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A geyser is a hot spring, occurring in a volcanic region, which at regular or irregular intervals throws a jet of hot water and/or steam into the air. From the mouth of a geyser, a tube or channel penetrates deeply into the earth, and fills with water which percolates through from the surface. The water near the base of the tube is heated considerably by hot magma, but the tube is so long and narrow that convection cannot take place freely; the temperature of the water at the base thus continues to rise, while that at the top of the tube is relatively cool. At the same time, the boiling point of the water at the base of the tube is raised by pressure to well above 100 degrees centigrade before steam is formed. When this takes place, the water above is forced upwards and flows away. The sudden reduction of pressure at the bottom of the tube lowers the boiling point of water, and is now converted into steam, which forms in such volume that the entire column of water above it is ejected into the air.
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