Kitabı oku: «A Girl's Ride in Iceland», sayfa 9
CHAPTER XII.
VOLCANOES
In the foregoing pages it may seem strange that hardly any allusion has been made to the special characteristic of Iceland, viz., its volcanic structure, or to the numerous lava floods which, bursting forth in furious molten streams, have from time to time devastated its surface, leaving in their track a chaos of disrupted rocks, chasms, vast fissures, and subterranean caverns.
Our trip to Iceland was, however, unfortunately so limited in duration as to preclude, save in our four days' ride to the Great Geyser tract, any extension of travel in the various volcanic regions. Hence the omission. I have therefore extracted the following data relative to its principal volcanoes and their eruptions from such books of reference1 as have been available to me.
The annexed compilation will, I think, explain to such of my readers as are not acquainted with the geological strata of Iceland, its sterile nature, the extreme poverty of its inhabitants, and the constant terror under which their existence is passed, lest a fresh outbreak of lava should sweep away both them and their homesteads. It is somewhat singular, that although Iceland may be looked upon as a veritable mass of volcanoes and hot springs – for with the exception of some 4000 square miles of habitable ground, it may be said literally to rest on underground fires, and while the various eruptions of Etna, Vesuvius, and other volcanoes have for centuries been watched and recorded in the public papers with interest – it is only comparatively recently that the awe-inspiring volcanic eruptions of Iceland have been brought into notice. For instance, while full fifty pages in Ree's 'Cyclopædia' are devoted to the subject of volcanoes, those of Iceland are barely touched upon; yet their eruptions are by far the most devastating on record. So limited, indeed, formerly were the researches of science in these ice-clad regions, that for long Hecla was quoted as its only volcano.
Now that the Island has attracted the further notice of geologists, it has been shown that there exist no less than twenty volcanic mountains, all of which have been in active eruption within historic times, and nearly one hundred eruptions have been chronicled as having taken place in the Island.
Although Hecla is doubtless the best known of the Iceland volcanoes, it is by no means the largest; that of 'Askja' (a basket), far surpasses it in size. This latter volcano lies in a great central desert termed 'Odaxa-hraun' or 'Misdeed Lava Desert,' covering a space of 1200 square miles, and a most appropriate name it is, for the devastation caused by its last flood of lava is indescribable.
In one of the convulsions of this mountain in 1875, a quantity of lava five miles in circumference was disrupted, sinking into the mountain to a depth of 710 feet, and causing an earthquake which was felt all over the island. In one region, viz., that of the 'Myvatn's Orœfi' or 'Midge Lake Desert,' a fissure was opened which extended over 20 miles in a north-easterly direction, through which molten lava flowed continuously for four months after the earthquake. Although this fissure is at least 30 miles from Askja, so great was the column of fire thrown up by the eruption, that it was visible for four successive days at Reykjavik, 100 miles distant. The study of an Icelandic map will show the numerous volcanic ranges of mountains which intersect the island in almost every direction.
To the north there will be seen a wonderful volcanic tract. So vast, in fact, that Professor Johnstrup has termed it the Fire Focus of the North. To the north-east, again, is found a large lake, called 'Myvata,' or 'Midge Lake,' with a volcanic range of mountains which stretch from north to south; the most famous of these are 'Leivhnukr,' and 'Krafla,' which, after years of quiescence, poured forth such an amount of lava into the adjoining lake that for many days its waters stood at boiling heat. Other volcanoes in this region eject with terrible force a quantity of boiling mineral pitch, throwing up the dark matter completely enveloped in steam, accompanied by horrible rumbling noises.
Sir George Mackenzie, in his travels in Iceland, thus describes one of the deposits: —
'It is impossible,' he says, 'to convey any idea of the wonders of its terrors, or the sensations of a person even of strong nerves standing on a support which but feebly bears him, and below which fire and brimstone are in incessant action, having before his eyes tremendous proof of what is going on beneath him; enveloped in thick vapour, his ears stunned with thundering noises – such a situation can only be conceived by one who has experienced it.'
