The phenomena of volcanic activity are numerous and varied. A great portion of the material upheaved in an eruption is lava. The kinds of rock composing this are largely silica and silicates, those containing a relatively small percentage of silica being called basic and those containing considerable silica acid. The acid variety of lava is usually the lighter colored; sometimes it does not move from the lava vent and when it does it generally proceeds a short distance only, solidifyin in a thick mass be cause of its viscosity. The basic is much more liquid and covers the slopes of the mountain or spreads over the adjacent plains. Examples of the latter are the basic flows that cover 200,000 square miles in the Deccan and bury most of the Columbia and Snake River plateau. When lava is imperfectly fused and large quan tities of steam are present, the lava becomes, as it approaches nearer the surface, filled with bubbles, which continue to expand, lightening the lava and fadlitating its upward movement. At the surface the lava is torn apart by the steam, the fragments being hurled high in the air. These fragments are known as cinders or pumice and, when finer, ashes. Mudflows, such as destroyed Herculaneum, are merely mixtures of volcanic ash with water from steam or fall ing rain. Violently explosive eruptions are most common in volcanoes that have long inter vening periods of quiet, because the vents be come choked. The Krakatoa (q.v.) eruption of 1883 was extremely violent. The volcano on the Lipari Islands is in almost constant erup tion but vessels passing do not fear danger. There are also numerous accessory phenomena, such as earthquakes (see EARTHQUAKE) ; elec tric and magnetic disturbances and various acoustic manifestations. The explanations of
these phenomena have not yet been satisfac torily reached. One of the most remarkable examples was the disturbance of the magnetic field throughout the world which accompanied the Pelee eruption of 8 May 1902. Contrary currents of air also occur, some advancing be. fore the clouds of the eruption, others moving toward the volcano, apparently into vacua caused by the abrupt explosion of steam. The geysers of Yellowstone Park, the solfataras (vents from which proceed sulphurous fumes) of Italy, etc., are signs of a decreasing volcanic activity in the areas where they occur. Consult Darwin, 'Volcanic Islands' ((Voyage of the Beagle' 1839) ; Lye11, (Principles of Geology,' (Vol. I, 1872); Green 'Vestiges of a Molten Globe) (1874) ; Dana y. D., (aaracteristics of Volcanoes' (New Vork 1890) ; judd. J. W.. (Volcanoes' (New York 1881) •, Scrope, (Vol canoes) (1872) ; Mallet, (Volcanic Energy' ((Philosophical Transactions' of the Royal Society 1873) ; Russell, I. C., (Volcanoes of North America) (New York 1897) ; Heilprin, 'Mont Pelee and the Tragedy of Martinique' (1903) ; Geikie, Sir A., 'Ancient Volcanoes of Great Britain' (London 1897) ; Hull, E., (Vol canoes, Past and Present' (London 1892) ; Hitchcock, C. H., 'Hawaii and Its Volcanoes' (Honolulu 1909) ; Iddings, J. P., 'The Prob lems of Volcanism) (New Haven 1914) ; Bon ney, T. G., 'Volcanoes' (London 1898). Con sult also articles on individual volcanoes, Pelee, Krakatoa, etc.