661 



VOLCANO. 



VOLCANO. 



0*2 



by the volcanic ashes, and we may therefore conclude, that the enor- 

 mous evolution of electricity accompanying it arises from their friction, 

 as in the hydro-electric machine. It is probably the greatest example 

 in nature of the production of frictional electricity 



Mass of Volcanic Product*. Now the first thing which arrests the 

 attention in regard to the circumstances which accompany the products 

 of volcanic eruptions, is the enormous mass of materials ejected at 

 particular points. In 48 hours, in 1538, the Monte Nuovo, 440 feet 

 high and 8000 feet in circumference, was thrown up in a place whkh 

 may be regarded aa a new vent of the Neapolitan volcanic region. In 

 1759 a new vent was opened west of Mexico, a new volcanic mountain 

 (Jorullo) was thrown up to the height of 1695 feet, and an area of 

 three or four miles was covered with its lavas. (See col. 670.) Between 

 July and August, in 1831, Graham's Island had been raised from the 

 sea-bed, 100 fathoms deep, to a height of 107 feet above the sea, with 

 a circumference of 3240 feet ; in September its height was 100 to 230 

 feet, and its circumference 2300. In the winter of 1831-2 the whole 

 vast heap of ashes had been dispersed by the waves, and nothing 

 remained of this short-lived volcano but a dangerous shoal. Subse- 

 quently this has been lowered, and a comparatively slight elevation 

 above the average level of the neighbouring sea-bed is now under deep 

 water. The lava currents from many volcanoes are of the same gigan- 

 tic proportions. In 1737 Vesuvius poured forth 33,587,058 cubic feet ; 

 in 1 794, 46,098,766 cubic feet ; and ^Etna, in 1669, gave forth 93,838,950 

 cubic feet, which would make a considerable hill ; for it would cover a 

 space of ground one-quarter of a mile across with a conical mound 180 

 feet high. The accumulated effects of two years' eruptions of Skaptaa 

 Jokul, in Iceland, appear to have filled valleys and hikes and broad 

 plains with floods of melted rock. The lava is said to have flawed in 

 one direction 50, and in another 40, miles, with breadths of 15 and 7 

 miles respectively, and with a depth averaging about 100 feet, but in 

 places reaching 600 feet. If these data have any claim to be regarded 

 as fair approximations (they are so regarded by Lyell and other 

 writers), the mass of lava poured out ia two years by this modem 

 volcano exceeds a hundredfold that of the Plutonic rocks which appear 

 in the chain of the Malvern Hills. It would cover all the coal-fields of 

 the British Islands with a plateau of basaltic rock 20 feet thick, or bury 

 London under a mountain rivalling the cone of Teneriile. In the 

 eruptions of Tomboro, in Sumbawa, in 1815, ashes and scoriae were 

 thrown out sufficient to form three mountains equal to Mont Blanc, or 

 to cover the whole of Germany two feet deep. The volume of muddy 

 and watery eruptions from volcanoes can seldom be accurately mea- 

 sured. Humboldt speaks of mud eruptions, called " Moya," as frequent 

 in the volcanic system of the Andes, and they are abundant enough to 

 fill valleys and stop the channels of rivers. 



From such data as can be collected there appears no sign of any 

 general decay in the magnitude of the volcanic eruptions taken gene- 

 rally, though in respect to any particular volcano the contrary may be 

 inferred. 



Eruptive Furcet. If the quantity of matter ejected by volcanoes be 

 taken as a measure of the amount of unbalanced pressure which required 

 and obtained relief, the force with which it was ejected may be regarded 

 as a measure of the intemily of this pressure. Accurate observations 

 on this point are needed. If, as recorded by Sir W. Hamilton, stones 

 were thrown so high above Vesuvius as to occupy 1 1 seconds of time 

 in falling to the level of the crater, this gives an upward velocity of 

 350 feet in a second at the level of the crater, and a height of about 

 2000 feet ; but the mountain being above 3000 feet high, we must 

 estimate the pressure at the level of the sea as competent to sustain a 

 column of matter of the ordinary weight of lava (gay twice and a half 

 that of water) nearly a mile in height. This would equal the pressure 

 of between 300 and 400 atmospheres. 



Lava which had flowed in 1798, was traced by Humboldt to the 

 summit of the Peak of Teneriffe, and must therefore have been sus- 

 tained (unless the lava were, as is probable, of a lighter kind) by double 

 the pressure. These pressures appear great, but in no degree 

 improbable if judged by the well-known effects of steam. A tempera- 

 ture of 800 Fahr. would give the steam pressure for a height 2000 feet 

 above the cone of Vesuvius ; and go rapidly does this power augment 

 with additional heat, that less than 100u Fahr. may be sufficient to 

 give steam a force equal to balance the whole column of lava in the 

 Peak of Teneritfe. Now these are temperatures which appear to fall 

 within the observed heats of gome of the lava currents, for these have 

 been found to melt silver and to perform heating effects greater than 

 those of red-hot iron. Steam-power, generated by the admission of 

 water to the hot interior parts of the earth, appears entirely adequate 

 to the " eruptive forces " actually witnessed in volcanoes. It is much 

 in favour of this being really the agency employed, that we find in 

 explosive eruptions such considerable bodies of aqueous vapour 

 erupted during most parts of the paroxysm ; that some eruptions have 

 yielded little else than steam, and others chiefly hot water. Moreover, 

 on considering attentively the distribution of volcanoes over the globe, 

 we find the active volcanoes most frequently by the side of the sea, or 

 by other considerable bodies of water ; and the extinct volcanoes in the 

 vicinity of ancient lakes, or desiccated branches of the ancient ocean. 