The extent of the sulphur beds too in this region are beyond calculation: they reproduce themselves every few years. In the vicinity of 'Krafla' is a curious rock, composed of obsidian, a substance which closely resembles black glass.
To the south of the Island is another volcano, termed the 'Kotlugja,' or 'Cauldron Rift,' lying among glaciers known as the 'Myrdals Jökull,' whose eruptions, thirteen of which have been noted, are considered to have done more mischief than any others in the Island. Between the Myrdals and the 'Orœja Jökla' lies one of the most noted volcanoes of Iceland – the 'Skaptar-Jökull,' whose eruption in 1783 is chronicled in all works on Iceland, as the prodigious floods of lava it poured forth in that year were unparalleled in historic times. The molten streams rushing seaward, down the rivers and valleys, the glowing lava leaping over precipices and rocks, which in after years, when they have cooled down, resemble petrified cataracts, and now form one of the grand scenic attractions of the Island.
In Mrs Somerville's 'Physical Geography,' she vividly describes this eruption, narrating how, commencing in May 1783, it continued pouring forth its fiery streams with unabated fury until the following August. So great was the amount of vapour, that the sun was hidden for months, whilst clouds of ashes were carried hundreds of miles out to sea. The quantity of matter ejected on this occasion was calculated at from fifty to sixty thousand millions of cubic yards. The burning lava flowed in a stream in some places 20 to 30 miles broad, filling up the beds of rivers, and entering the sea at a distance of 50 miles from where the eruption occurred. Some of the rivers were not only heated to boiling point, but were dried up, and the condensed vapour fell as snow and rain. Epidemic disease followed in the wake of this fearful lava flood. It was calculated that no less than 1300 persons, and 150,000 sheep and cattle perished, 20 villages were destroyed. The eruption lasted two years.
Mr Paulson, a geologist, who visited Iceland eleven years later, found smoke still issuing from the rocks in the locality.
The heat of this eruption not only re-melted old lavas, and opened fresh subterranean caverns, but one of its streams was computed to course the plains to an extent of 50 miles, with a depth of 100 feet, and 12 to 15 feet broad. Another stream was calculated at 40 miles long, and 7 wide. Men, their cattle and homesteads, their churches and grazing lands, were burnt up, whilst noxious vapours not only filled the air, but even shrouded the light of the sun.
The terrible convulsions which occurred in Iceland during the year 1783, were greater than those recorded at any other period. About a month previously to the convulsion of 'Skaptar-Jökull,' a submarine volcano burst out at sea, and so much pumice stone was ejected that the sea was covered with it for 150 miles round, ships being stopped in their course, whilst a new island was thrown up, which the King of Denmark claimed, and named Nyöe, or New Island. Before the year had elapsed, however, it as speedily disappeared, leaving only a reef of rocks some 30 fathoms under water to mark its site.
But what of Hecla? which is 5000 feet high, and is situated close to the coast at the Southern end of a low valley, lying between two vast parallel table lands covered with ice.
If the eruptions of Hecla are not considered to have been quite so devastating as those just recorded of the 'Skaptar-Jökull,' their duration has been longer, some of them having lasted six years at a time.
When Sir George Mackenzie visited Hecla, he found its principal crater 100 feet deep, and curiously enough, it contained a quantity of snow at the bottom. There are many smaller craters near its summit, the surrounding rocks, consisting chiefly of lava and basalt, are covered with loose stones, scoria, and ashes.
A record of the eruptions of Hecla has been chronicled since the 10th century, and they number 43. One of its most violent convulsions occurred in the same year as that of the 'Skaptar-Jökull,' viz., in 1783. At a distance of two miles from the crater, the lava flood was one mile wide, and 40 feet deep, whilst its fine dust was scattered as far as the Orkney Islands, 400 miles distant.
