Cosmos: A Sketch of the Physical Description of the Universe Part 23

[footnote] *[This mountain contains two funnel-shaped craters, apparently resulting from two set of eruptions: the western nearly circular, and having in its center a cone of eruption, from the summit and sides of which are no less than seventy vents, some in activity and others extinct. It is probable that the larger number of the vents were produced at periods anterior to history. Caubney, op. cit., p. 488.] -- Tr.

As the nature of rocks, or the mixture (grouping) of simple minerals into granite, gneiss, and mica slate, or into trachyte, basalt, and dolorite, is independent of existing climates, and is the same under the most varied lat.i.tudes of the earth, so also we find every where in inorganic nature that the same laws of configuration regulate the reciprocal superposition of the strata of the earth's crust, cause them to penetrate one another in the form of veins, and elevate them by the agency of elastic forces. This constant recurrence of the same phenomena is most strikingly manifested in volcanoes.

When the mariner, amid the islands of some distant archipelago, is no longer guided by the light of the same stars with which he had been familiar in his native lat.i.tude, and sees himself surrounded by palms and other forms of an exotic vegetation, he still can trace, reflected in the individual characteristics of the landscape, the forms of Vesuvius, of the come-shaped summits of Auvergne, the craters of elevation in the Canaries and Azores, or the fissures of eruption in Iceland. A glance at the satellite of our planet will impart a wider generalization to this a.n.a.logy of configuration.

by means of the charts that have been drawn in accordance with the observations made with large telescopes, we may recognize in the moon, where water and air are both absent, vast craters of elevation surrounding or supporting conical mountains, thus affording incontrovertible evidence of the effects produced by the reaction of the interior on the surface, favored by the influence of a feebler force of gravitation.

Although vocanoes are justy termed in many languages "fire-emitting mountains," mountains of this kind are not formed by the gradual acc.u.mulation of ejected currents of lava, but their origin seems rather to be a general consequence of the sudden elevation of soft ma.s.ses of trachyte or labradoritic augite. The amount of the elevating force is manifested p 229 by the elevation of the volcano, which varies from the inconsiderable height of a hill (as the volcano of Cosima, one of the j.a.panese Kurile islands) to that of a cone above 19,000 feet in height. It has appeared to me that relations of height have a great influence on the occurrence of eruptions, which are more frequent in low than in elevated volcanoes. I might instance the series presented by the following mountains: Stromboli, 2318 feet; Guacamayo, in the province of Quixos, from which detonations are heard almost daily (I myself often heard them at Chillo, near Quito, a distance of eighty-eight miles); Vesuvius, 3876 feet; Aetna, 10871 feet; the Peak of Teneriffe, 12,175 feet; and Cotopaxi, 19,069 feet. If the focus of these volcanoes be at an equal depth below the surface, a greater force must be required where the fused ma.s.ses have to be raised to an elevation six or eight times greater than that of the lower eminences. While the volcano Stromboli (Strongyle) has been incessantly active since the Homeric ages, and has served as a beacon-light to guide the mariner in the Tyrrhenian Sea, loftier volcanoes have been characterized by loong intervals of quiet. Thus we see that a whole century often intervenes between the eruptions of most of the colossi which crown the summits of the Cordilleras of the Andes.

Where we meet with exceptions to this law, to which I long since drew attention, they must depend upon the circ.u.mstance that the connections between the volcanic foci and the crater of eruption can not be considered as equaly permanent in the case of all volcanoes. The channel of communication may be closed for a time in the case of the lower ones, so that they less frequently come to a state of eruption, although they do not, on that account, approach more nearly to their final extinction.

These relations between the absolute height and the frequency of volcanic eruptions, as far as they are externally perceptible, are intimately connected with the consideration of the local conditions under which lava currents are erupted. Eruptions from the crater are very unusual in many mountains, generally occurring from lateral fissures (as was observed in the case of Aetna, in the sixteenth century, by the celebrated historian Bembo, when a youth*), whenever the sides p 230 of the upheaved mountain were least able, from their configuration and position, to offer any resistance.

