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NOT only because of the great power and terrifying grandeur of violent eruptions, but also because of their destruction of life and property, volcanoes stand out in the popular mind as among the most real and important of all geological phenomena. But great volcanic outbursts, like violent earthquakes, are in truth only outward, sensible, relatively minor manifestations of the tremendous earth-changing forces below the surface. They are far less important as geological agencies than the mighty interior forces which cause parts of continents to be slowly upraised and the rocks folded, or even than the incessant action of streams whereby the lands are cut down. Even as an igneous agency, volcanoes are notably less important than the development and shifting of molten materials within the earth’s crust. Volcanic action is, however, not only conspicuous, but it is also of real significance as a means of changing the earth, such action having taken place since very early known geologic time. After bringing out the main facts and principles of volcanoes, aided by descriptions of specific eruptions, we shall turn to a consideration of igneous activity within the earth’s crust.
Mount Vesuvius in Italy is perhaps the most famous active volcano in the world. Its eruptions have been more or less carefully studied for a longer time than any other. The eruption in the year 79 A. D. was really a tremendous explosion causing a large part of the old crater to be blown away, and sending immense volumes of rock fragments, mostly finely divided (so-called “ashes”) into the air which completely buried the small city of Pompeii. Water from the great clouds of condensing steam, mixed with “ashes,” formed muddy floods which overwhelmed Herculaneum. Little or no lava was erupted. Since that time the crater has been more or less active and the present cone, 4,000 feet high, has been built up. During the last fifty years the greatest eruptions took place in 1872 and 1906, when, streams of molten rock flowed down the sides of the mountain.
Fig. 16.—Map showing the distribution of active and recently active volcanoes of the world.
One of the greatest volcanic explosions ever recorded was that of the island of Krakatoa, between Sumatra and Java, in 1883. The greater part of the island was blown away and there was water 1,000 feet deep, where just before the island stood hundreds of feet high. About a cubic mile of rock material was sent into the air mostly in the form of fine dust—some of it for seventeen miles—and completely hid the sun, causing total darkness during the eruption. Dust fell over an area of several hundred thousand square miles. Several days after the explosion ships more than 1,000 miles away were dust covered. Such enormous quantities of a light porous lava called “pumice” fell and floated upon the sea that navigation was badly obstructed many miles from the volcano. Extremely fine dust gradually spread through the whole earth’s atmosphere, causing the extraordinary red sunsets for several months. The sound of the explosion was heard for hundreds of miles. Great sea waves 50 to 100 feet high were stirred up and they swept inland for several miles over the low-lying coast lands of neighboring Java and Sumatra, overwhelming hundreds of villages and drowning tens of thousands of people.
Fig. 17.—The great hole left after the top of Mt. Katmai in southern Alaska was blown off in 1912 by one of the most tremendous volcanic explosions in the annals of human history. The water in the lake is hot. (After Griggs, National Geographic Magazine.)