The mountain itself is composed of sand, grit, and ashes, several kinds of pumice stone being thrown out of it. It also ejects a quantity of a species of black jaspars, which look as if they had been burned at the extremities, while in form they resemble trees and branches. All the different kinds of lava found in volcanoes are to be met with here, such as agate, pumice stone, and both black and green lapis obsidian. These lavas are not all found near the place of eruption, but at some distance, and on their becoming cold form arches and caverns, the crust of which being hard rock. The smaller of the caverns are now used by the Icelanders for sheltering their cattle. The largest of the caves known is 5034 feet long and from 50 to 54 feet broad and from 34 to 36 feet high.
It is believed by some geologists that a subterranean channel connects the volcanic vent of Hecla with the great central one of Askja. This theory is based on the fact that a number of lava floods have burst forth simultaneously at different times at great distances from the volcanoes, leading to the supposition that innumerable subterranean channels exist in the neighbourhood.
The eruptions attributed to the volcano of Hecla vary much in number, some authorities saying there have been 40. Mrs Somerville quotes them at 23, and Mr Locke, in his 'Guide to Iceland,' at 17 in number. In the latter's work is given a table of most of its principal eruptions. One of these was of a singular nature; huge chasms opened in the earth, and for three days the wells and fountains became as white as milk, and new hot springs sprang into existence.
The twelfth eruption of this mountain was also of unusual violence. It occurred in January 1597. For twelve days previously to the outbreak loud reports were heard all over the Island, while no less than eighteen columns of fire were seen ascending from it during its eruption. The ashes it threw out covered half the Island.
The seventeenth eruption commenced on the 2d September 1845, and continued for seven months. On this occasion the ashes were carried over to Shetland, and the columns of smoke rising from the mountain reached a height of 14,000 Danish feet.
Such is a brief description of the tremendous forces which dominate Iceland. Here Nature works in silence for long periods beneath the crust of the earth, and then, with little or no forewarning, bursts forth in uncontrollable fury, ruthlessly devastating with its fiery streams whatever impedes its course.
Who can wonder that, under such existing terrors, the scanty inhabitants of the Island are a sad and dejected race. A people with death and terror continually at their doors can hardly be otherwise; whilst competitive industry, energy, and hopeful prosperity are alike suppressed by the constant devastations which occur.
With respect to the Thermal Springs, these must be considered as products of the same underground fires, and which form a second characteristic of Iceland.
These Springs may be divided into three kinds, viz., those of unceasing ebullition, those which are only sometimes eruptive, and wells which merely contain tepid water, though supposed to have been formerly eruptive.
Professor Bunsen, who passed eleven days by the side of the Great Geyser in Iceland, attributes the phenomenon to the molecular changes which take place in water after being subjected to heat. In such circumstances, water loses much of the air condensed in it, and the cohesion of the molecules is thereby increased, and a higher temperature required to boil it. In this state, when boiled, the production of vapour is so instantaneous as to cause an explosion.
Professor Bunsen found that the water at the bottom of the great Icelandic Geyser had a higher temperature than that of boiling water, and that this temperature increasing, finally caused its eruption.
In America, among the hot springs warmed by subterranean vapours, such as those springing from the sides of 'Nuerode Chilian,' the hot springs gush out through a bed of perpetual snow.
Among the hot springs of Iceland, Mr G. Loch gives an interesting description of those known as the 'Northern Geyser' and its tributary springs. One of these, the 'Uxhaver' or 'Ox Spring' is named from an Ox having fallen into it, and in a short time having been thrown out in the form of boiled beef. This hot spring emanates from an oval basin, 30 feet in circumference, and 4 feet in diameter. Its spurts are very regular, occurring about every 6 minutes, and about 10 feet high. After a spurt the water in the basin is lowered from 4 to 6 feet, but quickly refills, whilst the water thrown up is clear as crystal, and its spray glistening in the sun's rays has a most beautiful effect.
