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THE EARTH'S INTERNAL FIRES. ?
THEIR PROBABLE ORIGIN. PAST AND PRESENT VOLCANIC ERUPTIONS.
ine voicanic eruptions which devastated large portions of the island** of Martinique and St. Vincent a few weeks ago empha sized the fact that ?many problems relat
ing to tnese awe-inspiring upheavals 01 ^ature have not yet been solved by scieij :ist3. Numerous theories have been fornm ated to account for the origin of tdie im mense forces which arc generated within the earth, and which have from time to time proved so terribly destructive to liuinan life. Some of these conjectures are jf a conflicting character, while others have been proved by more recent scientific rc iearch to be wholly untenable. With the ob ject of obtaining an authoritative statement Df the results of modem investigations re lative to the origin and distribution of the internal fires, which are the primary cause of volcanic outbreaks, a representative of Hie Register sought an interview with Mr. W. Howchin, F.G.S., lecturer at the Ade laide University on geology and palaeonto logy. — Things Scientists Do Not Know.— 'Can you explain how the internal fires ire generated in the earth, and by what means they are kept burning?' was the first question put to Mr. Howchin.— 'Those ire matters upon which it is impossible to speak with any degree of certainty,'* was bis reply. 'Various theories have been rormulated to account for the phenomenon, but they are all founded upon conjecture. No one-has explored the interior of the earth, and as we cannot hope that any one will ever do so, we must be content with such evidence as geologists are able feo furnish as a result of their examination of the outer crust of the globe. It is an established fact that the temperature of the earth increases from the surface down wards at the rate of one degree in 60 to 120 feet; and the old idea— which has now bee- discarded by science a-s wholly unten able— was that the centre of the globe was a ball of fire, with a comparatively thin shell of solid matter on the surface. Among modern scientists there is a divergence of opinion. Some think that the centre of ttie earth, like its superficial portions, is solid, and that there are intermediate beds of molten rock, kept at a high temperature by pressure. Another conjecture is that the forces which are constantly operating upon the surface of the globe cause the crust of the earth to fokl or buckle; and that the intense pressure thus brought to bear upon the lower strata 13 transmitted into heat of sufficient intensity to melt the rock. It may be that this theory is correct, and that volcanoes are merely outlets from local areas of molten matter. In that case we would expect that the volcanic areas would be simultaneous with the areas of greatest crust movement; and as the greatest disturbances occur near coastlines that might account for the dis tribution of volcanoes along the margins of the great ocean basins. As 1 have already indicated, science is not prepared to speak dogmatically upon any of these points. We know that volcanoes depend upon internal heat as their prime source oF energy; but the modus operandi by which the heat is created is still an obscure problem. Many eruptions are probably intensified by sea water having been brought into contact with'the superheated rocks at a consider able depth from the surface. The steam thus generated would cause enormous pres sure upon the crust of the earth.' —Contortions of the Earth's Crust.— 'My statements relative to the folding of the -earth's crust are not mere conjecture, however, because id is a Weil-known fact that the. surface of the globe is seldom sta tionary. For a-me reason portions of it are almost always rising or falling. The Alps, Andes, and' Himalayas have all been raised to tlieir present magnitudes above the surrounding councry since the geologi cal period represented by the deposits of eocene limestone in the River Murray val ley and along the shores of St. Vincent and Spencer's Gulfs. This fact indicates the extent of the contortions which have occur red from time to time. In past ages there have been slow upheavals of some portions of the South Australian coast, while other portions have subsided. When the excava tions were being made at Port Adelaide for Fletcher's dock a feiv years ago an old shore surface about 20 ft. below the present level was exposed, with river wash sepa rating a lower marine bed from the higher one. This upper sea beach is now several feet above high water mark.' —The Distribution of Volcanic Zones.