Principles of Geology Part 36

Whatever theory we adopt, we must always explain the abrupt termination of the dikes and layers of trachyte and basalt in the steep walls of the escarpments surrounding the great crater by supposing the removal of part of the materials once prolonged farther inward towards the centre.

If, according to the elevation-crater hypothesis, a series of sheets of lava and ashes originally spread over a level and even surface have been violently broken and uplifted, why do not the opposite walls of the chasm correspond in such a manner as to imply by their present outline that they were formerly united? It is evident that the precipices on opposite sides of the crateriform hollow would not fit if brought together, there being no projecting ma.s.ses in one wall to enter into indentations in the other, as would happen with the sides of many mineral veins, trap-dikes, and faults, could we extract the intrusive matter now separating them, and reunite the rocks which have been fractured and disjoined.

The highest crater of the peak has merely disengaged sulphureous vapors ever since it has been known to Europeans; but an eruption happened in June, 1798, not far from the summit, and others are recorded, which poured out streams of lava from great heights, besides many which have broken out nearer the level of the sea. All these, however, seem to be dependent on one great centre of eruption, or on that open channel communicating between the interior of the earth and the atmosphere, which terminates in the highest crater of the peak.

We may consider Teneriffe, then, as having been from a remote period the princ.i.p.al and habitual vent of the volcanic archipelago of the Canaries.

The discharges which have taken place in the contiguous isles of Palma, Lancerote, and the rest, may be of a subsidiary kind, and have probably been most frequent and violent when the greater crater has been partially sealed up, just as the violent eruptions of Ischia or that of Monte Nuovo coincided with the dormant state of Vesuvius.

SANTORIN.

The Gulf of Santorin, in the Grecian Archipelago, has been for two thousand years a scene of active volcanic operations. The largest of the three outer islands of the group (to which the general name of Santorin is given) is called Thera (or sometimes Santorin), and forms more than two-thirds of the circuit of the gulf (see Map, fig. 63, p. 442). The length of the exterior coast-line of this and the other two islands named Therasia and Asp.r.o.nisi, taken together, amounts to about thirty miles, and that of the inner coast-line of the same islands to about eighteen miles. In the middle of the gulf are three other islands, called the Little, the New, and the Old "Kaimenis," or "Burnt Islands."

The accompanying map has been reduced from a recent survey executed in 1848 by Captain Graves, R. N., and shortly to be published by the Admiralty.

[Ill.u.s.tration: Fig. 63.

Map of Santorin in the Grecian Archipelago, from a Survey in 1848, by Captain Graves, R. N.

_The soundings are given in fathoms._

A, Shoal formed by submarine volcanic eruption in 1650.

B, Northern entrance.

C, Mansell's Rock.

D, Mount St. Elias, 1887 feet high.

[Ill.u.s.tration: Fig. 64.

Section of Santorin, in a N. E. and S. W. direction, from Thera through the Kaimenia to Asp.r.o.nisi.]

[Ill.u.s.tration: Fig. 65.

Part of the section, fig. 64, enlarged.]

Pliny informs us that the year 186, B. C., gave birth to the Old Kaimeni, also called Hiera, or the "Sacred Isle," and in the year 19 of our era "Thia" (the Divine) made its appearance above water, and was soon joined by subsequent eruptions to the older island, from which it was only 250 paces distant. The Old Kaimeni also increased successively in size in 726 and in 1427. A century and a half later, in 1573, another eruption produced the cone and crater called Micra-Kaimeni, or "the Small Burnt Island." The next great event which we find recorded occurred in 1650, when a submarine outbreak violently agitated the sea, at a point three and a half miles to the N. E. of Thera, and which gave rise to a shoal (see A in the map) carefully examined during the late survey in 1848 by Captain Graves, and found to have ten fathoms water over it, the sea deepening around it in all directions. This eruption lasted three months, covering the sea with floating pumice. At the same time an earthquake destroyed many houses in Thera, while the sea broke upon the coast and overthrew two churches, exposing to view two villages, one on each side of the mountain of St. Stephen, both of which must have been overwhelmed by showers of volcanic matter during some previous eruptions of unknown date.[607] The accompanying evolution of sulphur and hydrogen issuing from the sea killed more than fifty persons, and above 1000 domestic animals. A wave, also, 50 feet high, broke upon the rocks of the Isle of Nia, about four leagues distant, and advanced 450 yards into the interior of the Island of Sikino. Lastly, in 1707 and 1709, Nea-Kaimeni, or the New Burnt Island, was formed between the two others, Palaia and Micra, the Old and Little isles. This isle was composed originally of two distinct parts; the first which rose was called the White Island, composed of a ma.s.s of pumice, extremely porous.