[footnote] *Petri Bembi Opuscula ('Aetna Dialogus'), Basil, 1556, p. 63: "Quicquid in Aetnae matris utero coulescit, nunquam exit ex cratere superiore, quod vel eo inscondere gravis materia non queat, vel, quia inferius alia spiramenta sunt, non fit opus. Despumant flammis urgentibus ignei rivi pigro fluxu totas delambentes plagas, et in lapidem induresc.u.n.t."

Cones of eruption are sometimes uplifted on these fissures; the larger ones, which are erroneously termed 'new volcanoes', are ranged together in line marking the direction of a fissure, which is soon reclosed, while the smaller ones are grouped together covering a whole district with their dome-like or hive-shaped forms. To the latter belong the 'hornitos de Jorullo',I the cone of Vesuvius erupted in October, 1822, that of Awatscha, according to Postels, and those of the lava-field mentioned by Erman, near the Baidar Mountains, in the peninsula of Kamtschatka.

[footnote] See my drawing of the volcano of Jorullo, of its 'hornitos', and of the uplifted 'malpays', in my 'Vues de Cordilleres', pl. xliii., p. 239.

[Burckhardt states that during the twenty-four years that have intervened since Baron Humboldt's visit to Jorullo, the 'hornitos' have either wholly disappeared or completely changed their forms. See 'Aufenthalt und Reisen in Mexico in 1825 und 1834'.] -- Tr.

When volcanoes are not isolated in a plain, but surrounded, as in the double chain of the Andes of Quito, by a table-land having an elevation from nine to thirteen thousand feet, this circ.u.mstance may probably explain the cause why no lava streams are formed* during the most dreadful eruption of ignited scoriae accompanied by detonations heard at a distance of more than a hundred miles.

[footnote] * Humboldt, 'Essaii sur la Geogr. des Plantes et Tableau Phys.

des Regions Equinoxiales', 1807, p. 130, and 'Essai Geogn. sur le Gis.e.m.e.nt des Roches', p. 321. Most of the volcanoes in Java demonstrate that the cause of the perfect absence of lava streams in volcanoes of incessant activity is not alone to be sought for in their form, position, and height.

Leop. von Buch, 'Descr. Phys. des Iles Canaries', p. 419; Reinwardt and Hoffmann, in Poggened., 'Annalen.', bd. xii., s. 607.

Such are the volcanoes of Popayan, those of the elevated plateau of Los Pastos and of the Andes of Quito, with the exception, perhaps, in the case of the latter, of the volcano of Antisana. The height of the cone of cinders, and the size and form of the crater, are elements of configuration which yield an especial and individual character to volcanoes, although the cone of cinders and the crater are both wholly independent of the dimensions of the mountain. Vesuvius is more than three times lower than the Peak of Teneriffe; its cone of cinders rises to one third of the height of the whole mountain, while the cone of cinders of the Peak is only 1/22d of its alt.i.tude.

[footnote] * [It may be remarked in general, although the rule is liable to exceptions, that the dimensions of a crater are in an inverse ratio to the elevation of the mountain. Daubeney, op. Cit., p. 444.] -- Tr.

