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Hot springs
on the ocean floor are called hydrothermal vents. The most numerous and
spectacular hydrothermal vents are found along world’s mid-ocean
ridges. The heat source for these springs is the magma (molten rock) beneath
the volcanic ridge system. Geothermal activity beneath 2000 to 5000 meters
of seawater is markedly different than on land because of the high pressure
at the bottom of the ocean. As seawater descends into the region of partly
molten rock beneath the mid-ocean ridge, it heats up to 300-400°C
and becomes extremely corrosive. This hot fluid is capable of dissolving
the surrounding basaltic rock and leaching out metals and other elements.
This 300-400°C
fluid is also very buoyant and begins rising rapidly back to the surface,
and eventually reenters the ocean at hydrothermal vents.
The most
spectacular kind of hydrothermal vent are called "black smokers",
where a steady stream of "smoke" gushes from a chimney-like
structures. The "smoke" consists of tiny metallic sulfide particles
that precipitate out of the hot vent fluid as it mixes with the cold seawater.
Plumes from such vents can be traced in the ocean for hundreds of meters
upwards and hundreds of kilometers horizontally. The chimneys are made
out of sulfide minerals that precipitate out of the vent fluid and can
grow 10's of meters high. Many large ore deposits now found on land were
formed at hydrothermal vents millions or even billions of years ago. Black
smokers are an example of focused vents, in which almost all the vent
fluid comes out of one small pipe.
Sometimes
the hot fluids rising from depth are mixed with cold seawater and spread
out before they emerge back onto the seafloor. These are called diffuse
vents and are usually only a few tens of degrees above the near freezing
deep ocean water. Diffuse vent areas have warm water exiting the seafloor
over a large area and consequently do not build sulfide chimneys. However,
they still contain high levels of hydrogen sulfide and other compounds
that specialized microbes can use for energy. This is the basis for an
ecosystem that is largely independent of the sun and gives rise to the
specialized vent animals such as large tubeworms and clams. The relatively
low temperature allows the animals to remain immersed in the nutrient
rich water and allows the diffuse vent sites to develop into complex ecosystems.
Often chimneys with focused, high-temperature venting are surrounded by
areas of diffuse, low-temperature venting.
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