 Why they should be in the immediate proximity of the ocean or of 

 lakes, will appear in the sequel. But while such proximity undoubtedly 

 facilitates the operation of water in volcanic phenomena, that operation 



appears not to depend upon it ; volcanoes may receive water oil account 

 of their vicinity to it, but that vicinity is not occasioned by the 

 necessary agency of water in their eruptions. 



The general type of a volcanic eruption appears to be as follows : 

 The ground is rocked by frequent earthquakes ; special movements ami 

 noises happen in and about the volcanic mountain ; clouds of steam 

 rise from the crater, followed and mixed with showers of ashes and 

 scoriae driven up by the exploding and expanding vapour ; the tube of 

 the crater becomes filled by melted, or at least flowing matter, which 

 undulates upward and downward with the irregular pressure of the 

 vaporous or gaseous matter ; these burst in large bubbles through it, 

 scattering it into granular duat and ashes, till the lava overtops or 

 breaks through the loose conical walla of the crater, and flows abun- 

 dantly, so as partially or wholly to relieve for a time the unbalanced 

 internal pressure. 



Folcanic Proilitcts. The substances thrown out during volcanic 

 eruptions, whether stony, liquid, or gaseous disclose more or leas com- 

 pletely the nature and condition of the interior masses of the globe, 

 at deptha greatly exceeding the dimensions of the greatest volcano or 

 mountain known, but still very small in comparison to the earth's 

 radius, and belonging to the mere outer crust of the globe. The 

 lava or melted rock is generally referrible to a very small number of 

 aggregations, in which felspar, augite (or hornblende), and oxide of 

 iron are the most important ingredients, the ruass being modified by 

 additional minerals, as leucite, idocrasu, olivine, garnet, epidote. 

 stilbite, heulandite, and many others. Combinations of sulphur, and 

 of uranium, copper, lead, arsenic, and manganese, also occur in various 

 proportions; but these metallic bodies do not play an important 

 part in volcanic phenomena. The so-called ashea and scoria; consist 

 of the same substances as the lava, the most prolific repositories of 

 the rarer minerals being always in cavities of the lava or scoriform 

 aggregations. [The characters of these mineral substances have been 

 described under then- respective names in NAT. HIST. Div.] 



In these particulars modern lava will bear comparison with ancient 

 Plutonic rocks, for they are composed of similar mineral aggregates, 

 modified by many of the same rarer crystallisations, which mostly 

 occur in the cavities of their mags. The difference of most importance 

 between Plutonic rocks (granite, &c.) and volcanic rocks (trachyte, &c.) 

 is in the degree of their consolidation ; and this difference appears 

 quite intelligible by a comparison of the various appearance and 

 character of lava which has cooled and become solid under different 

 circumstances. Lava cooled in air under slight pressure is often 

 cellular ; cooled under the pressure of water (as in the case of the 

 current which passed through Torre del Greco into the sea), it is more 

 compact ; when vitreous, and much distended by steam, it becomes 

 vesicular pumice. We may therefore believe that lavas which remain 

 and grow solid under great pressure about the internal base of the 

 volcano are of a more dense nature than those which come to the 

 surface, and may thus closely resemble, or be even identical with, 

 some of the older Plutonic rocks, which thus regarded, and from other 

 evidence, appear to be in fact uneruptcd lavru. 



The foregoing account of volcanic rocks applies, principally, to one 

 class only of lavas, the stony or common lavas, the most abundant 

 product of volcanoes ; but there is a second, the vitreous or glassy la vat, 

 of which the mineral called obsidian, or volcanic glass, is the type, 

 and of which pearlstone and pitchstone and a few other minerals are 

 examples, but partly in a different condition. While the lavas of the 

 greater number of volcanoes belong to the former class (though mingled 

 we believe in those of every one with some glassy lava), those of 

 certain volcanoes are exclusively vitreous, of which that of the volcano 

 in the Island of Bourbon is an example. In the flowing state the 

 nature of these two classes of lavas is very different. The vitreous 

 lavas are then essentially in a state of dry igneous fusion, comparable 

 to that of artificial glass, or of metals ; but the stony lavas, even while 

 fresh erupted and flowing, are not properly in a melted state, being in 

 fact as truly aggregates of distinct mineral substances while in that 

 state as they are when they have ceased to flow, acquired the character 

 of ordinary solids, and become cold. They are, while they retain the 

 flowing property, as Mr. G. Poulett Sorope first observed and maintained 

 (in his work referred to below) more than a third part of a century since, 

 a sort of mud, consisting of the crystalline grains of the minerals found 

 to constitute them, allowed to slip or glide over each other by tho 

 intervention of water or of aqueous vapour in a peculiar condition of 

 condensation and adhesion to the surfaces of the solid particles, though 

 at a red- or even at a white-heat. Mr. Sorope's views, though almost 

 contemptuously rejected at the period of their enunciation, havo 

 received great support from the subsequent discovery of the possible 

 existence of water in a liquid state at high temperatures (as they 

 might hare received it from a contemporary one of its convertibility 

 into vapour of its own volume), and from the now admitted necessity 

 of the agency of water in the production of all the crystalline aggregates 

 constituting the Plutonic rooks, including granite itself, which has been 

 referred to by Mr. Scrope in a confirmatory paper communicated to thu 

 Geological Society in 1856. A remarkable proof, among many otheiv, 

 that the stony lavas are not really in a fused condition, is afforded by 

 the fact urged by Mr. Scrope, that the radiation of heat by them i-i 

 comparatively so slight, evincing that they cannot possess that intensely 

 high temperature which would be requisite to impart to their mineral 