The smaller springs in this so-called 'Uxhaver' group are collected in a bed of rock 280 feet from the principal Geyser, and it is singular that although separated from it by only 300 yards of boggy ground, the springs in each bed of rock seem to have a distinct source of supply, for they are not affected by each other's spoutings. It is impossible even to enumerate the various hot springs of Iceland, as they are spread over all its volcanic region.
I must here bring my little book to a close, and if it has done no more than make my readers desire to make a personal acquaintance with this wonderful little Island, so full of natural curiosities, so abounding in ancient history, so isolated, and so quaint, it will have served its object.
APPENDIX
WHAT IS A GEYSER?
Having been requested by my daughter to add to her little book a short explanatory chapter on the marvellous phenomenon of Nature she saw in Iceland, commonly called a Geyser, I herewith subjoin the results of a few of the observations and reflections I made while visiting the great geysers of the volcanic districts of Wyoming and Montana, in the autumn of 1884.
In order to make the matter perfectly clear, let me say at the very outset that a geyser is simply a volcano from which a quantity of superheated boiling water, saturated with mineral matter, is paroxysmally ejected high into the air. Instead of, as in the case of fire volcanoes, the ejected matters being smoke, flame, lava, scoria, pumice stone, and scalding mud. Moreover, while the eruptions from all volcanoes are intermittent – that is to say, every kind of volcano has alternating periods of activity and repose – the eruptions from geysers further differ from the fire and flame ejections of burning mountains, with their other attendant phenomena, in occurring at definite periods of time, and being of equally definite durations. It is this life-like periodicity in the geyser's mode of action which makes it as awe-inspiring to behold as it is puzzling to explain.
That hot water should issue in a continuous and but little varying sized stream from the bowels of the earth, with a force sufficient to carry it high into the air, has nothing whatever wonderful about it. Such a natural phenomenon may be witnessed at many places. For example, it may be seen doing so everyday at the white foaming, frothing, natural mineral water sprudel of Nauheim, or at any artificially bored artesian well, such as the celebrated one at Paris. Nor does the mere intermittence of water issuing from the bowels of the earth suffice to surprise one. For such natural phenomena are seen at Bolder-Born, in Westphalia; the Lay-Well, at Torbay; the Giggleswick Well, in Yorkshire; and even on a small scale at St Anthony's Well, Arthur's Seat, Edinburgh; all which occurrences are readily explicable on ordinary hydraulic principles, and quite different things from geyser action, which try to explain it as you will, always runs into a volcanic groove. Yet the periodicity of a geyser's action cannot be said to be entirely due to volcanic agency. For the mere action of heat on the solids of the earth's crust, or even of heat in simple conjunction with water, according to either Mackenzie or Tyndall's theories,2 even did they suffice to give a satisfactory explanation of the action of the geysers in Iceland, are assuredly totally inadequate to explain the action of all those of the Yellowstone Park. For the simple reason that the vapours escaping from some of them are so strongly impregnated with hydrochloric, sulphurous, and sulphuric acid gases, as well as with sulphuretted hydrogen, as to compel one to believe that chemical action plays a not unimportant part in the production of the phenomena there witnessed. Moreover, the solids brought up by the water closely resemble in chemical composition the lava ejected from burning mountains, inasmuch as, besides containing a large percentage of silica and alumina, they likewise consist of lime, potash, soda, magnesia, and iron, as well as of a small proportion of other metals, as was guessed at by the beautifully varied green, rose, yellow, and purple hues of the beds of the streamlets flowing from the craters of the geysers. The geysers of the Yellowstone, although situated at the height of 7765 feet above the level of the sea, nevertheless lie in valleys, for the mountains surrounding them are much higher still.
