Volcanoes: What They are and What They Teach

by John W. Judd

Von Richthofen and the geologists who most strongly maintain the generalisations which he has made concerning the order of appearance of volcanic products, go much farther than we have ventured to do, and insist that in all volcanic districts a constant and unvarying succession of different kinds of lavas can be made out. It appears to us, however, that the exceptions to the law, as thus precisely stated, are so numerous as to entirely destroy its value.

The generalisation that in most volcanic districts the first ejected lavas belong to the intermediate group of the andesites and trachytes, and that subsequently the acid rhyolites and the basic basalts made their appearance, is one that appears to admit of no doubt, and is found to hold good in nearly all the volcanic regions of the globe which have been attentively studied.

The Tertiary volcanic rocks of our own country, those of North Germany, Hungary, the Euganean Hills, the Lipari Islands, and many other districts in the Old World, together with the widespread volcanic rocks of the Rocky Mountains in the New World, all seem to conform to this general rule.

THEORY OF VOLCANIC MAGMAS.

In connection with this subject, it may be well to refer to the ideas on the composition of volcanic rocks which were enunciated by Bunsen, and the theoretic views based on them by Durocher. Bunsen justly pointed out that all volcanic rocks might be regarded as mixtures in varying proportions of two typical kinds of materials, which he named the 'normal trachytic' and the 'normal pyroxenic' elements respectively. The first of these corresponds very closely in composition with the acid volcanic rocks or rhyolites, and the second with the basic volcanic rocks or basalts. Durocher pointed out that if quantities of these different materials existed in admixture, the higher specific gravity of the basic element would cause it gradually to sink to the bottom, while the acid element would rise to the top. Carrying out this idea still further, he propounded the theory that beneath the earth's solid crust there exist two magmas, the upper consisting of light acid materials, the lower of heavy basic ones; and he supposed that by the varying intensity of the volcanic forces we may have sometimes one or the other magma erupted and sometimes varying mixtures of the two.

The study of volcanic rocks in recent years has not lent much support to the theoretic views of Durocher concerning the existence of two universal magmas beneath the earth's crust; and there are not a few facts which seem quite irreconcilable with such a theory. Thus we find evidence that in the adjacent volcanic districts of Hungary and Bohemia, volcanic action was going on during the whole of the latter part of the Tertiary period. But the products of the contemporaneous volcanic outbursts in adjacent areas were as different in character as can well be imagined. The volcanic rocks all over Hungary present a strong family likeness; the first erupted were trachytes, then followed andesites and dacites in great abundance, and lastly rhyolites and basalts containing felspar. But in Bohemia, the lavas poured out from the volcanoes during the same period were firstly phonolites and then basalts containing nepheline and leucite. It is scarcely possible to imagine that such very different classes of lavas could have been poured out from vents which were in communication with the same reservoirs of igneous rock, and we are driven to conclude that the Hungarian and Bohemian volcanoes were supplied from different sources.

SEPARATION OF LAVAS IN RESERVOIRS.

But the undoubted fact that in so many volcanic regions the eruption of andesitic and trachytic rocks, which are of intermediate composition, is followed by the appearance of the differentiated products, rhyolite and basalt, which are of acid and basic composition respectively, lends not a little support to the view that under each volcanic district a reservoir of more or less completely molten rock exists, and that in these reservoirs various changes take place during the long periods of igneous activity. During the earlier period of eruption the heavier and lighter elements of the contents of these subterranean reservoirs appear to be mingled together; but in the later stages of the volcanic history of the district, the lighter or acid elements rise to the top, and the heavier or basic sink to the bottom, and we have separate eruptions of rhyolite and basalt. We even find some traces of this action being carried still further. Among the basalts ejected from the volcanoes of Northern Germany, Bohemia, Styria, Auvergne, and many other regions, we not unfrequently find rounded masses consisting of olivine, enstatite, augite, and other heavier constituents of the rock. These often form the centre of volcanic bombs, and are not improbably portions of a dense mass which may have sunk to the bottom of the reservoirs of basaltic materials.

In consequence of the circumstance that the eruption of lavas of intermediate composition usually precedes that of other varieties, we usually find the central and older portions of great volcanoes to be formed of andesites, trachytes, or phonolites, while the outer and newer portions of the mass are made up of acid or basic lavas. This is strikingly exemplified in the great volcanoes of the Auvergne and the Western Isles of Scotland, in all of which we find that great mountain masses have, in the first instance, been built up by extrusions of lava of the intermediate types, and that through this central core fissures have been opened conveying basic lavas to the surface. From these fissures great numbers of basaltic lava-streams have issued, greatly increasing the height and bulk of the volcanic cones and deluging the country all around.

The lavas of intermediate composition—the andesites, trachytes, and phonolites—possess, as we have already seen, but very imperfect liquidity as they flow from the volcanic vents. Hence we find them either accumulating in great dome-shaped masses above the vent or forming lava-streams which are of great bulk and thickness, but do not flow far from the orifices whence they issue. The more fusible basaltic lavas, on the other hand, spread out evenly on issuing from a vent, and sometimes flow to the distance of many miles from it. This difference in the behaviour of the intermediate and basic lavas is admirably illustrated in the volcanic districts of the Auvergne and the Western Isles of Scotland.