OCR Text |
Show THE MECHANICAL ASPECT OF EEUPTIONS. 129 ever essential the presence of water may be to the total process of erup-tivity, something more is obviously needed, and this additional want is apparently well satisfied by a local rise of temperature in the rocks to be erupted. For it cannot be insisted upon too strenuously that from a dynamical standpoint the problem to be explained is the passage of lava-forming materials from a dormant to an energetic condition. And when we resolve this very general statement into a more special and definite one, we find that it means the passage of solid materials into the liquid condition and (as will be indicated further on) a decrease of density. Whatever may be the ulterior cause of volcanicity, a rise of temperature in the erupting masses seems to be an indispensable condition, and in assuming it we are apparently doing nothing more than taking the most obvious facts and giving them the plainest and simplest interpretation. III. The third general consideration has reference to the mechanics of eruptions. The fact that lavas are generated at the depth of several miles below the surface being given, how do they reach the surface 1 A study of the geological relations of eruptive masses furnishes a decisive answer to this question. The power of lava to penetrate and burrow into solid rock would never have been credited or even suspected had we not the proof of it in the rock exposures. The opening of fissures and the rise of lava into the gaps is one of the commonest and most intelligible methods. All volcanic areas are traversed by dikes, and near the centers of eruption they are exceedingly numerous. But what is most suggestive is the fact that many lavas, after rising part-way to the surface, suddenly tear open the strata and diffuse themselves between the beds, forming subterranean lakes at levels far above their original source. These intrusive lavas are exceedingly common, so much so, that they appear to have constituted in all ages a notable proportion of volcanic movements. But when a vent i's established through which lavas can find escape, •we have still to consider the propelling force which urges them onwards or upwards. A very common view, long entertained by many geologists, is that the escape of lavas is analogous to what takes place when a bottle of warm champagne is suddenly uncorked. So comprehensible and plausible is this explanation that its wide acceptance is not surprising. In some 9 h p |