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National Geographic : 1902 Jun
Contents
VOLCANOES Two other factors may assist this pro cess: The shrinking of the earth, which allows the molten mass to rise through fissures and generate steam more rap idly, and the change of load at the surface, caused by deposition or deg radation, which thus disturbs the law of equilibrium. Of eruptions in general Professor Russell says : ''The cause of the rise of the molten rock in a volcano is still a matter of discussion. Certain geologists contend that steam is the sole motive power, while others consider that the lava is forced to the surface owing to pressure on the reservoir from which it comes. The view perhaps most favorably enter tained at present in reference to the gen eral nature of volcanic eruptions is that the rigid outer portion of the earth be comes fractured, owing principally to movements resulting from the shrink ing of the cooling inner mass, and that the intensely hot material reached by the fissures, previously solid owing to pressure, becomes liquid when pressure is relieved, and is forced to the surface. As the molten material rises, it invades the water-charged rocks near the sur face and acquires steam or the gases re sulting from the decomposition of water, and a new force is added, which pro duces the most conspicuous and at times the most terrible phenomena accompa nying eruptions. " The recent volcanic outbreaks on Martinique and St. Vincent were erup tions of the explosive type The volca noes have been dormant for years, and the lava in the summit portion of their conduits was cold and hard. Move ments in the earth's crust caused a fresh ascent of lava from deep below the sur face, the molten material came in con tact with water in the rocks it invaded, and steam explosions resulted. " These explosions were similar to what would happen if water should be poured on a mass of molten slag such as comes from an iron furnace."' SOME ERUPTIONS OF THE PAST Prof. William M. Davis and Mr. William H. Snyder, in their excellent " Physical Geography ' (Ginn & Co.), give a number of interesting instances of volcanic eruptions. I quote the fol lowing : " Monte Nuovo (New Mountain) is a small volcano that was formed on the north side of the Gulf of Naples, in Italy, in 1538. Earthquakes occurred thereabouts for two years before the eruption, when in a week's time a cone was built up 440 feet high, half a mile in diameter at the base, and with a crater over 400 feet deep. Masses of lava 'as large as an ox' were shot into the air by the bursting of great bubbles of gas or steam that ascended through the lava in the vent. Finer ashes fell over the country for several miles around. The people of the neigh boring villages fled in terror from their homes. "A great eruption took place in Mex ico in 1750, when the volcano Jorullo (pronounced Ho-rul-yo) was built on the central plateau, burying fertile fields of sugar cane and indigo. The outburst was preceded by earthquakes; the erup tion continued half a year, building six cones and pouring out extensive lava flows. The highest cone, Jorullo, rose 700 feet above the plateau. The flows retained a perceptible heat for over 20 years. " Many examples might be given of marine eruptions. In 1867 a shoal was discovered among the Tonga Islands of the Pacific (latitude 200 20' south, lon gitude 175° 20' west), the surrounding sea floor being about i ,ooo fathomsdeep. In 1877 smoke was seen ascending from the sea surface over the shoals. In Oc tober, 1885, an island had been formed two miles long and 200 feet high. At this time a terrific eruption was in progress, enormous clouds of constantly changing form rising over the island. The shocks of the explosion were felt 207
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