Popular Science Monthly/Volume 17/August 1880/Fouque's Santorin and its Eruptions
|M. FOUQUÉ'S SANTORIN AND ITS ERUPTIONS.|
M. FOUQUÉ, a professor in the College of France, has recently o published an elaborate work on the volcano of Santorin, which is considered one of the most remarkable and instructive examples of volcanic phenomena on the earth. He possesses excellent qualifications for this work, for he has made special researches on the volcano on three different occasions: first, when sent by the French Academy of Sciences in 1866; and twice afterward, in 1867 and 1875, under a commission from the Minister of Public Instruction. His account embraces the detailed recital of the observations which he made on the ground, and the description of his labors in the laboratory, analyses, and microscopic examinations, and contributes much to our knowledge on mooted questions respecting volcanic action.
The products thrown out in the recent eruption—which took place in 1866—like those of most other volcanic manifestations, may be divided into two categories: 1. Volatile matters exhaled in the form of gases and more or less easily condensible vapors; and, 2. Lavas overflowing in imperfect fusion, or thrown out as ashes and scoriæ. The gases varied greatly in composition during the course of the eruption. At the beginning, when they had not been subjected to the action of the air, they were rich in combustible elements, particularly in free hydrogen, arising in some cases from the separation of water into its two elements. At a later stage, the gases varied incessantly in composition, and the combustible elements at last entirely disappeared. The emitted gases consisted chiefly of free hydrogen, marsh-gas, hydrochloric, sulphuric, and carbonic acids, sulphuretted hydrogen, and nitrogen. Water played a prominent part in the different phases of the eruption. In the state of vapor it was present in all the emissions of volatile matters whatever was their temperature, and it might be regarded as the immediate cause of the explosions. No dry smoke-vent was observed. Coming in its liquid state from hot springs, the temperature and flow of the water varied with the state of the sea.
Chloride of iron, expelled as a vapor, was found associated with hydrochloric acid; but the hydrochlorate of ammonia, commonly abundant in eruptions, was almost wanting. This fact lends support to the opinion that the ammoniacal product of volcanoes is of organic origin, and is brought by the atmosphere over vents charged with hydrochloric acid. The distance and the small extent of cultivated lands explain the rarity of ammonia in the air at Santorin.
Over the central eruptive mouths, the points of the volcano where the incandescence was most lively, the spectroscope showed the presence of volatilized salts of soda and potash. After the eruptive manifestations had ceased, the salts which were deposited around the orifices of the vents were collected and analyzed, and appeared to consist largely of the chlorides of sodium and potassium, the sulphates of soda and potash, and the carbonates of soda and magnesia. Sea-water after evaporation leaves a residuum of analogous composition. These results are interesting in their bearing on the theory that the water of the sea is the immediate ordinary agent of the eruptions. All of the elements usually occurring at volcanic vents were found to be present at the points which were the seat of a volatilization of alkaline salts, lending support to the opinion which had been previously drawn from studies made at Etna, that the vents at their highest temperature present at once all the chemical elements of the volatilized bodies in the volcanoes, and that the cooler vents are gradually impoverished as their temperature becomes insufficient for the reduction of the eruptive materials to vapors.
Some of the volatilized substances in the volcanic conduits are susceptible of reacting on each other and producing fixed compounds. Thus are engendered the hydrated oxides of iron, specular iron-ore, free sulphuric acid, alum, and sulphate of lime, which are met around the vents.
Certain crystallized silicates, however, generally originate under different conditions. Although they are formed of elements which we are in the habit of considering fixed, they are found in the volcanic vents, on the surface of the rocks, under such conditions that they could have been produced only through volatilization. Such silicates have been gathered at Santorin in the interior of tubular cavities like fulgurites, which have evidently been traversed by vapors at a very high temperature. The deposited crystals are of anorthite, sphene, and pyroxene under the forms of fassaïte and augite.
Other crystallized silicates are met occasionally in the lavas. They arise from the transformation of the calcareous masses which are taken from the subsoil in the region and carried out in the lavas, and consist chiefly of anorthite, fassaïte, the melanite garnet, and Wollastonite. The quartz and mica-schist, which, like the limestone, appear as inclosed lumps in the lava of Santorin, do not seem to have suffered any action from the matter surrounding them, notwithstanding the high temperature it possessed.
The mineralogical study of the lavas of 1866 offered great difficulties, on account of the small size and strong adherence of the mineral integrants. It was not practicable to extract the crystals by picking them out, and M. Fouque was obliged to seek new processes. One, founded on the employment of a powerful electro-magnet to draw out the ferruginous minerals, permitted the isolation of the feldspar; and other minerals, harder to deal with, were separated by the use of concentrated hydrofluoric acid.
The crystallization of minerals in the recent lava took place in two stages. In the first stage were developed crystals which frequently attained the length of about a fiftieth of an inch, and of which the other dimensions exceeded a tenth of that size; and, in the other stage, crystals of notably small dimensions were produced. The latter crystals, or microliths, unite the others and follow their contour. Before the microscope was applied to the study of the rocks, only the larger crystals were recognized, and the matter which in volcanic rocks envelops crystals large enough to be perceived through the glass was supposed to be wholly uncrystalline. In this matter, however, the microliths abound in immense quantities. The discovery of them has been one of the most signal triumphs of microscopic micrography.