Gorce, the Jesuit, who was then in Santorin, says that the rock "cut like bread," and that, when the inhabitants landed on it, they found a mult.i.tude of full-grown fresh oysters adhering to it, which they ate.[608] This ma.s.s was afterwards covered, in great part, by the matter ejected from the crater of a twin-island formed simultaneously, and called Black Island, consisting of brown trachyte. The trachytic lava which rose on this spot appears to have been a long time in an intumescent state, for the New Kaimeni was sometimes lowered on one side while it gained height on the other, and rocks rose up in the sea at different distances from the sh.o.r.e and then disappeared again. The eruption was renewed at intervals during the years 1711 and 1712, and at length a cone was piled up to the height of 330 feet above the level of the sea, its exterior slope forming an angle of 33 with the horizon, and the crater on its summit being 80 yards in diameter. In addition to the two points of subaerial eruption on the New and Little Kaimenis, two other cones, indicating the sites of submarine outbursts of unknown date, were discovered under water near the Kaimenis during the late survey.

In regard to the "White Island," which was described and visited by Gorce in 1707, we are indebted to Mr. Edward Forbes for having, in 1842, carefully investigated the layer of pumiceous ash of which it is const.i.tuted. He obtained from it many sh.e.l.ls of marine genera, Pectunculus, Arca, Cardita, Trochus, and others, both univalve and bivalve, all of recent Mediterranean species. They were in a fine state of preservation, the bivalves with the epidermis remaining, and valves closed, showing that they had been suddenly destroyed. Mr. Forbes, from his study of the habits of the mollusca living at different depths in the Mediterranean, was able to decide that such an a.s.semblage of species could not have lived at a less depth than 220 feet, so that a bodily upheaval of the ma.s.s to that amount must have taken place in order to bring up this bed of ashes and sh.e.l.ls to the level of the sea, and they now rise five or six feet above that level.[609]

We may compare this partial elevation of solid matter to the rise of a hardened crust of scoriae, such as is usually formed on the surface of lava-currents, even while they are in motion, and which, although stony and capable of supporting heavy weights, may be upraised without bursting by the intumescence of the melted matter below. That the upheaval was merely local is proved by the fact that the neighboring Kaimenis did not partic.i.p.ate in the movement, still less the three more distant or outer islands before mentioned. The history, therefore, of the Kaimenis shows that they have been the result of intermittent action, and it lends no support to the hypothesis of the sudden distension of horizontal beds blown up like a bladder by a single paroxysmal effort of expansive gases.

It will be seen by the accompanying map and sections, that the Kaimenis are arranged in a linear direction, running N. E. and S. W., in a manner different from that represented in the older charts. In their longest diameter they form at their base a ridge nearly bisecting the gulf or crater (see sections, figs. 64, 65).

On considering these facts we are naturally led to compare the smaller and newer islands in the centre of the gulf to the modern cone of Vesuvius, surrounded by the older semicircular escarpment of Somma, or to liken them to the Peak of Teneriffe before described, as surrounded by its "fosse and bastion." This idea will appear to be still more fully confirmed when we study the soundings taken during the late hydrographical survey. Thera, which const.i.tutes alone more than two-thirds of the outer circuit, presents everywhere towards the gulf, high and steep precipices composed of rocks of volcanic origin. In all places near the base of its cliffs, a depth of from 800 to 1000 feet of water was found, and Lieut. Leycester informs us[610] that if the gulf, which is six miles in diameter, could be drained, a bowl-shaped cavity would appear with walls 2449 feet high in some places, and even on the southwest side, where it is lowest, nowhere less than 1200 feet high; while the Kaimenis would be seen to form in the centre a huge mountain five and a half miles in circ.u.mference at its base, with three princ.i.p.al summits (the Old, the New, and the Little Burnt Islands) rising severally to the heights of 1251, 1629, and 1158 feet above the bottom of the abyss. The rim of the great caldron thus exposed would be observed to be in all parts perfect and unbroken, except at one point where there is a deep and long chasm or channel, known by mariners as "the northern entrance" (B, fig. 63) between Thera and Therasia, and called by Lieut. Leycester "the door into the crater." It is no less than 1170 feet deep, and const.i.tutes, as will appear by the soundings (see map), a remarkable feature in the bed of the sea. There is no corresponding channel pa.s.sing out from the gulf into the Mediterranean at any other point in the circuit between the outer islands, the greatest depth there ranging from 7 to 66 feet.

