Cosmos: A Sketch of the Physical Description of the Universe, Vol. 1

by Alexander von Humboldt

A volcano, properly so called, exists only where a permanent connection is established between the interior of the earth and the atmosphere, and the reaction of the interior on the surface then continues during long periods of time. It may be interrupted for centuries, as in the case of Vesuvius Fisove,* and then manifest itself with renewed activity.

[footnote] *Ocre Fisove (Mons Vesuvius) in the Umbrian language. (Lassen 'Deutung der Eugubinischen Tafeln in Rhein. Museum', 1832, s. 387.) The word 'ochre' is very probaby genuine Umbrian, and means, according to Festus, 'mountain'. Aetna would be a burning and shining mountain, if Voss is correct in stating that [Greek work] is an Hellenic sound, and is connected with [Greed word] and [Greek word]; but the intelligent writer Parthey doubts this Hellenic origin on etymological grounds, and also because etna was by no means regarded as a luminous beacon for ships or wanderers, in the same manner as the ever-travailing Stromboli (Strongyle), to which Homer seems to refer in the Odyssey (xii., 68, 202, and 219), and its geographical position was not so well determined. I suspect that tna would be found to be a Sicilian word, if we had any fragmentary materials to refer to. According to Diodorus (v., 6), the Sicani, or aborigines preceding the Sicilians, were compelled to fly to the western part of the island, in the consequence of successive eruptions extending over many years. The most ancient eruption of Mount Aetna on record is that mentioned by Pindar and Schylus, as occurring under Hiero, in the second year of the 75th Olympiad. It is probable that Hesiod was aware of the devastating eruptions of Aetna before the period of Greek immigration. There is, however, some doubt regarding the work [Greek word] in the text of Hesiod, a subject into whci I have entered at some length in another place. (Humboldt, 'Examen Crit. de le Geogr.', t. i., p. 168.)

In the time of Nero, men were disposed to rank Aetna among the volcanic mountains which were graduallybecoming extinct,* and subsequently Aelian** even maintained that mariners could no longer see the sinking summit of the mountain from so great a distance at sea.

[footnote] *Seaeca. 'Epist.', 79.

[footnote] ** Aelian, 'Var. Hist.', viii., 11.

Where these evidences — these old scaffoldings of eruption, I might almost say — still exist, the volcano rises from a crater of elevation, while a high rocky wall surrounds, like an amphitheater, the isolated conical mount, and forms around it a kind of easing of highly elevated p 228 strata. Occasionally not a trace of this inclosure is visible, and the volcano, which is not always conical rises immediately from the neighboring plateau in an elongated form, as in the case of Pichincha,* at the foot of which lies the city of Quito.

[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 latitudes 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 latitude, 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 analogy 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 accumulation of ejected currents of lava, but their origin seems rather to be a general consequence of the sudden elevation of soft masses 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 Japanese 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 masses 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 circumstance 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.