In a much higher volcano than that of Teneriffe, the Rueu Pichincha, other relations occur p 231 which approach more nearly to that of Vesuvius. Among all the volcanoes that I have seen in the two hemispheres, the conical form of Cotopaxi is the most beautifully regular. A sudden fusion of the snow at its cone of cinders announces the proximity of the eruption. Before the smoke is visible in the rarefied strata of air surrounding the summit and the opening of the crater, the walls of the cone of cinders are sometimes in a state of glowing heat, when the whole mountain presents an appearance of the most fearful and portentous blackness. The crater, which, with very few exceptions, occupies the summit of the volcano, forms a deep, caldron-like valley, which is often accessible, and whose bottom is subject to constant alterations. The great or lesser depth of the crater is in many volcanoes likewise a sign of the near or distant occurrence of an eruption. Long, narrow fissures, from which vapors issue forth, or small rounding hollows filled with molten ma.s.ses, alternately open and close in the caldron-like valley; the bottom rises and sinks, eminences of scoriae and cones of eruption are formed, rising sometimes far over the walls of the crater, and continuing for years together to impart to the volcano a peculiar character, and then suddenly fall together and disappear during a new eruption. The openings of these cones of eruption, which rise from the bottom of the crater, must not, as is too often done, be confounded with the crater which incloses them. If this be inaccessible from extreme depth and from the perpendicular descent, as in the case of the volcano of Rucu Pichincha, which is 15,920 feet in height, the traveler may look from the edge on the summit of the mountains which rise in the sulphurous atmosphere of the valley at his feet; and I have never beheld a grander or more remarkable picture than that presented by this volcano. In the interval between two eruptions, a crater may either present no luminous appearance, showing merely open fissures and ascending vapors, or the scarcely heated soil may be covered by eminences of scoriae, that admit of being approached without danger, and thus present to the geologist the spectacle of the eruption of burning and fused ma.s.ses, which fall back on the ledge of the cone of scoriae, and whose appearance is regularly announced by small wholly local earthquakes. Lava sometimes streams forth from the open fissures and small hollows, without breaking through or escaping beyond the sides of the crater. If, however, it does break through, the newly-opened terrestrial stream generally flows in such a quiet and well-defined course, that the deep valley, which we term the crater, remains accessible p 232 even during periods of eruption. It is impossible, without an exact representation of the configuration -- the normal type, as it were, of fire-emitting mountains, to form a just idea of those phenomena which, owing to fantastic descriptions and an undefined phraseology, have long been comprised under the head of 'craters, cones of eruption', and 'volcanoes'.

The marginal ledges of craters vary much less than one would be led to suppose. A comparison of Saussure's measurements with my own yields the remarkable result, for instance, that in the course of forty-nine years (from 1773 to 1822), the elevation of the northwestern margin of Mount Vesuvius ('Rocca del Palo') may be considered to have remained unchanged.*

[footnote] *See the ground-work of my measurements compared with those of Saussure and Lord Minto, in the 'Abhandlungen der Akademie der Wiss. zu Berlin' for the years 1822 and 1823.

Volcanoes which, like the chain of the Andes, lift their summits high above the boundaries of the region of perpetual snow, present peculiar phenomena.

The ma.s.ses of snow, by their sudden fusion during eruptions, occasion not only the most fearful inundations and torrents of water, in which smoking scoriae are borne along on thick ma.s.ses of ice, but they likewise exercise a constant action, while the volcano is in a state of perfect repose, by infiltration into the fissures of the trachytic rock. Cavities which are either on the declivity or at the foot of the mountain are gradually converted into subterranean resevoirs of water, which communicate by numerous narrow openings with mountain streams, as we see exemplified in the highlands of Quito. the fishes of these rivulets multiply, especially in the obscurity of the hollows; and when the shocks of earthquakes, which precede all eruptions in the andes, have violently shaken the whole ma.s.s of the volcano, these subterranean caverns are suddenly opened, and water, fishes, and tufaceous mud are all ejected together. It is through this singular phenomenon* that the inhabitants of the highlands of Quito became acquainted with the existence of the little cyclopic fishes, termed by them the prenadilla.

[footnote] *Pimelodes cyclopum. See Humboldt, 'Recueil d'Observations de Zoologie et d'Anatomie Comparee', t. i., p. 21-25.