We may conceive, therefore, if at some former time the whole ma.s.s of Santorin stood at a higher level by 1200 feet, that this single ravine or narrow valley now forming "the northern entrance," was the pa.s.sage by which the sea entered a circular bay and swept out in the form of mud and pebbles, the materials derived by denudation from wasting cliffs. In this manner the original crater may have been slowly widened and deepened, after which the whole archipelago may have been partially submerged to its present depth.

That such oscillations of level may in the course of ages have taken place, will be the more readily admitted when we state that part of Thera has actually sunk down in modern times, as, for example, during the great earthquake before alluded to, which happened in 1650. The subsidence alluded to is proved not only by tradition, but by the fact that a road which formerly led between two places on the east coast of Thera is now twelve fathoms under water.

MM. Boblaye and Virlet mention,[611] that the waves are constantly undermining and encroaching on the cliffs of Therasia and Asp.r.o.nisi, and shoals or submarine ledges were found, during the late survey, to occur round a great part of these islands, attesting the recent progress of denudation. M. Virlet also remarks, in regard to the separation of the three islands forming the walls of the crater, that the channels between them are all to the W. and N. W., the quarter most exposed to the waves and currents.

Mr. Darwin, in his work on volcanic islands, has shown that in the Mauritius and in Santiago, there is an external circle of basaltic rocks of vast diameter, in the interior of which more modern eruptions have taken place, the older rocks dipping away from the central s.p.a.ce in every direction, as in the outer islands of Santorin. He refers the numerous breaches, some of them very wide in the external ramparts of those islands, to the denuding action of the sea. Every geologist, therefore, will be prepared to call in the aid of the same powerful cause, to account for the removal of a large part of the rocks which must once have occupied the interior s.p.a.ce, in the same manner as they attribute the abstraction of matter from elliptical "valleys of elevation," such as those of Woolhope and the Wealden in England, to the waves and currents of the sea.

Thera, Therasia, and Asp.r.o.nisi are all composed of volcanic matter, except the southern part of Thera, where Mount St. Elias rises to three times the height of the loftiest of the igneous rocks, reaching an elevation of 1887 feet above the sea.[612] This mountain is formed of granular limestone and argillaceous schist, and must have been originally a submarine eminence in the bed of the Mediterranean, before the volcanic cone, one side of the base of which now abuts against it, was formed. The inclination, strike, and fractures of the calcareous and argillaceous strata of St. Elias have no relation to the great cone, but, according to M. Bory St. Vincent, have the same direction as those of the other isles of the Grecian Archipelago, namely, from N. N. W. to S. S. E. Each of the three islands, Thera, Therasia, and Asp.r.o.nisi, is capped by an enormous ma.s.s of white tufaceous conglomerate, from forty to fifty feet thick, beneath which are beds of trachytic lava and tuff, having a gentle inclination of only 3 or 4. Each bed is usually very narrow and discontinuous, the successive layers being moulded or dove-tailed, as M. Virlet expresses it, into the inequalities of the previously existing surface, on which showers of cinders or streams of melted matter have been poured. Nothing, therefore, seems more evident than that we have in Santorin the basal remains of a great ruined cone, or flattened dome; and the absence of dikes in the cliffs surrounding the gulf would indicate that the eruptions took place originally, as they have done in the last two thousand years, not near the margin but in the centre of the s.p.a.ce now occupied by the gulf. The central portions of the dome have since been removed by engulfment, or denudation, or by both these causes.

An important fact is adduced by M. Virlet, to show that the gentle dip of the lava-streams in the three outer islands towards all points of the compa.s.s, away from the centre of the gulf, has not been due to the upheaval of horizontal beds, as conjectured by Von Buch, who had not visited Santorin.[613] The French geologist found that the vesicles or pores of the trachytic ma.s.ses were lengthened out in the several directions in which they would have flowed if they had descended from the axis of a cone once occupying the centre of the crater. For it is well known that the bubbles of confined gas in a fluid in motion a.s.sume an oval form, and the direction of their longer axis coincides always with that of the stream.

