also:Ecuador, no fewer than 267 explosions in the course of an See also:hour, the vapour here being associated, as is so often the case, with ashes
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During a violent eruption the vapour may be suddenly shot upwards as a See also:vertical See also:column of enormous height, penetrating the passing clouds
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For a See also:short distance above the vent the superheated steam sometimes exists as a transparent vapour, but it soon suffers partial condensation, forming clouds, which, if not dispersed by winds, accumulate over the mountain
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When the vapour is See also:free from ash it forms See also:rolling balls of fleecy See also:cloud, but usually it carries in See also:mechanical association more or less finely divided lava as volcanic dust and ashes, whereby it becomes yellow, See also: The emission of vast volumes of vapour at high tension naturally produces much atmospheric disturbance, often felt at great distances from the centre of eruption . See also:Electrical Excitement.—It is probably to the uprushing current of vapour that much of the electrical excitement which invariably accompanies an eruption may be referred . The See also:friction of the steam rushing in jets through the volcanic vent must produce electrical disturbance, and indeed an active volcano has been aptly compared to a hydroelectric See also:machine of gigantic See also:power . Another cause of excitement may be found in the mutual friction of the ejected cinders and ashes as they rise and fall in showers through the air . Much trituration of volcanic material may go on in the crater and elsewhere during the eruption, whereby the solid lava is reduced to a See also:fine dust . Other means of generating See also:electricity are found in the chemical reactions effected in the volcano and in the sudden condensation of the emitted vapour . L . Palmieri, in the course of his investigations at the See also:observatory on Vesuvius, found that the vapours free from cinders carried a See also:positive See also:charge, whilst the cinders were negative . The electrical phenomena attending an eruption are often of great intensity and splendour . The dark ash-laden clouds of vapour are shot through and through by volcanic See also:lightning, sometimes in rapid See also:horizontal flashes, then in oblique forked streaks, or again in tortuous lines compared to fiery serpents, whilst the See also:borders of the cloud may be brilliant with electric scintillations, often forming balls and stars of fire . During the great eruption of Krakatoa remarkable phenomena were observed by See also:ships in the Strait of Sunda, luminous balls like " St Elmo's fire " appearing at the See also:mast-heads and the yard-arms, whilst the volcanic mud which See also:fell upon See also:rigging and See also:deck was strongly phosphorescent . Quite distinct from any electrical phenomena is that intermittent reddish glare which is often seen at See also:night in clouds See also:hanging over an active crater, and which is simply a glow due to reflection from the incandescent lava and stones in the volcanic cauldron below . Volcanic See also:Rain and Mud.—The condensation of the vast volumes of steam exhaled during an eruption produces torrents of rain, which, mingling to a greater or less extent with the volcanic ashes, forms a hot muddy stream known in See also:Italy as lava d'acqua and lava di fango, and in See also:South See also:America as moya . Deluges of such mud-lava may See also:rush violently down the mountain-See also:side and spread over the neighbouring See also:country with terribly destructive effect, whence they are greatly dreaded by those who dwell at the See also:base of a volcano . The solidified. volcanic mud, often mingled with larger fragments of lava, is known as See also:tuff or tufa . Herculaneum was buried beneath a See also:flood of mud swept down from Vesuvius during the Plinian eruption of 79, and the hard tufaceous crust which thus sealed up the See also:ill-fated See also:city came in turn to be covered by lava-flows from subsequent eruptions: hence the difficulty of excavating at Herculaneum compared with similar See also:work at Pompeii, where there was probably much less mud, since the city, having been at a greater distance from the volcanic centre, was overwhelmed in great measure by loose ashes, capable of removal with comparative ease . It sometimes happens that volcanic mud is formed by the mingling of hot ashes not directly with rain but with water from streams and lakes, or even, as in See also:Iceland, with melted snow . A torrent of mud was one of the earliest symptoms of the violent eruption of Mont Pele in See also:Martinique in 1902 . This mud had its source in the Etang Sec, a crater-See also:basin high up on the S.W. side of the mountain . By the explosive discharge of ashes and vapours mingled with the water of the See also:tarn there was produced a vast volume of hot muddy matter which on the 5th of May suddenly escaped from the basin, when a huge torrent of boiling black mud, charged with blocks of rock and moving with enormous rapidity, rolled like an See also:avalanche down the See also:gorge of the See also:Riviere See also:Blanche . If a stream of lava obstructs the drainage of a volcano, it may give rise to floods . Ejected Blocks.—When a volcano after a long period of re-pose starts into fresh activity, the materials which have accumulated in the crater, including probably large blocks from the disintegration of the crater-walls, have to be ejected . If the lava from the last eruption has consolidated as a plug in the See also:throat of the volcano, the conduit may be practically closed, and hence the first effort of the renewed activity is to expel this obstruction . The hard See also:mass becomes shattered by the explosions, and the angular fragments so formed are hurled forth by the outrushing stream of vapour .
