VOLCANOES AS NATURE’S SAFETY VALVES ARE USEFUL.

Star (Christchurch), Issue 19271, 7 January 1931, Page 6

VOLCANOES AS NATURE’S SAFETY VALVES ARE USEFUL.

To-day’s Article

By The Late Professor u

A. W. Bickerton.

What are the processes which take place in Nature’s subterranean laboratory to produce such startling outbursts as volcanoes? Volcanic eruptions are akin to earthquakes. Volcanoes are called the “ safety valves ” of earthquakes, and to some extent this is a true term—though the “ safety ” is sometimes more theoretic than actual. The last time I saw Vesuvius it was belching out at regular intervals vast, black, solid-looking masses of smoke, rather like enormous mattresses. But the presence of this safety valve had not averted the dreadful earthquake of Messina, whose ruins we passed. Both earthquakes and volcanoes usually occur along lines of weakness, where the shrinking of the earth’s interior makes wrinkles in the earth’s surface.

JJISTORICALLY, there have been two main periods of volcanic activity. The first of these was when the cooling surface of the globe solidified, shrank and split, exercising stupendous pressure in the process, so that the liquid interior was forced up in the form of mountain ranges. After this came a comparatively quiet time, when the shrinkage of the interior and exterior of the globe was about equal. Then followed the present volcanic period. The earth’s crust is now thick and cool, but the interior is continually losing heat by conduction through the surface; it is shrinkng and causing the surface to wrinkle like an apple when it loses its moisture.

Thus the earth’s crust buckles and crumples, splits and slides, forming the vast slicken-side surfaces. It forces up the mountain chains and lowers the adjacent oceanbeds, often giving very deep water close to high mountains, as on the coast of America, New Zealand, and the long stretch of coast from Kamschatka to the islands of Japan. As the land in these districts slides to fit the new physical conditions, the enormous subterranean pressure melts the rock which is driven upwards as lava, gradually filling the volcanic craters. Sometimes this lava overflows at regular periods, as in ihe volcanoes of Hawaii. Violent Explosions. The lines of volcanic activity nearly always run along the borders of seas and lakes. In volcanic eruptions, too, there are always vast ejections of steam. What happens is this. The water from lakes or seas percolates through the ground until it reaches the molten rocks. Here it is converted into steam at enormous temperature and pressure. For years, perhaps, the water continues to feed this gigantic terrestrial boiler, and at the same time (it may be) the slipping process continues to generate heat and stoke it. At last the pressure becomes so great that there is an explosion. If the line of least resistance is along the vent of the y “cano, there is a normal eruption. But it he top of the vent is encrusted with solid lava, the steam may burst through m m m hi hi m m si m ® in m m ® @ ® ® m m si m m \

the side of the volcano, in which case a new cone is formed. Some of these explosions are remarkably violent. Autuco, in Chili, is said to h ive thrown stones a distance of 36 miles. Cotopaxi once hurled a huge mass of rock weighing 200 tons a distance of nine miles. In some cases the ejections have taken the form of volcanic dust. In 1835 Coseguina, in Nicaragua, emitted such dense volumes of dust that the surrounding country was plunged in complete darkness within a radius of 35 miles. Vesuvius's Greatest Outbreak. Vesuvius has erupted violently from time to time ever since the fearful outbreak in A.D. 79. This eruption, the greatest within the period of human history, suddenly blew out the southern half of the ancient crater, which was believed to be extinct. At every important eruption since that date the same thing, on a lesser scale, has taken place. The hardened cake of lava forming the floor of the crater is burst open, >nd with it there generally disappears a good deal of the upper part of the cone, and sometimes a large part of the crater w. 11. A remarkable thing happened in 153£. Within 24 hours a new volcano, Monte Nuovo, was formed on the shoi of •.he Bay of Naples. Successive explosions had bored a cavity underground, and whe.i burst through the surface such quantities of stones and ashes were thrown out from it that there appeared a hill 440 ft high and more than a mile and a half In circumference at the base. The craters of some of the extinct volcanoes I have seen in New Zealand are most interesting. They are more than twenty miles in diameter, and are now, in most cases, inland lakes with an island 'n the centre where the last ejection took place. There are traces of early volcanic activity in the British Isles. Snowdon, Aran Mowddwy and Cader Idris were all active in the remote past. Central Scotland was another volcanic district, and there are even traces of volcanic activity in the South of England. (Anglo-American N.S. —Copyright.)