On a review, therefore, of all the facts now brought to light respecting Santorin, I attribute the moderate slope of the beds in Thera and the other external islands to their having originally descended the inclined flanks of a large volcanic cone, the princ.i.p.al orifice or vents of eruption having been always situated where they are now, in or near the centre of the s.p.a.ce occupied by the gulf or crater--in other words, where the outburst of the Kaimenis has been witnessed in historical times. The single long and deep opening into the crater is a feature common to all those remnants of ancient volcanoes, the central portions of which have been removed, and is probably connected with aqueous denudation. This denuding process has been the work of ages when the sea was admitted into an original crater, and has taken place during the gradual emergence of the island from the sea, or during various oscillations in its level.

The volcanic island of St. Paul in the midst of the Indian Ocean, lat.

38 44' S., long. 77 37' E., surveyed by Capt. Blackwood in 1842, seems to exemplify the first stage in the formation of such an archipelago as that of Santorin. We have there a crater one mile in diameter, surrounded by steep and lofty cliffs on every side save one, where the sea enters by a single pa.s.sage nearly dry at low water. In the interior of the small circular bay or crater there is a depth of 30 fathoms, or 180 feet. The surface of the island slopes away on all sides from the crest of the rocks encircling the crater.[614]

[Ill.u.s.tration: Fig. 66.

Cone and crater of Barren Island, in the Bay of Bengal. Height of the central cone (according to Capt Miller, in 1834), 500 feet.]

_Barren Island._--There is great a.n.a.logy between the structure of Barren Island in the Bay of Bengal, lat. 12 15', and that of Santorin last described. When seen from the ocean, this island presents, on almost all sides, a surface of bare rocks, rising, with a moderate acclivity, towards the interior; but at one point there is a cleft by which we can penetrate into the centre, and there discover that it is occupied by a great circular basin, filled by the waters of the sea, and bordered all around by steep rocks, in the midst of which rises a volcanic cone, very frequently in eruption. The summit of this cone is about 500 feet in height, corresponding to that of the circular border which incloses the basin; so that it can be seen from the sea only through the ravine. It is most probable that the exterior inclosure of Barren Island (_c_, _d_, fig. 67) is nothing more than the remains of a truncated cone _c_, _a_, _b_, _d_, a great portion of which has been removed by engulfment, explosion, or denudation, which may have preceded the formation of the new interior cone, _f_, _e_, _g_.[615]

[Ill.u.s.tration: Fig. 67.

Supposed section of Barren Island, in the Bay of Bengal.]

MUD VOLCANOES.

_Iceland._--Mr. R. Bunsen, in his account of the pseudo-volcanic phenomena of Iceland, describes many valleys where sulphurous and aqueous vapors burst forth with a hissing sound, from the hot soil formed of volcanic tuff. In such spots a pool of boiling water is seen, in which a bluish-black argillaceous paste rises in huge bubbles. These bubbles on bursting throw the boiling mud to a height of fifteen feet and upwards, acc.u.mulating it in ledges round the crater or basin of the spring.

_Baku on the Caspian._--The formation of a new mud volcano was witnessed on the 27th of November, 1827, at Tokmali, on the peninsula of Abscheron, east of Baku. Flames blazed up to an extraordinary height for a s.p.a.ce of three hours, and continued for twenty hours to rise about three feet above a crater, from which mud was ejected. At another point in the same district where flames issued, fragments of rock of large size were hurled up into the air, and scattered around.[616]

[Ill.u.s.tration: Fig. 68.

Mud cones and craters of Hinglaj near Beila, district of Lus, 120 miles northwest of mouth of Indus. From original drawing by Capt. Robertson.

(See Map, p. 460.)]

_Sicily._--At a place called Macaluba, near Girgenti in Sicily, are several conical mounds from ten to thirty feet in height, with small craters at their summits, from which cold water, mixed with mud and bitumen, is cast out. Bubbles of carbonic acid and carburetted hydrogen gas are also disengaged from these springs, and at certain periods with such violence, as to throw the mud to the height of 200 feet. These "air volcanoes," as they are sometimes termed, are known to have been in the same state of activity for the last fifteen centuries; and Dr. Daubeny imagines that the gases which escape may be generated by the slow combustion of beds of sulphur, which is actually in progress in the blue clay, out of which the springs rise.[617] But as the gases are similar to those disengaged in volcanic eruptions, and as they have continued to stream out for so long a period, they may perhaps be derived from a more deep-seated source.

