Hydrothermal
vents
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Hot springs on the ocean floor are called hydrothermal vents. The most numerous and spectacular hydrothermal vents are found along worlds 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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