VOLCANOES.* IT says much for the philosophy of the human
race that volcanoes are not the objects of terror they might very well be. Yet such is the trust dwellers in volcanic countries repose in the presumption that a certain volcano will not exceed in violence its usual activity, that they continue to dwell on its flanks—ignoring even the customary warnings—till molten lava, hot mud, or ashes drive them out of home and house. This indifference has become even more noticeable of late, now that delicate instruments foretell the coming disturb- ance. People realise that the volcano, terrible when aroused, is comparatively slow in its operations. They know, too, that volcanic activity is a safety-valve in the immediate district for something far more terrible and appalling,—the earthquake. The instant the earth quakes, the utter destruction of all feeling or hope of security, of being able to stand upright even, demoralises the calmest. The average volcano, there- fore, which smokes away steadily, ceases to be an object of interest to any but the stranger and the scientific visitor. It is as much a part of the scenery as any other mountain, and nothing more, except that there is a cloud over it in the day- time and it is lit up in the night-time in a somewhat striking manner. It is certainly an instance of " where ignorance is bliss 'tis folly to be wise," for no man who has grasped the significance of the immense forces at work, held in equilibrium by laws inconceivably vast yet most delicately balanced, can feel the trusting security of the majority. Both these feelings are natural, and both have their uses and compensations ; the one is just as likely to escape as the other, who goes poking about in craters and building observatories outside them, and is contented in a sort of fatalism. On the other hand, the geologist has the satisfaction, afforded to followers of no other science, of seeing the original forces at work that he recognises as having built up the world of his conceptions. He can theorise, and point at the same moment to something going on that makes his surmises obviously true to any eye.
The great attraction of volcanoes is that we can all see for our- selves the forces at work, and judge from the shape of the cone and the materials ejected, how "a burning mountain" works. Their slope varies from 3Q to 40°, the lava-producing volcanoes possessing enormous circumferences and a gradual slope ; while those elegant symmetrical cones that produce chiefly mud and ashes, and are continuously and less violently active, are many of them remarkably slender. The craters of the volcanoes of the Sandwich Islands simply consist of vast reservoirs of molten lava, banked up by successive accumula- tions of previous eruptions,—Kilanea, in Hawaii, having a permanent lava-lake, which rarely overflows its banks, re- sembling, in fact, a " guage " to a boiler, the boiler in this case being the molten interior of the earth. An unusually violent explosion in a volcano that has laboriously built into itself a symmetrical cone with a bore too narrow for emer- gencies, blows off the top, or " truncates " it, and converts it into a "ringed " volcano. Vesuvius is an example of this, and centuries ago was undoubtedly more artistic than now. It has assumed its present shape daring the last two centuries only. The volcanoes of the moon are the largest we know of, and their outlives, so well seen by a moderate glass at the first quarter, belong to the same type as the compara- tively feeble craters of the Sandwich Islands,—simply vents for molten lava. Lunar craters are many of them fifty miles and more in diameter, and from the floor of these vast craters rise lesser cones comparable with terrestrial summits.
The volcanoes of the earth, dead or alive, are distributed in certain well-known lima, and their position is never far from the sea, or what was b‘ a in the Tertiary epoch. There are exceptions—Kenia, Kilimanjaro, Cotopaxi, Jorullo, Demavend —but the most active volcanoes of to-day are mid-oceanic ; and it is absurd to suppose this arrangement to be an acci- dental coincidence. The great chains of volcanoes, too, • Volcanoes Past and Present. By Edward Hull, F.H.S. With Illustrations. The " Contemporary Science Series." London : Walter Scott.
