WHY VOLCANOES ERUPT.

Grey River Argus, 21 July 1922, Page 4

WHY VOLCANOES ERUPT.

FAMOUS SCIENTIST EXPLAINS. Dr Milton A. Nobles, the American geologist, has recently made the remarkable statement that an earthquake era which began on March 15 will ultimately submerge a large part of Europe. This fearsome prophecy, of which we are reminded by recent forebodings in New Zealand, gives interest to the following article by Sir E. Fay Lankester, in which he gives the scientific explanation of these great earth upheavals. The extract is taken from Sir Ray’s fascinating book, ‘‘Secrets of Earth and Sea.’ The crater or basin formed by a volcano starts with the opening of a fissure in the earth’s surface communicating by a pipe-like passage with very deeply-seated molten matter and steam. Whether the molten matter thus naturally “tapped” is only a local, though vast accumulation, or is universally distributed at a given depth below the earth’s crust, and at how many miles from the surface, is not known. It seems to be certain that the great pressure of the crust of the earth (from 5 to 25 miles thick) must prevent the heated matter below it from becoming either liquid or gaseoift, whether the heat of that mass be due to the cracking of the earth’s erust, and the friction of the moving surfaces as the crust cools : nd shrinks or is to be accounted for bv the original high temperature of the entire mass of the terrestial globe. “As Through a Safety Valve.” It is only when the gigantic pressure is relieved by the cracking or fissuring of the closed case called “the crust of the earth,” that the enclosed deep-lying matter of immensely high temperature liquidities, or even vaporises, and rushes into the up-leading fissure. Steam and gas thus “set free drive everything before them, carrying solid masses along with them, tearing, rending, shaking “the foundations of the hills.’’ ami issuing in terrific jets from the earth’s surface, as through a safety valve, into the astonished world above. Often in a few hours they choke their own path by the destruction they produce and the falling in of the walls of their briefly-opened channels. Then there is a lull of hours, days, or even centuries, and after that again, a movement of the crust, a “giving” of the blockage of the deep, vertical pipe, and a renewed rush and jet of expanding gas and liquefying rock. The general scheme of this process and its relations to the structure and properties of the outer crust and inner mass of the globe is still a matter of discussion, theory, and verification; but whatever conclusions geologists mayreach on these matters, the main fact of importance is that steam and gasses issue from these fissures with enormous velocity and pressure, and that “a vent” of this kind, once established, continues, as a rule, to serve intermittently for centuries, nnd, indeed, for vast periods to which we can assign no definite limits. The solid matter ejected becomes piled up around the vent as a mound, its outline taking the graceful catenary curves of rest and adjustment to which are due the great beauty of volcanic cones. The apex of tho cone is blown ! away at intervals by the violent blasts issuing from the vent, and thus we have formed the “crater,” varying in the area enclosed by its margin, and in the depth and appearance of the eup so produced. At a rate depending on the amount of solid matter ejected by the crater, the mound will grow in the course of time to be a mountain, and ; often secondary craters or temporary I openings, connected at some depth with i the main passage leading to the central vent, will form on the sides of the ' mound or mountain. Sometimes the old crater will cease to grow in consequence of the blocking of its central vent and the formation of one or more subsidiary vents, the activity of which may blast away or smother the cup-like edge of the first crater. Such a history has been that of Vesuvius. In geologic ages—perhaps some thousands of centuries ago—Vesuvius was probably a perfect cone some 7000 ft high, rising by characteristically ; accelerated upgrowth from a circle of 10 miles or more in diameter to its delicate central peak, hollowed out at the summit by a small crater a couple of hundred yards across. Its eruptions at that time were neither excessive nor violent. Then came a period of greatly increased energy —the steam jet blew with such violence that it shattered and j dispersed the cone, lowering the mountain to 3700 ft in height, truncating it ! and leaving a proportionately widened i crater of a mile and a-half in diameter. And then the mountain reposed for long centuries.