The minerals observed in the microliths of the general lava of 1866 are feldspar and titaniferous oxide of iron. The predominant feldspar in large crystals is the labrador, but anorthite, and some oligoclase and sanidine are also found in the same condition. The microlithic feldspar is albite with a considerable proportion of oligoclase, and the whole is cemented with a vitreous matter which represents the residue of the crystallization, the part of the rock to which the latter owed its fluidity, at the moment when the minerals which had made a portion of it were already crystallized. This amorphous matter is of a composition similar to that of albite, but a little richer in silica and potassa; and it is curious that a substance of such a composition should have constituted the part of the rock which remained longest melted.
The order of crystallization of the minerals was as follows: 1. Magnetic oxide of iron in large crystals; 2. Apatite; 3. Silicated magnesian irons (augite and hypersthene); 4. Feldspars in large crystals; 5. Microlithic granules of titaniferous oxide of iron; 6. Feldspathic microliths.
The general lava of the eruption of 1866 contains as inclosed masses lavas, the mineral composition of which differs considerably from that of the medium which surrounds them, of types which have a considerable part in the constitution of the ancient lavas of the volcano. The minerals of the recent lava are generally penetrated by foreign matters, microscopic inclusions which are sometimes crystalline, sometimes composed of amorphous matter. The latter inclusions are portions of the surrounding matter which remains inclosed in the crystals at the moment they are formed, and nearly always contain a bubble of gas, in which M. Fouqué found minute quantities of matter analogous in its properties to organic matter. Jets of ashes were cast out in many of the spurts of the eruption. These ashes are lava pulverized by the quick passage of gases and vapors through it while it was still more or less fluid. Its condition as to crystallinity depends on the condition of the lava at the moment it became an ash. The more fluid the lava, the more like pumice is the ash. If the lava has become charged with crystalline substances, especially if they are microliths, the ashes will offer the same characteristics. In the present case the ashes, being derived from lavas which were so crystalline that they were hardly fluid, were rich in crystals and microliths.
Remarkable movements of the soil have been produced by the eruptions within a limited space around the principal cones of the volcano, and have notably modified the surface of the land to that extent. These manifestations have always been special subjects of study with geologists.
Besides the phenomena already described, the complete development of an eruption involves: 1. The opening of the ground; 2. The formation of a cone or crater; 3. The production of a stream of lava. All these manifestations took place during-the last eruption at Santorin. Eruptions analogous to those of 1866 have taken place in the Bay of Santorin since the beginning of the historical period, and have given rise to the islands which are known as the Kamenis. The bay itself was created by a catastrophe which was anterior to history, for no writer of antiquity mentions it. Yet remains of habitations have been discovered in the lava, with numerous objects and domestic utensils, which lead to the conclusion that a civilized population, who had already developed artistic tastes, were its witnesses and victims. Judiciously conducted excavations and microscopic examinations of their potteries have furnished much information concerning these ancient people. They were laborers and fishermen; they had flocks of goats and sheep, cultivated grain, made meal, extracted the oil from olives, wove cloths, fished with nets, and lived in houses with walls of squared stone and wooden beams. Most of their tools were of stone, the common ones of lava, others of flint or obsidian. They were acquainted with gold and probably with copper, but those metals were rare with them. Woods were abundant, although there now exists only a single tree, a kind of palm, in all the archipelago. The vine, which is now cultivated almost exclusively, seems to have been unknown at that period.
The islands of Théra, Thérasia, and Aspronisi are the remains of a large island which existed before the formation of the bay. Their soil is composed of three classes of rocks: metamorphic rocks (marbles and mica-schists), volcanic rocks of subaërial formation, and volcanic rocks of submarine formation. Volcanic products of subaërial formation are the only ones in view in the larger part of the group. They appear as compact lavas, scoriæ, pumice, and in dikes. Fragments of the rocks in the dikes have been subjected to examination like that which was given to the lavas of the recent eruption, and the study has been applied to the solution of the problem of the specification of the triclinic feldspars. By it M. Fouqué has been led to conclude that the four varieties so described are distinct.
M. Fouqué accounts for the origin of Santorin by supposing that there was an island composed of marbles and mica-schists, against which submarine eruptions took place; a considerable upheaval then occurred. The eruptions having become subaërial, masses of matter were thrown out from different vents, and produced a large island, the slopes of which became wooded and its valleys fertile, while its summits continued to be rugged with lavas. A violent convulsion, accompanied and followed by formidable explosions and showers of pumice, hollowed out the bay. Finally, eruptions which have taken place since the beginning of the historical period have produced the Kamenis.
The phenomena of this volcano, which is one of those that were cited by Leopold von Buch, the principal advocate of the theory, as examples of the mode of formation which he suggested, have been carefully studied in view of their bearing upon the theory of craters of elevation. They appear, in the light of M. Fouqué's investigations, to contradict this theory at every point, to sustain the old arguments which have been brought against it, and to offer other features which are irreconcilable with it; so that the conclusion is reached that the theory must be definitively abandoned.—Revue Scientifique.