When the discharge is violent, the vapour, as it rushes impetuously up the volcanic duct, may See also:tear fragments of rock from its walls and project them to a considerable distance from the vent
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Such ejected blocks, by no means uncommon in the See also:early stages of an eruption, are often of large See also:size and naturally vary according to the See also:character of the rocks through which the duct has been opened
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They may be irregular masses of igneous rocks, possibly lavas of earlier eruptions, or they may be stratified, sedimentary and fossiliferous rocks representing the See also:platform on which the volcano has been built, or the yet more deeply seated fundamental rocks
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By Dr H
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J
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See also:Johnston-Lavis, who specially studied the ejected blocks of Vesuvius, the volcanic materials broken from the See also:cone are termed " See also:accessory " ejecta, whilst other fragmentary materials he conveniently calls " accidental " products, leaving the See also:term essential " ejecta for plastic lava, ashes, crystals, &c
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Masses of Cretaceous or Apennine See also:limestone ejected from Somma are scattered through the tuffs on the slopes of Vesuvius; and See also:objects carved in such altered limestone are sold to tourists as " lava " ornaments
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Under the See also:influence of volcanic heat and vapours, the ejected blocks suffer more or less alteration, and may containin their cavities many crystallized minerals
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Certain blocks of See also:sandstone ejected occasionally at Etna are composed of See also: If the lava has become granulated it is termed " volcanic See also:sand "; when in a finer state of See also:division it is called ash, or if yet more highly comminuted it is classed as dust; but the latter terms are sometimes used interchangeably . The pulverized material, consisting of lava which has been broken up by the explosion, or triturated in the crater, is often discharged in prodigious quantity, so that after an eruption the country for See also:miles around the volcano• may be covered with a coating of fine ash or dust, sometimes nearly white, like a fall of snow, but often of greyish See also:colour, looking rather like See also:Portland See also:cement, and in many cases becolnirg reddish by oxidation of the ferruginous constituents . Even when first ejected the ash is sometimes See also:cocoa-coloured . This finely divided lava insinuates itself into every crack and cranny, reaching the interior of houses even when windows and doors are closed . A heavy fall of ash or cinders may cause great structural damage, crushing the See also:roofs of buildings by sheer See also:weight, as was markedly the case at Ottajano and See also:San Guiseppe during the eruption of Vesuvius in April 1906 . On this occasion the dry ashes slipped down the sides of the volcanic cone like an avalanche, forming great ash-slides with ridges and furrows rather like barrancos, or ravines, caused by rain . The See also:burial of Ottajano and San Ginseppe in 1906 by Vesuvian ejecta, mostly lapilli, has been compared with that of Pompeii in 79 . Deposits of volcanic sand and ashes retain their heat long after ejection, so that rain will cause them to evolve steam, and if the rain be heavy and sudden it may produce explosions with emission of great clouds of vapour . The fall of ash is at first prejudicial to vegetation, and is often accompanied or followed by See also:acid rain; but ultimately the ash may prove beneficial to the See also:soil, chiefly in consequence of the alkalis which it contains . The " May dust " of See also:Barbados was a rain of volcanic ash which fell in May 1812 from the eruption of the Soufriere in St See also:Vincent . It is estimated that the amount of dust which during this eruption fell on the surface of Barbados, too m. distant from the eruptive centre, was about 3,000,000 tons . The distance to which ash is carried depends greatly on the atmospheric conditions at the See also:time of the eruption .
