Volcanoes: What They are and What They Teach

by John W. Judd

FORMATION OF SINTER-TERRACES.

When springs charged with silica or carbonate of lime rise upon the slope of a hill composed of loose volcanic materials, they give rise to the remarkable structures known as sinter- and travertine-terraces (see [fig. 80]). The water flowing downwards from the vent forms a hard deposit upon the lower slope of the hill, while the continual deposition of solid materials within the vent tends to choke it up. As a new vent cannot be forced by the waters through the hard rock formed below, it is originated a little higher up. Thus the site of the spring is gradually shifted farther and farther back into the hill. As deposition takes place along the surfaces over which this water flows, terraces are built up enclosing basins. Of structures of this kind we have remarkable examples in the sinter-terraces of Rotomahana in New Zealand and the travertine-terraces of the Gardiner's River in the Yellowstone Park district of the Rocky Mountains.

Fig. 80.—Diagram illustrating the mode of formation of Travertine and Sinter Terraces on the sides of a hill of tuff.
Click on image to see original negative view.

CHAPTER VII.
THE SUCCESSION OF OPERATIONS TAKING PLACE AT VOLCANIC CENTRES.

That a volcanic vent, when once established, may display intense activity during enormous periods of time, there cannot be the smallest reason for doubting; for the accumulation of materials around some existing volcanic centres must certainly have been going on during many thousands, perhaps millions, of years. To us, whose periods of observation are so circumscribed, it may therefore at first sight appear a hopeless task to trace the 'life-history of a volcano,' to discover the stages of its development, and to indicate the various episodes which have occurred during the long periods it has been in existence. But when it is remembered that we have the opportunity of studying and comparing hundreds of such volcanoes, exhibiting every varying phase of their development, we shall see that such an attempt is by no means so unpromising as it at first sight appears to be. In the present chapter, we shall give an account of the results which have already been obtained by inquiries directed to this object.

CYCLES OF VOLCANIC PHENOMENA.

There is not the smallest room for doubt that during the past history of our globe, exhibitions of subterranean energy have occurred at many different parts of its surface. There is further evidence that at the several sites where these displays of the volcanic forces have taken place, the succession of the outbursts has run through a regular cycle, gradually increasing in intensity to a maximum, and then as gradually dying away.

A little consideration will show that the first portion of this cycle of events is the one which it is most difficult to examine and study. The products of the earlier and feeble displays of volcanic activity, at any particular centre, are liable to be destroyed, or masked, during the ejection of overwhelming masses of materials in the later stages of its more matured energy. That the feeble displays of volcanic force now exhibited in some localities will gradually increase in intensity in the future, and eventually reach the grandest stage of development, there can be no reason for doubting. But, unfortunately, we are quite unable to discriminate these feeble manifestations, which are the embryonic stages in the development of grand exhibitions of the volcanic forces, from slight outbursts which die away and make no farther sign.

From what has been proved concerning the true nature of volcanic action, however, it is certain that the first step towards the exhibition of such action, at any particular locality, must be the production of an aperture in the earth's crust. Only by means of such an aperture can the vapours, gases, and rocky materials reach the surface, and give rise to the phenomena there displayed. There is reason to believe that all such apertures are really of the nature of fissures, or cracks, which have been opened through the superjacent strata by the efforts of the repressed subterranean forces.