Hydrothermal circulation in its most general
sense is the circulation of hot water (Ancient
Greek ὕδωρ, water, and θέρμη, heat
). Hydrothermal circulation occurs most often
in the vicinity of sources of heat within
the Earth's crust.
In general, this occurs near volcanic activity,
but can occur in the deep crust related to
the intrusion of granite, or as the result
of orogeny or metamorphism.
== Seafloor hydrothermal circulation ==
Hydrothermal circulation in the oceans is
the passage of the water through mid-oceanic
ridge systems.
The term includes both the circulation of
the well-known, high-temperature vent waters
near the ridge crests, and the much-lower-temperature,
diffuse flow of water through sediments and
buried basalts further from the ridge crests.
The former circulation type is sometimes termed
"active", and the latter "passive".
In both cases, the principle is the same:
Cold, dense seawater sinks into the basalt
of the seafloor and is heated at depth whereupon
it rises back to the rock-ocean water interface
due to its lesser density.
The heat source for the active vents is the
newly formed basalt, and, for the highest
temperature vents, the underlying magma chamber.
The heat source for the passive vents is the
still-cooling older basalts.
Heat flow studies of the seafloor suggest
that basalts within the oceanic crust take
millions of years to completely cool as they
continue to support passive hydrothermal circulation
systems.
Hydrothermal vents are locations on the seafloor
where hydrothermal fluids mix into the overlying
ocean.
Perhaps the best-known vent forms are the
naturally occurring chimneys referred to as
black smokers.
== Volcanic and magma related hydrothermal
circulation ==
Hydrothermal circulation is not limited to
ocean ridge environments.
The source water for hydrothermal explosions,
geysers, and hot springs is heated groundwater
convecting below and lateral to the hot water
vent.
Hydrothermal circulating convection cells
exist any place an anomalous source of heat,
such as an intruding magma or volcanic vent,
comes into contact with the groundwater system.
== Deep crust ==
Hydrothermal also refers to the transport
and circulation of water within the deep crust,
in general from areas of hot rocks to areas
of cooler rocks.
The causes for this convection can be:
Intrusion of magma into the crust
Radioactive heat generated by cooled masses
of granite
Heat from the mantle
Hydraulic head from mountain ranges, for example,
the Great Artesian Basin
Dewatering of metamorphic rocks, which liberates
water
Dewatering of deeply buried sedimentsHydrothermal
circulation, in particular in the deep crust,
is a primary cause of mineral deposit formation
and a cornerstone of most theories on ore
genesis.
=== Hydrothermal ore deposits ===
During the early 1900s, various geologists
worked to classify hydrothermal ore deposits
that they assumed formed from upward-flowing
aqueous solutions.
Waldemar Lindgren (1860–1939) developed
a classification based on interpreted decreasing
temperature and pressure conditions of the
depositing fluid.
His terms: "hypothermal", "mesothermal", "epithermal"
and "teleothermal", expressed decreasing temperature
and increasing distance from a deep source.
Recent studies retain only the epithermal
label.
John Guilbert's 1985 revision of Lindgren's
system for hydrothermal deposits includes
the following:
Ascending hydrothermal fluids, magmatic or
meteoric water
Porphyry copper and other deposits, 200–800
°C, moderate pressure
Igneous metamorphic, 300–800 °C, low to
moderate pressure
Cordilleran veins, intermediate to shallow
depths
Epithermal, shallow to intermediate, 50–300
°C, low pressure
Circulating heated meteoric solutions
Mississippi Valley-type deposits, 25–200
°C, low pressure
Western US uranium, 25–75 °C, low pressure
Circulating heated seawater
Oceanic ridge deposits, 25–300 °C, low
pressure
== See also ==
Volcanogenic massive sulfide ore deposit
Geothermal
Hydrothermal synthesis