Ashes from Vesuvius in an eruption in the year 472 were carried, it is said, as far as See also:Constantinople
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During an eruption of See also:Cotopaxi, on the 3rd of See also:July 188o, observed by E
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See also:Whymper, an enormous black column of dust-laden vapour was shot vertically upwards with such rapidity that in less than a See also:minute it rose to a height estimated at 20,000 ft. above the crater-rim, or nearly 40,000 ft. above See also:sea-level, when it was dispersed by the See also:wind over a very wide See also:area
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It is believed that the amount of dust in this discharge must have been more than 2,000,000 tons
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Enormous quantities of dust ejected from Krakatoa in 1883 were carried to prodigious distances, samples having been collected at more than a thousand miles from the volcano; whilst the very fine material in ultra-microscopic grains which remained suspended for months in the higher regions of the atmosphere seems to have enjoyed an almost See also:world-wide See also:distribution, and to have been responsible for the remarkable sunsets at that period
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The ash falling in the immediate vicinity of a volcanic vent will generally be coarser than that carried to a distance, since the particles as they are wafted through the air undergo a See also:kind of sifting
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See also:Professor J
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W
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See also:Judd, who made an exhaustive examination of the products of the eruption of Krakatoa, found that the dust near the volcano was comparatively coarse, dense and rather dark-coloured, in consequence of the presence of numerous fragments of
heavy, dark, crystalline minerals, whilst the dust at a distance was excessively fine and perfectly white
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According to this observer, the particles tended to fall in the following See also:order: See also:magnetite, pyroxenes, See also:felspar, See also:glass
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The finely comminuted material, carried to a great height in the atmosphere, consisted largely of delicate threads and attenuated plates of vitreous matter, in many cases hollow and containing air-bubbles
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The greater part of the dust was formed by the mutual See also:attrition of fragments of brittle See also:pumice as they rose and fell in the crater, which thus became a powerful "dast-making See also:
The explosions at Krakatoa were, however, exceptionally violent, having been sufficient to project some of the finely pulverized lava to an altitude estimated to have been at least 3o m
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It is usually impossible during a great eruption to determine the height of the column of " smoke," since it hangs over the country as a See also:pall of darkness
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The great black cloud, which was so characteristic a feature in the terrible eruptions in the See also:West Indies in 1902, was formed of steam with See also:sulphur dioxide and other gases, very heavily charged with incandescent sand or dust, forming a dense mixture that in some respects behaved like a liquid
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Unlike the Krakatoa dust, which was derived from a vitreous pumice, the solid matter of the black cloud was largely composed of fragments of crystalline minerals
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According to Drs See also: In consequence of the See also:peculiar constitution of the magma, the order in which minerals See also:separate and solidify from it on cooling does not necessarily correspond with the inverse order of their relative fusibility . The lava differs from the magma before eruption, inasmuch as water and various volatile substances may be expelled on extrusion . The rapid See also:escape of vapour from the lava contributes to the explosive phenomena of an eruption, whilst the See also:rate at which the vapour is disengaged depends largely on the viscosity of the magma . The lava on its immediate issue from the volcanic vent is probably at a white heat, but the temperature is difficult of determination since the molten matter is usually not easy of approach, by See also:reason of the enshrouding vapour . Determinations of temperature are generally made at a short distance from the exit, when the lava has undergone more or less cooling, or on a small stream from a subordinate vent . A . See also:Bartoli, using a See also:platinum electric resistance See also:pyrometer, found that a stream of lava near a bocca, or orifice of emission, on Etna, in the eruption of 1892, had at a See also:depth of one See also:foot a temperature of See also:lobo° C . In the lavas of Vesuvius and Etna thin wires of See also:silver and of See also:copper have frequently been melted . Probably the lava at the surface of the stream has a temperature of something like 11o0° C., but this must not be assumed to be its temperature at the volcanic See also:focus . C . Doelter, in some experiments on the melting-point of lava by means of an electric furnace, found that a lava from Etna softened at from 962° to 970° C. and became fluid at 1o10° to 1040°, whilst a Vesuvian lava softened at 1030° to 1060° and acquired fluidity at 1080° to 1o9o° . These_ results were obtained at See also:ordinary atmospheric pressure, but it has been assumed that the melting-point of lava at a great depth would, through pressure alone, exceed that obtained in the laboratory . On the other See also:hand the presence of water and of certain volatile fluxes in the magma lowers the fusing-point, and hence the extruded lava from which these have largely escaped may be much less fusible