On the night between the 19th and 20th of June, 1698, when the summit of Carguairazo, a mountain 19,720 feet in height, fell in, leaving only two huge ma.s.ses of rock remaining of the ledge of the crater, a s.p.a.ce of nearly thirty-two square miles was overflowed and devastated by streams of liquid tufa and argillaceous mud ('lodazales'), containing large quant.i.ties of dead fish.

p 233 In like manner, the putrid fever, which raged seven years previously in the mountain town of Ibarra, north of Quito, was ascribed to the ejection of fish from the volcano of Imbaburu.*

[footnote] *[It would appear, as there is no doubt that these fishes proceed from the mountain itself, that there must be large lakes in the interior, which in ordinary season are out of the immediate influence of the volcanic action. See Daubeney, op. cit., p. 488, 497.] -- Tr.

Water and mud, which flow not from the crater itself, but from the hollows in the trachytic ma.s.s of the mountain, can not, strictly speaking, be cla.s.sed among volcanic phenomena. They are only indirectly connected with the volcanic activity of the mountain, resembling, in that respect, the singular meteorological process which I have designated in my earlier writings by the term of 'volcanic storm'. The hot stream which rises from the crater during the eruption and spreads itself in the atmosphere, condenses into a cloud, and surrounds the column of fire and cinders which rises to an alt.i.tude of many thousand feet. The sudden condensation of the vapors, and, as Gay-Lussac has shown, the formation of a cloud of enormous extent, increase the electric tension. Forked lightning flashes from the column of cinders, and it is then easy to distinguish (as at the close of the eruption of Mount Vesuvius, in the latter end of October, 1822) the rolling thunder of the volcanic storm from the detonations in the interior of the mountain. the flashes of lightning that darted from the volcanic cloud of steam, as we learn from Olafsen's report, killed eleven horses and two men, on the eruption of the volcano of Katlagia, in Iceland, on the 17th of October, 1755.

Having thus delineated the structure and dynamic activity of volcanoes, it now remains for us to throw a glance at the differences existing in their material products. The subterranean forces sever old combinations of matter in order to produce new ones, and they also continue to act upon matter as long as it is in a state of liquefaction from heat, and capable of being displaced. The greater or less pressure under which merely softened or wholly liquid fluids are solidified, appears to const.i.tute the main difference in the formation of Plutonic and volcanic rocks. The mineral ma.s.s which flows in narrow, elongated streams from a volcanic opening (an earth-spring), is called lava. where many such currents meet and are arrested in their course, they expand in width, filling large basins, in which they become solidified in superimposed strata. These few sentences describe the general character of the products of volcanic activity.

p 234 Rocks which are merely broken through by the volcanic action are often inclosed in the igneous products. Thus i have found angular fragments of feldspathic syenite imbedded in the black augitic lava of the volcano of Jorullo, in Mexico; but the ma.s.ses of dolomite and granular limestone, which contain magnificent cl.u.s.ters of crystalling fossils (vesuvian and garnets, covered with mejonite, nepheline, and sodalite), are not the ejected products of Vesuvius, these belonging rather to very generally distributed formations, viz., strata of tufa, which are more ancient than the elevation of the Somma and of Vesuvius, and are probably the products of a deep-seated and concealed submarine volcanic action.*

[footnote] *Leop. von Buch, in Poggend., 'Annalen', bd. x.x.xvii., s. 179.

We find five metals among the products of existing volcanoes, iron, copper, lead, a.r.s.enic, and selenium, discovered by Stromeyer in the crater of Volcano.*

[footnote] *[The little island of Volcano is separated from Lipari by a narrow channel. It appears to have exhibited strong signs of volcanic activity long before the Christian era, and still emits gaseous exhalations.

Stromeyer detected the presence of selenium in a mixture of sal ammoniac and sulphur. Another product, supposed to be peculiar to this volcano, is boracic acid, which lines the sides of the cavities in beautiful white silky crystals. Daubeney, op. cit., p. 257.] -- Tr.

The vapors that rise from the 'fumarolles' cause the sublimation of the chlorids of iron, copper, lead, and ammonium; iron glanceI and chlorid of sodium (the latter often in large quant.i.ties) fill the cavities of recent lava streams and the fissures of the margin of the crater.

