GEYSER, gi'zer (Icel. Geysir, name of a fa mous hot spring in Iceland, from geysa, gjosa, to gush). An eruptive thermal spring. A true gey ser has an underground passage communicating with a source of water-supply and usually ter minating at the surface in a basin built up by a deposition of sinter. From the surface vent eruptions of hot water accompanied by subter ranean rumblings take place at more or less regular intervals. In the powerful outbursts the water is shot upward with a loud to a height of 100 feet or more; this display continues for a brief time, and then subsides until the next period of activity. The occur rence of geysers is limited to regions of recent volcanic activity, where hot springs and mud springs are accompanying phenomena. The gey sers of Iceland have been known for many centuries, while those of Yellowstone Park and of North Island, New Zealand, were discovered only in the last century. The most prominent examples in Iceland are the Great Geyser, the Little Geyser, and the Strokhr; the first has a pipe nearly ten feet in diameter, and erupts at intervals of a day or more, hurling the water like an immense fountain to a height exceeding 100 feet. In Yellowstone Park, there are at least 70 eruptive geysers, and nearly 3000 vents of mud volcanoes, fumaroles, and hot springs, most of which occur in four basins., The sur face is covered with terraces and elevations sur rounding the openings, beautifully ornamented with snowy deposits of silica. Among the most remarkable of these geysers are the Giant, which throws a column of water five feet in diameter to a. height of 200 feet, playing continuously for an hour and a half; Old Faithful, which spouts with great regularity every sixty-five minutes, sending the water to a height of 125 feet; Castle Geyser, issuing from a chimney twelve feet high; Excelsior, which has a basin 200 feet in diameter, and spouts at intervals of eight years; the Giantess, which is said to throw a column twenty feet in diameter; the Beehive; and the Grand Geyser. The terraces of Rotomahana, New Zea
land, once rivaling those of Yellowstone Park, were destroyed by a volcanic eruption in 1886.
The investigations of Bunsen in the geyser region of Iceland, confirmed as they were by laboratory experiment, have been generally ac cepted by geologists as affording a satisfactory explanation of the origin and activity of erup tive thermal springs. By seepage from the sur face the geyser tube (a) is filled with a column of water, which, at a considerable depth, receives heat from buried lava flows or other volcanic sources. When the temperature in the lower part of the tube is raised to such a point that the water boils in spite of the superincum bent column, a portion of the water is changed into steam, and by expansion causes an overflow at the surface. Thus relieved of pressure, a large quantity of water flashes into steam, and ejects the whole- column violently into the air. If the circulation of the waters be impeded by throwing stones into the geyser tube, the erup tion can often be hastened. Geysers in many cases were originally hot springs, from which they have gradually developed by building and extending their tubes. Hot alkaline springs carry silica in solution, which is readily pre cipitated along the path of the flowing water; as the tube becomes longer, the difference in temperature between the upper and lower por tions increases, until sufficient to cause an erup tion. In course of time geysers must lose their activity, and again become hot springs, or the flow of water may be entirely checked by struc tural changes in the tubes. Consult: United States Geological Survey of the Territories, 5th and 6th Annual Reports (Washington, 1872-73) ; Bunsen, On the Intimate Connection Existing Between the Pseudo-Volcanic Phenomena of Ice land (London, 1848) ; Malfroy, On Geyser Action at Rotorua (1891).