than the See also:original magma . Determinations of the melting-points of various glasses formed by the fusion of certain igneous rocks have been made by J . A . See also:Douglas, with the meldometer of Professor J . Joly . The results givetemperatures ranging from 126o° C. for See also:rhyolite to o7o° for See also:dolerite from the Clee Hills in See also:Shropshire . The melting-points of the rocks in a glassy condition as here given are, however, See also:lower than those of the corresponding rocks in a crystalline state . It should be noted that all determinations of the melting-points of minerals and rocks involving ocular inspection of the See also:physical state of the material are liable to considerable See also:error, and the only accurate method seems to be that of determining the point at which absorption of heat abruptly occurs—the latent heat of fusion . This has been done in the refined investigations by Mr A . L . See also:Day and his colleagues in the Geophysical Laboratory of the See also:Carnegie Institution at See also:Washington . It is believed that the temperature of lava in the volcanic conduit may be in some cases sufficiently high to fuse the neighbouring rocks, and so melt out a passage through them in its ascent . The See also:wall-rock thus dissolved in the magma will not be without influence on the See also:composition of the lava with which it becomes assimilated . Many interesting observations are on record with regard to the See also:heating effect of lava on metals and other objects with which it may have come in contact . Thus, after the destruction of Torre del See also:Greco by a current of lava from Vesuvius in 1794, it was found that See also:brass in the houses under the lava had suffered decomposition, the copper having become crystallized ; whilst silver had been not only fused but sublimed . This indicates a temperature of upwards of l000° C . Panes of glass in the windows at Torre del Greco on the same occasion suffered devitrification . Notwithstanding the high temperature of lava on emission, it cools so rapidly, and the consolidated lava conducts heat so slowly, that See also:vegetable structures may be involved in a lava-flow without being entirely destroyed . A stream of lava on entering a See also:wood, as in the sylvan region on Etna, may See also:burn up the undergrowth but leave many of the larger trees with their trunks merely carbonized . On Vesuvius a lava-flow has been observed to surround trees while the foliage has been apparently uninjured . A vertical trunk of a coniferous tree partially enveloped in See also:Tertiary See also:basalt occurs at Gribon in the Isle of See also:Mull, as described by See also:Sir A . See also:Geikie and others; plant-remains in basalt from the Bo'ness coalfield in See also:Linlithgow-See also:shire have been noticed by H . M . Cadell; and attention has been called by B . Hobson to a specimen of scoriaceous basalt, from Mexico, which shows the impression of ears of See also:maize and even See also:relics of the actual grains . In consequence of the slow transmission of heat by solid lava, the crust on the surface of a stream may be crossed with impunity whilst the matter is still glowing at a short distance below . See also:Lichens may indeed grow on lava which remains highly heated in the interior . The solidified surface of a See also:sheet of lava may be smooth and shining, sometimes quite satiny in sheen, though locally wrinkled and perhaps even See also:ropy or hummocky, the irregularities being mainly due to superficial movement after partial solidification . The " corded lava " has a surface similar to that often seen on blast-furnace slag, and is suggestive of a tranquil flow . After a lava stream has become crusted over on cooling, the subjacent lava, still moving in a viscous condition, tends to tear the crust, forming irregular blocks, or clinkers, which are carried forward by the flow and ultimately See also:left in the form of confused heaps, perhaps of considerable magnitude . The front of a stream may See also:present a wall of scoriaceous fragments looking like a huge See also:pile of See also:coke . As the clinkers are carried along, on the surface of the lava, they produce by mutual friction a crunching See also:noise; and the sluggish flow of the lava-stream laden with its See also:burden has been compared with that of a See also:glacier . Since the upper part of the stream moves more rapidly than the lower, which is retarded by cooling in contact with the See also:bed-rock, the superficial clinkers are carried forward and, rolling over the end, may become embedded in the lava as it advances . Scoriae formed on the top of a stream may thus find their way to the base . Rock-fragments or other detrital matter occurring in the path of the lava will be caught up by the flow and become involved in the lower part of the molten mass; whilst the rocks over which the lava travels may suffer more or less alteration by the heat of the stream . The rapidity of a lava flow is determined partly by the slope of the bed over which it moves and partly by the consistency of the lava, this being dependent on its chemical composition and on the conditions of cooling .
In an eruption of Mauna Loa, in See also:Hawaii, in 1855, the lava was estimated to flow at a rate of 40 M. an hour; and at an eruption of Vesuvius in 1805 a velocity of more than 5e M. an hour, at the moment of emission, was recorded
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The rapidity of flow is, how-ever, rapidly checked as the stream advances, the retardation being very marked in small flows
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Where lava travels down a steep incline there is naturally a great tendency to form a rugged surface, whilst a quiet flow over a See also:flat See also:plane favours smoothness
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If the lava meet a precipice it may form a cascade of great beauty, the clinkers rapidly rolling down with a clatter, as described by Sir W
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