[footnote] *Regarding the chemical origin of iron glance in volcanic ma.s.ses, see Mitscherlich, in Poggend., 'Annalen', bd. xv., s. 630; and on the liberation of hydrochloric acid in the crater, see Gay-Lussac, in the 'Annals de Chimique et de Physique', t. xxii., p. 423.

The mineral composition of lava differs according to the nature of the crystalline rock of which the volcano is formed, the height of the point where the eruption occurs, whether at the foot of the mountain or in the neighborhood of the crater, and the condition of temperature of the interior. Vitreous volcanic formations, obsidian, pearl-stone, and pumice, are entirely wanting in some volcanoes, while in the case of others they only proceed from the crater, or, at any rate, from very considerable heights. These important and involved relations can only be explained by very accurate crystallographic and chemical investigations. My fellow-traveler in Siberia, Gustav Rose, and subsequently Hermann Abich, have already been able, by their fortunate and ingenious researches, to throw much light on the structural relations of the various kinds of volcanic rocks.

p 235 The greater part of the ascending vapor is mere steam. When condensed, this forms springs, as in Pantellaria,Iwhere they are used by the goatherds of the island.

[footnote] *[Steam issues from many parts of this insular mountain, and several hot springs gush forth from it, which form together a lake 6000 feet in circ.u.mference. Daubeney, op. cit.] -- Tr.

On the morning of the 26th of October, 1822, a current was seen to flow from a lateral fissure of the crater of Vesuvius, and was loong supposed to have been boiling water; it was, however, shown, by Monticelli's accurate investigations, to consist of dry ashes, which fell like sand, and of lava pulverized by friction. The ashes, which sometimes darken the air for hours and days together, and produce great injury to the vineyards and olive groves by adhering to the leaves, indicate by their columnar ascent, impelled by vapors, the termination of every great eqrthquake. This is the magnificent phenomenon which Pliny the younger, in his celebrated letter to Cornelius Tacitus, compares, in the case of Vesuvius, to the form of a lofty and thickly-branched and foliaceous pine. That which is described as flames in the eruption of scoriae, and the radiance of the glowing red clouds that hover over the crater, can not be ascribed to the effect of hydrogen gas in a state of combustion. They are rather reflections of light which issue from molten ma.s.ses, projected high in the air, and also reflections from the burning depths, whence the glowing vapors ascend. We will not, however, attempt to decide the nature of the flames, which are occasionally seen now, as in the time of Strabo, to rise from the deep sea during the activity of littoral volcanoes, or shortly before the elevation of a volcanic island.

When the questions are asked, what is it that burns in the volcano? what excites the heat, fuses together earths and metals, and imparts to lava currents of thick layers a degree of heat that lasts for many years? it is necessarily implied that volcanoes must be connected with the existence of substances capable of maintaining combustion, like the beds of coal in subterranean fires.

[footnote] *See the beautiful experiments on the cooling of ma.s.ses of rock, in Bischof's 'Warmelehre', s. 384, 443, 500-512.

According to the different phases of chemical science, bitumen, pyrites, the moist admixture of finely-pulverized sulphur and iron, pyrophoric substances, and the metals of the alkalies and earths, have in turn been designated as the cause of intensely active volcanic phenomena. The great chemist, Sir Humphrey Davy, to whom we are indebted for the knowledge of the most combustible metallic p 236 substances, has himself renounced his bold chemical hypothesis in his last work ('Consolation in Travel, and last Days of a Philosopher') -- a work which can not fail to excite in the reader a feeling of the deepest melancholy. the great mean density of the earth (5.44), when compared with the specific weight of pota.s.sium (0.865), of sodium (-.972), or of the metals of the earths (1.2), and the absence of hydrogen gas in the gaseous emanations from the fissures of craters, and from still warm streams of lava, besides many chemical considerations, stand in opposition with the earlier conjectures of Davy and Ampere.*