Large amounts of hot water are presumed to fill underground cavities. The water, upon further heating, is violently ejected when a portion of its suddenly flashes into steam. This cycle can be repeated with remarkable regularity, as for example, Old Faithful Geyser in Yellowstone National Park , which has erupted on an average of about once every 65 minutes. Fumaroles , which emit mixtures of steam and other gases , are fed by conduits that pass through the water table before reaching the surface of the ground. This accounts for the intense chemical activity and brightly colored rocks in many thermal areas.
Hot springs occur in many thermal areas where the surface of the Earth intersects the water table.
The temperature and rate of discharge of hot springs depend on factors such as the rate at which water circulates through the system of underground channelways, the amount of heat supplied at depth, and the extent of dilution of the heated water by cool ground water near the surface. For example, geothermal heat warms more than 70 percent of the homes in Iceland, and The Geysers geothermal field near Santa Rosa, in Northern California produces enough electricity to meet the power demands of San Francisco. The Geysers area is the largest geothermal development in the world. In addition to being an energy resource, some geothermal waters also contain sulfur, gold, silver, and mercury that can be recovered as a byproduct of energy production. In summer 1987, maximum ground temperatures were near 85 degrees C and maximum fumarole temperatures were about 92 degrees C, slightly above the boiling point of water at 3,100 meters.