— In reply to an enquiry whether he could explain why it is tiiat in some countries volcanoes are almost always in a state of eruption, while other parts of the world have enjoyed immunity from such calami ties from time immemorial, Mr. Howchin pointed out that with few exception's the existing voicanoes are situated within what are termed volcanic zones. Most of them are either within or around the edges of the great ocean basins. There is evidence to show that Australia at one time was within the Pacific Ocean volcanic area,' and that in addition to MountGambier numerous other craters and fissure*- must have been in a state of active eruption in the ea-tern portion of the continent. Although the Pacific area is still one of the most exten sive volcanic zones on the globe, Australia is not now within it. The area extends from New Zealand through the New Hebri des to Japan. From that point it continues in the form of an immense loop, which em braces the Kamtscliatka Peninsula and por tions of the coast of North and South America. The West India isles are within tliesame line of activity. Another area, in which the most prominent features are Mount Etna and Mount Vesuvius, covers the south-eastern portion of Europe. There is also a disturbed area without volcanoes in Asia Minor. In the Andes (South America) there are no fewer th.in 51 known volcanoes, of which 36 are active nnd 15 about which there ifl some uncertainty. In Quito there are 10 active ond 7 of doubt ful activity; Peru and Bolivia, 9 and 3 re spectively; Chili, 17 and 5.' —Mount Gambior's Last Eruption.— 'Can you form any opinion as Li the time which has elapsed since Mount Gam bier was last in a state of eruption?' rsked the interviewer.— 'It is not possible to name any period of years,' replied i\Ir. Howchin, 'but there' is ample evidence to riiow that the old crater, which can still be seen, was
active within a comparatively recent gee^ logical epoch. For. instance, the bones o£ animals similar to the existing Australian marsupials and dingoes have been found under the volcanic deposits in the vicinity; of Mount Gambier. which shows that erup tions have taken place since those classes pi; animals have been indigenous to Australia. Of course it is impossible -to form any opinion as to the length of time whick has elapsed since the animals referred to first made their appearance on this conti nent. _ The mount has not shown any signs of activity since it was discovered by Euro peans, and the condition of the surroundiiij; country indicates that a considerable period must have elapsed since the last outflow 01! lava. Portions of a semi-circular crater may still be seen at the summit of Mount Gam bier, and also a series of subsidiary depres sions outside the crater proper, which are now filled with water. One side of the old crater has entirely disappeared, but on tlie other side the original face of the lunucl is still intact.' —Are the Fires Beneath Mount Gambier Extinct?— 'Would it be possible for aa extirlct vol cano such as that at Mount Gambier to be come active again after so long. a period 0': quiescence?'— 'Not. only is such a contin gency possible,' waslhereply, 'bat there are several recorded instances of such occurren ces. In fact in many cases it is impossible to decide whether a volcano is extinct or merely dormant, For instance, in the fir=t ? century of the Christian era the old Somma valcano would naturally have been regarded as extinct, its fires had never been known to kindle within the period covered by human, memory or tradition; and its crater was a vast wilderness) of wild vines and brushwood. Yet in a few days in the autumn of the year 79 half of the original crater was blown away by a terrific Eerier of explosions, and a new ontiet, which is now known as Vesuvius, was formed with the limits of tho earlier crater. Since that rime volcanic action has been intermittent ly exhibited up to the present day. Some of the subsequent intervals of inactivity have been eo considerable that Vesuvius might almost have been again regarded as an extinct volcano. For 131 years, between 1500 and 1631, the eruptions had ceased so completely that the crater once more be came choked with copsewood. A few springs of hot salt water were the only me morials which remained to indicate what had been the former condition of the mount. But in 1631 the most powerful of all known explosions of Vesuvius, with the exception of that of 79, abruptly closed the long period of quiescence referred to. The approach of such eruptions is not always in dicated by any, premonitory symptom?, and many of the most tremendous explosions liave occurred without any perceptible warning. In cases where a crater has been for a long time quiescent a large amount of solidified lava accumulates in the up]-er portion of the vent or funnel, and forms an immense plug which prevents the ascent of any steam or molten matter. In tha event of a sudden increase of the internal heat, or it may be as the result of a 6lo\v accumulation of energy, a vast amount of pressure is exerted by the superheated ma terial which is imprisoned below. Finally an explosion occurs which either Wows out the plug in the old vent or forms new fis sures io tlie fades of the eld cone. This ao counts for the largo quantities of rocks, stones, and other solid \ debris which are ejected during the early stages of an erup tion.' —How Craters are Formed.