Further Reading

Keary, P., Keith Klepeis, and Fredick J. Vine. Global Tectonics. Oxford: Blackwell, 2008. Shearer, Peter M. Introduction to Seismology. Cambridge:

Cambridge University Press, 2009. Sheriff, Robert E. Encyclopedic Dictionary of Applied Geophysics. 4th ed. Tulsa, Okla.: Society of Exploration Geophysicists, 2002. Turcotte, Donald L., and Gerald Schubert. Geodynam-ics. 2nd ed. Cambridge: Cambridge university Press, 2002.

Vanicek, Petr, and Nikolaos T. Christou. Geoid and Its Geophysical Interpretations. New York: CRC Press, 1994.

geyser Geysers are springs in which hot water or steam sporadically or episodically erupts as jets from an opening in the surface, in some cases creating a tower of water hundreds of feet (tens of meters) high. Geysers are often marked on the surface by a cone of siliceous sinter and other minerals that precipitated from the hot water, known as geyser cones. Many also have deposits of thermophilic (heat-loving) bacteria that can form layers and mounds in stromatolitic buildups. Geysers form where water in pore spaces and cracks in bedrock gets heated by an underlying igneous intrusion or generally hot rock, causing it to boil and erupt, then the lost water is replaced by other water that comes in from the side of the system. In this way a circulation system is set up that in some cases is quite regular with a predictable period between eruptions. The most famous geyser in the world is Old Faithful in Yellowstone National Park, Wyoming, which erupts every 20 to 30 minutes.

Thermal springs in which the temperature is greater than that of the human body are known as hot springs. They are found in places where porous structures such as faults, fractures, or karst terrains channel meteoric water (derived from rain or snow) deep into the ground where it warms, and also where it can escape upward fast enough to prevent it from cooling by conduction to the surrounding rocks. Most hot springs, especially those with temperatures above 140°F (60°C), are associated with regions of active volcanism or deep magmatic activity, although some hot springs are associated with regions of tectonic extension without known magmatism. Active faulting is favored for the development of hot springs since the fluid pathways tend to become mineralized and closed by minerals that precipitate out of the hot waters, and the faulting is able to break repeatedly and reopen these closed passageways.

When cold, descending water heats up in a hot spring thermal system, it expands and the density of the water decreases, giving it buoyancy. Typical geo-thermal gradients increase about 120-140°F per mile (25-30°C per km) in the Earth, so for surface hot springs to attain temperatures of greater than 140°F (60°C), it is usually necessary for the water to circulate to at least two miles (two or three kilometers) depth. This depth may be less in volcanically active areas where hot magmas may exist at shallow crustal levels, reaching several hundred degrees at two miles (three kilometers) depth. Hot springs may boil when the temperatures of the waters reach or exceed 212°F (100°C), and if the rate of upward flow is fast enough to allow decompression. In these cases boiling water and steam may be released at the surface, sometimes forming geysers.

Hot springs are often associated with a variety of mineral precipitates and deposits, depending on the composition of the waters that come from the springs. This composition is typically determined by the type of rocks the water circulates through and is able to leach minerals from, with typical deposits including mounds of travertine, a calcium carbonate precipitate, siliceous sinters, and hydrogen sulfides.

Hot springs are common on the seafloor, especially around the oceanic ridge system where magma is located at shallow levels. The great pressure of the overlying water column on the seafloor elevates the boiling temperature of water at these depths, so that vent temperatures may exceed 572°F (300°C). Submarine hot springs often form 10-foot (several meter) or taller towers of sulfide minerals with black clouds of fine metallic mineral precipitates emanating from the hot springs. These systems, known as black smoker chimneys, host some of the most primitive known life-forms on Earth, some of which derive their energy from the sulfur and other minerals that come out of the hot springs rather than from sunlight.

Geysers and hot springs may be the surface outflows of hot waters that flow from deep within the

Beehive Geyser at Yellowstone National Park, Wyoming (National Park Service)

Earth. Many geysers and hot springs contain water that fell as rain, seeped into the Earth where it got heated, then rose again to the surface. Other hot springs and geysers have water that came from deeper levels in the Earth's crust, are known as hydrothermal fluids. Most heated subsurface waters contain dissolved minerals or other substances. Known as hydrothermal solutions, these waters are important because they dissolve, transport, and redistribute many elements in the Earth's crust and are responsible for the concentration and deposition of many ores, including many gold, copper, silver, zinc, tin, and sulfide deposits. These mineral deposits are known as hydrothermal deposits.

Hydrothermal solutions are typically derived from one or more sources, including fresh or saline groundwater, water trapped in rocks as they are deposited, water released during metamorphic reactions, or water released from magmatic systems. The minerals, metals, and other compounds dissolved in hydrothermal solutions often come from the dissolution of the rocks through which the fluids migrate or are released from magmatic systems. Hydrothermal solutions commonly form during the late stages of crystallization of a magmatic body, and these fluids contain many of the chemical elements that do not readily fit into the atomic structures of the minerals crystallizing from the magma. These fluids tend to be enriched in lead, copper, zinc, gold, silver tin, tungsten, and molybdenum. Many hydrothermal fluids are also saline, with the salts derived from leaching of country rocks. Saline solutions are much more effective at carrying dissolved metals than nonsaline solutions, so these hydrothermal solutions tend to be enriched in dissolved metals.

As hydrothermal solutions move up through the crust, they cool from as high as 1,112°F (600°C), and at lower temperatures the solutions cannot hold as much dissolved material. Therefore as the fluids cool, hydrothermal veins and ore deposits form, with different minerals precipitating out of the fluid at different temperatures. some minerals may also precipitate out when the fluids come into contact with rocks of a certain composition, with a fluid-wall rock reaction.

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