Thermal springs in which the temperature is
greater than that of the human body are known as hot
springs. Hot springs are found in places where porous structures
such as faults, fractures, or karst terrains are able to
channel meteoric water (derived from rain or snow) deep into
the ground where it can get heated, 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 repeatedly break and reopen these
closed passageways.
When cold descending water gets heated in a hot spring
thermal system it expands, and the density of the water
decreases, giving it buoyancy. Typical geothermal gradients
increase about 155°F–170°F per mile (25°C–30°C per km) in
the Earth, so for surface hot springs to attain temperatures of
greater than 60°C it is usually necessary for the water to circulate
to at least two or three kilometers’ depth. This depth
may be less in volcanically active areas where hot magmas
may exist at very shallow crustal levels, even reaching several
hundred degrees at two or three kilometers’ depth. Boiling of
hot springs may occur when the temperatures of the waters
are greater than 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 be above
572°F (300°C). Submarine hot springs often form severalmeter
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, are
also host to some of the most primitive known life-forms on
Earth, some of which do not require sunlight but derive their
energy from the sulfur and other minerals that come out of
the hot springs.
See also BLACK SMOKER CHIMNEYS; GEOTHERMAL ENERGY;
GEYSER.
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