— 'How do you account for the fart thai volcanic outbreaks usually occur at the top of mountains instead oi breaking througii the valleys?'— 'In the first place,' replied Mr. Howchin, 'it is not correct t'ospenk ot'a volcano as a mountain in the ordinary sense of the term. As I have already . stated, volcanoes usually occur in or near the great ocean' basins, which may repre sent the lines of greatest weakness in the earth's crust. What is spoken of as n, mountain in this connection is really no thing more than an accumulation of lava and other matter which have been ejected from a fissure in the crust of the earth. A striking example of this is furnished by ' Mount Nuovo, situated near the Bay of Naples. In 1538 the site of this mountain was a level plain, through which a cavity was drilled by a series of explosions in tho year mentioned. In an iacredibly short time the immense quantities of lava, stones, . scoriae, nnd ashes which were ejected' through this new opening had formed 11 hill about 450 ft. hifdi and more than a mile and a half in circumference. The force exerted by the explosions keeps the original vent open and forms a huge cavity at the. top, which is termed a crater.' — Lava and other Volcano Products.— In reply to further questions, Mr. How chin stated that lava was simply molten rock, and that the volcanic ash which is a' prominent feature of almost all great eruptions is merely lava which lias been blown to pieces. The term lava is applied generally to all molten rocks, but the mat ter ejected during eruptions varies greatly in texture and appearance according to the condition of cooling. After this molten rock has cooled aud been exposed to the air for a considerable period it decomposes and forms rich soil, which almost invari ably produces luxuriant vegetation. The stones and 7'ocks which prove so destructive when scattered over the country in the vicinity of a volcano consist mainly of fragments which have been torn from the sides of the funnel and the crater during the eruption. Since the details of the1 eruption at St. Vincent have been publish ed speculation has been indulged in as to how the lakes are formed in extinct craters. Mr. Howchin explained that in most in stances the lakes are formed by the rain which falls on to the sloping sides of the cavity. He pointed out that the solidified lava of which the aides and bottom of craters are formed would not absorb Water to the' same extent as ordinary soil, and with tho exception of the decrease by evaporation- practically the whole of the water which found its way to the bot tom of the crater would remain there. Thus in the course of a few years a lake of considerable depth would be formed to add to the , terrors of those unfortunate enough to be in the vicinity of the volcano when the next outbreak occurred. The Blue Lake and other sheets of ? water at Mount Gambier have a different source of supply. They touch the level of the great underground drainage of the south-east, which in its enormous quantity arid extent indicates a source which mast be practi cally limitless— perhaps the Murray. It is by no means improbable that this water BlTatum had something to do with the initial activities of the volcanic force of the Routh-east of this slate. He also men tioned that volcanoes have been found to vary in depth from 7 to SO miles below the surface of the earth. —Australia's Line of Weakness.— It is interesting to notice that the earth quake which occurred, in South Australia about six years ago indicated the existence of a possible line of weakness along the south-east coast of Australia; It was as certained that the axis of the earth wave extended beneath the sea parallel with the shore, and was directed towards Bass Strait, the shock being most severe in the neighlwurhood of Kindlon and Robe. It has been suggested that this fact points to a probable cleavage in the rock, and per hnpg a slight settlement along the line which formed the axis of the earth move ment. Although there Is a distinction be tween volcanoes and earthquakes the causei of both are so closely correlated that it is impossible to separate them.