have arisen from lines of fissure, such as the Cordilleras of Quito, a series carried on through Jacora, Sahama, down to Tierra del Fuego. The mid-oceanic volcanoes can also be arranged, and the connection becomes obvious between such volcanic vents as the Sunda, Philippine Islands, Java, Japan, Aleutian Islands, and Kamtchatka. Thus, we are enabled to say that a line, including the volcanoes just men- tioned, stretches from this peninsular and the Aleutian Islands to the Malay Islands, of some 10,000 miles and with 150 active vents ; another, along the west coast of North and South America, of 8,000 miles and 100 active vents ; and a third, from Greenland to Tristan de Cunha, of 1,000 miles with 50 active vents, of which the Mediterranean volcanoes are branches ; and a line has even been traced along the eastern coast of Africa. It is in the period known as Tertiary to geologists, that volcanoes appear to have been most active, the deposits usually overlying Tertiary strata. (We have already noted the fact of volcanic action having taken place near either the present or the Tertiary sea- coast.) Exceptions there are : extinct volcanoes of the A.avergne burst through granite ; those of the Eifel, through Devonian rocks ; but these are very old volcanoes. Again, vast beds of lava have been thrown out in North America, between the Rocky Mountains and the Sierra Nevada, and, it seems to be pretty generally agreed, through innumerable fissures, there being no cone to account for so much volcanic material, which, by-the-way, is the result of a comparatively recent display of prehistoric action.
That immediately beneath the crust of the earth there is a reservoir of molten or semi-molten lava, seems almost beyond the possibility of a doubt, and there is an equal pro- bability that it is arranged in a certain order of density. The heavier lavas have been extended more recently, as a rule, and in the greater quantity. The inference to us is that these layers must be very thin, or the composition of the lavas should not change, comparatively speaking, so quickly. Some people may be puzzled, when the existence of this continuous sheet is affirmed, to account for the extinction of volcanoes. We are not aware that any one has offered a reasonable explanation of such phenomena. The varying thickness of the crust no doubt affects volcanic activity ; the thinner the crust, the more active the volcanic vents will be. Meanwhile, be it observed, some ancient volcanoes are so dead as to be barely recognisable as volcanoes, others have their craters full of water. The crater lakes of Europe, in the Auvergne and the Eifel, must be known to many ; while there are even more remarkable ones afield, that on the summit of Kilimanjaro, and the beautiful Crater Lake in Southern Oregon, also Jahoe in California. Volcanoes, dead or alive, form some of the most striking features in scenery. The Himalaya and the Alps, it is true, are not volcanic, but the Andes are, and from the aesthetic and the geo- logical standpoint are marvellous objects, and there is surely an indescribable impressiveness about Arctic and Ant- arctic volcanoes, the contrast between their fire and energy and their desolate surroundings is so extraordinary.
Mr. Hull has set himself two questions to answer, viz.,—Are we living in a period of specially energetic volcanic action F and What is the ultimate cause of volcanic action F The last decade, it must be allowed, has been one of unusual display. The eruption of Krakatoa, for instance, was enough to satisfy the most exacting ; one of the most remarkable incidents of this great disturbance was the air-wave which travelled round the globe, and was reflected back to its centre, and the passage of which was observed seven times. That terrible eruption was remarkable also for the dense clouds of ashes, seen even hundreds of miles away, and portions of which, after being shot up to an immense height, travelled the whole globe round, so that, far away as we were from Krakatoa, gorgeous after-glows held us spellbound, and the timid feared to gaze upon such blood-red skies. The rain of ashes is a far more terrible reality than it sounds, as, doubtless, Pompeians of A.D. 79 could have told us.
The ultimate cause of volcanic action is the issuing of the heated interior through such weak parts of the earth's shell as fissures, the abrupt edges of continents, and geological faults. The cooling and, therefore, contraction of the crust is, doubtless, the great agent. In early epochs of the globe's history, this was rapid, yet it is not certain that volcanic action, strictly speaking, was more violent than in Tertiary times. Mr. Hall answers his first question by affirming the decay of volcanic energy, and the probable extinction of it, owing to the lower rate of cooling ; if, indeed, this be so, why should a fairly recent epoch have been so active ? It seems to us that, with a lower rate of cooling, one might very well couple slowly accumulating pressures and tensions, the adjustments of which would be few and far between, compared to the movements in the earlier and softer crust, but far more shattering and convulsive in their effects. The earth loses, we believe, some infinitesimal fraction of a degree of heat every year, and it would not be too violent an illustration to say that were the earth to lose half-a-degree of heat, every volcano on the face of the globe would burst into furious activity as a consequence of the contraction following, and any greater loss of heat would probably mean a new Atlantis, the flooding of Europe, and thousands of new volcanoes in the Pacific.
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