Some idea of the force with which the water issues from the earth, may be formed from the fact that it is in some cases sufficient to carry a column of over six feet in diameter 200 feet high, for the space of twenty minutes at a time. And all know that 200 feet is nearly double the height of any ordinary church steeple. Moreover, the amount of solids brought up with the water may be imagined when I say that, in one of the boiling springs, the mixture so closely resembles thick milk gruel as to have given to it the name of the 'paint-pot,' and so loaded is its water with mineral matters, that they consolidate almost immediately after escaping from the spring's outlet. So thick indeed is it, that I kneaded some into the shape of a brick, which I have still in my possession. All the geyser water in this district is so charged with silicious earths that it consolidates sufficiently rapidly to form an upright rim around each geyser's vent. Just as a fringe of scoria and lava encircles the mouth of a burning mountain.
The rapidity with which the deposits form and solidify may be conjectured when I say that I saw trees growing close to some of the geysers whose stems and lower branches were so encrusted with geyserite as to give the idea that they were actually petrified. While again I saw an old horse shoe, which had only been fourteen days in the water, so completely enveloped with it that it looked exactly as if it had been hewn out of solid marble.
The mere glancing around, and noticing how the geysers had evidently, like human beings, but a transient existence, produced a somewhat strange sensation. For it was perfectly evident that they are born but to die. All of them appearing to spout themselves permanently out. For while on one side some seemed just as if they were starting into existence, on another were those apparently in the very zenith of their strength, while others again looked as if they were making but their last feeble efforts at existence, though it was evident, from the heaps of consolidated geyserite surrounding them, that they had but recently passed through halcyon days of youthful energy and manhood power. Every here and there again we came upon others from whose wide open empty mouths came forth neither a puff of steam nor a drop of water. They were dead, and not a few of them were so completely eviscerated as to allow of the explorer to descend with perfect safety into the bowels of the earth through their vents. Geyser activity is in fact but the last act in the drama of volcanic life: all around proved this. Close at hand were stupendous cliffs of pure obsidian – the black bottle glass manufactured in Nature's furnaces. Even half a mile of our road was macadamised with it. And so similar not only in chemical composition but in optical properties is this obsidian to actual glass, that a flat piece I picked up on the road, just after it had been splintered off a block by one of the wheels of our carriage, is as transparent as any piece of black bottle glass of equal thickness. These mountains of obsidian plainly tell how awfully stupendous must have been the heating process which called them into existence, as well as how big must be the cavities left in the bowels of the earth from which the materials constituting them were obtained. No doubt water scoops out caverns in the softer strata composing the earth's crust, but these can scarcely be thought to equal in extent the cavities made by volcanoes. Think, for example, of what a hole in the earth must have been left by the 50 miles long and 5 miles broad lava stream which flowed from Mauna Loa in 1859, and fell as a fiery cascade over a cliff into the sea, in sufficient amount to fill up a large bay.
The geyser basin is in many places actually honeycombed with various sized caverns, either directly due to volcanic action, or to water, or to both combined, and these caverns, though widely apart, may yet freely communicate with each other by means of subterranean river courses. I have myself followed one river course into the bowels of the earth for three miles and more, in the great Adelsberg Grotto, in Styria. I have rowed across the lake in the dismally dark cavern at Han, in the Ardennes. And even in our own Derbyshire, I have seen, half-a-mile from the entrance of the Speedwell Mine, a river, a water-fall, and a lake, all of which tell that such natural phenomena exist within the bowels of the earth as well as upon its surface. Moreover, the resounding echoes from the clatter of our horses' feet as they briskly trotted over some of the geyserite, as well as the heat we experienced through the thick leather soles of our boots as we walked across it, was unmistakable proof that but a thin layer of crust separated the surface of the globe we were traversing in Wyoming and Montana not alone from vast caverns, but likewise from still active subterranean fires.