_Beila in India._--In the district of Luss or Lus, south of Beila, about 120 miles N. W. of Cutch and the mouths of the Indus (see Map, fig. 71, p. 460), numerous mud volcanoes are scattered over an area of probably not less than 1000 square miles. Some of these have been well described by Captain Hart, and subsequently by Captain Robertson, who has paid a visit to that region, and made sketches of them, which he has kindly placed at my disposal. From one of these the annexed view has been selected. These conical hills occur to the westward of the Hara mountains and the river Hubb. (See Map, p. 460.) One of the cones is 400 feet high, composed of light-colored earth, and having at its summit a crater thirty yards in diameter. The liquid mud which fills the crater is continually disturbed by air-bubbles, and here and there is cast up in small jets.[618]

_Mineral composition of volcanic products._--The mineral called felspar forms in general more than half of the ma.s.s of modern lavas. When it is in great excess, lavas are called trachytic: they consist generally of a base of compact felspar, in which crystals of gla.s.sy felspar are disseminated.[619] When augite (or pyroxene) predominates, lavas are termed basaltic. They contain about 50 per cent. of silica, or much less than the trachytes, in which there is usually about 75 per cent. of that mineral. They also contain about 11 per cent. of protoxide of iron, and as much of lime, both of which are wanting, or only in insignificant quant.i.ties in the trachytic rocks.[620] But lavas occur of an intermediate composition between the trachytic and basaltic, which from their color have been called graystones. The abundance of quartz, forming distinct crystals or concretions, characterizes the granitic and other ancient rocks, now generally considered by geologists as of igneous origin; whereas that mineral is rarely exhibited in a separate form in recent lavas, although silica enters so largely into their composition. Hornblende, so common in hypogene rocks, or those commonly called "primary," is rare in modern lava; nor does it enter largely into rocks of any age in which augite abounds. It should, however, be stated, that the experiments of Mr. Gustav Rose have made it very questionable, whether the minerals called hornblende and augite can be separated as distinct species, as their different varieties seem to pa.s.s into each other, whether we consider the characters derived from their angles of crystallization, their chemical composition, or their specific gravity.

The difference in form of the two substances may be explained by the different circ.u.mstances under which they have been produced, the form of hornblende being the result of slower cooling. Crystals of augite have been met with in the scoriae of furnaces, but never those of hornblende; and crystals of augite have been obtained by melting hornblende in a platina crucible; but hornblende itself has not been formed artificially.[621] Mica occurs plentifully in some recent trachytes, but is rarely present where augite is in excess.

_Frequency of eruptions, and nature of subterranean igneous rocks._--When we speak of the igneous rocks of our own times, we mean that small portion which, in violent eruptions, is forced up by elastic fluids to the surface of the earth,--the sand, scoriae, and lava, which cool in the open air. But we cannot obtain access to that which is congealed far beneath the surface under great pressure, equal to that of many hundred, or many thousand atmospheres.

During the last century, about fifty eruptions are recorded of the five European volcanic districts, of Vesuvius, Etna, Volcano, Santorin, and Iceland; but many beneath the sea in the Grecian archipelago and near Iceland may doubtless have pa.s.sed unnoticed. If some of them produced no lava, others, on the contrary, like that of Skaptar Jokul, in 1783, poured out melted matter for five or six years consecutively; which cases, being reckoned as single eruptions, will compensate for those of inferior strength. Now, if we consider the active volcanoes of Europe to const.i.tute about a fortieth part of those already known on the globe, and calculate that, one with another, they are about equal in activity to the burning mountains in other districts, we may then compute that there happen on the earth about 2000 eruptions in the course of a century, or about twenty every year.

However inconsiderable, therefore, may be the superficial rocks which the operations of fire produce on the surface, we must suppose the subterranean changes now constantly in progress to be on the grandest scale. The loftiest volcanic cones must be as insignificant, when contrasted to the products of fire in the nether regions, as are the deposits formed in shallow estuaries when compared to submarine formations acc.u.mulating in the abysses of the ocean. In regard to the characters of these volcanic rocks, formed in our own times in the bowels of the earth, whether in rents and caverns, or by the cooling of lakes of melted lava, we may safely infer that the rocks are heavier and less porous than ordinary lavas, and more crystalline, although composed of the same mineral ingredients. As the hardest crystals produced artificially in the laboratory require the longest time for their formation, so we must suppose that where the cooling down of melted matter takes place by insensible degrees, in the course of ages, a variety of minerals will be produced far harder than any formed by natural processes within the short period of human observation.