All the preceding facts I have narrated must be borne in mind, in order that the theory of geyser action I am now about to propound may be readily understood. For unless the reader believes: —
1st. That cavities of various shapes and sizes exist in the earth's crust;
2d. That the earth possesses internal lakes as well as rivers;
3d. That there are vast internal fires still actively at work in the neighbourhood of geysers; and,
4th. That the smell of the acid vapours and sulphuretted hydrogen, as well as the mineral matters dissolved and suspended in the ejected waters, are proof positive of chemical activity, he will entirely fail to perceive the value of my remarks regarding the cause of a geyser's action being not only spasmodic but periodic.
On the next page is an explanatory diagramatic sketch, in which no attempt has been made at the impossible, namely, to apportion the size, the shape, or the situation of the cavities to each other. As they may in reality be close together, or miles apart. They may all be on the same level, or more likely not. They may be of nearly equal dimensions, or of varying sizes. It matters not one whit, for the purposes of the demonstration of the theory of geyser action now being adduced.
a. A cavernous reservoir, receiving its water supply by streamlet feeders (b) from the hills (a). b. A natural, and, it may be, circuitous syphon conduit, by which the water can only reach chamber (c) after it has filled tube (b) to the level of the syphon's top, consequently the supply of water to chamber (c) is intermittent, and only lasts until the water in chamber (a) has sunk down to the orifice of its syphon connection. c. Is supposed to be the chemical laboratory in which the decomposable minerals are, and it is further supposed to be heated by subterranean fires. In case the reader knows but little of chemistry, I may remark that all chemical changes are greatly accelerated by heat, and that superheated steam is a most powerful agent in expediting the decomposition of earthy and alkaline compounds.
In the case of these subterranean laboratories, it is utterly impossible for even the scientifically trained mind to conceive what the extent of the heat may be. All he knows is that it is probably far greater than suffices to resolve water into its gaseous elements – oxygen and hydrogen – and that even before this point is reached, superheated steam becomes a terrifically formidable explosive agent. Look at what it did at Ban-dai-san in Japan last year. It actually split a mountain three miles in circumference in twain, and blew one half of it right away into a valley as if it had been the mere outside wall of a house. And such was the force of the wind-shock it occasioned that all the trees growing on the opposite mountain's side were knocked down by it as if they had been mere nine-pins. (In 'Nature,' of the 17th January 1889, at p. 279, will be found an account of the scene of devastation when it was visited (in the month of October 1888) by my son Vaughan; the same who visited the geysers of the Yellowstone with me in 1884, and those of Iceland with his sister in 1887.)
In the case of the geyser, superadded to the superheated steam's explosive power, there will be in addition that of the gases liberated by the decomposition of the carbonates, sulphates, and chlorides (under the combined influence of heat and water) in chamber (c), which I call for the nonce the chemical laboratory. Not alone will all earthy and alkaline, but even metallic compounds, like iron pyrites, therein contained, be rapidly decomposed on the advent of the superheated water. And from their gaseous elements being held in a confined space, they will acquire an enormous explosive power. Consequently, there is no difficulty in understanding how that on obtaining entrance into chamber (e) by means of conduit (d), they will instantly proceed to expel from it all its water. And from the water finding no other outlet except by vent (f), it will rush through it, and, by virtue of the propelling force of the gases, be thrown up into the air in the form of a geyser. Whose activity will only last so long as the supply of water in chamber (f) remains unexhausted.
The above being a rough outline of the salient points of what I consider to be a rational, though, it may be, incomplete, theory of the geyser action I saw in the Yellowstone Park, I shall now add a concluding word on the probable mode of action of the so-called 'earth-sod emetic' that my daughter describes as having been given to the 'Stroker' geyser in Iceland in order to make it eject its water.
The mode of action of the sods, I think, is easily enough explained on the supposition that the geyser has a constriction at some point or another in its vent, and that the sods plug it up sufficiently to hold back the steam and water until they have accumulated sufficient power to blow out the obstructing body, and escape after it with a rush into the air. Precisely in the same way as a fermenting barrel of beer blows out its bung, and its fluid contents gush out, when its vent-hole accidentally becomes plugged up.
