Hydrothermal vents are a feature of the deep seafloor discovered less than 25 years ago. Fields of them are created when seawater percolates into sub-seafloor rocky structures, becomes superheated by the molten rock (magma) oozing up from the earth's interior, then forces its way back up to the seafloor, leaching mineral elements from the rock structures. As the mineral-- laden jet erupts through the seafloor its temperature can be as high as 700 deg F. "Black smokers" is the term often used to describe these plumes, but their color depends on the amount and type of materials ejected and can range from clear to white to pitch black.
Because the surrounding water temperature is about 37-46 deg F, rapid chilling of the jet's outer surface results in an immediate precipitation of minerals. Most rains down on the seafloor, but some forms "stacks" of concreted material.
Although composition may vary from site to site, the minerals mainly are sulfide compounds containing copper, cobalt, zinc, iron, lead, and silver. In many vent sites these metallic elements are ore quality, often better than that found in many mines ashore. Vent fields are not permanent, however; their natural process of creation and termination is continuous. This could make commercial mining activities difficult, uncertain, and expensive.
Hydrothermal vent fields mostly are found in the central rift valleys of the global mid-ocean ridge system, where water depths range from 6,000 to 8,000 feet. This globe-girdling mountain system is the world's longest. Running 40,000 miles north to south in the Atlantic, it crosses the bottom of the Indian Ocean and up across the South Pacific to intersect the North American continent near Mexico.
The earth's surface is made up of 12 to 14 moving crustal plates. There are continuous processes of differential motion, creation, and destruction along their edges. Where plates rub past each other, frictional forces cause major earthquake events. California's San Andreas Fault, boundary between Pacific and North American plates, is an example.
At the mid-ocean rift valleys, new seafloor crust is created continuously when molten rock forces it way up from the interior of the earth. This is seafloor spreading; because our planet is not swelling up, there have to be places where an equal amount of seafloor crust is being consumed. This happens in the global deep ocean trench systems. Here seafloor material is pushed beneath the continents to be recycled in the earth's molten interior. From creation to destruction, lifetime of seafloor crust is about 200 million years.
A fundamental discovery about life on our planet was made in 1977 when the Woods Hole manned submersible Alvin dove to a hydrothermal vent field in the eastern Pacific Ocean. The intention was to study geology of the area, but the major finding was an entirely new life system.
Previously it had been thought that all life on earth was solar dependent, powered by photosynthesis. Now a rich and extensive life system had been discovered that knew nothing of the sun. It was powered by chemosynthesis, in which life forms depend on the thermal energy and chemical elements in the plumes for support. Some scientists have suggested that this noxious and sunless environment is where life on earth may have begun.
Since 1977 submersibles and research ships have been used to study these living communities at numerous vent sites. More than 300 new species of life have been discovered in this world without light. It is hard not to make the analogy of a spaceship visiting an alien planet.
Studies continue. The more oceanographers look, the more hydrothermal vent fields they discover. It has been determined that these are not isolated phenomena but ubiquitous, regular features of the rift valleys. Also recently, major vent fields have been found on slopes of underwater volcanoes. In sum, these fissures pump enormous quantities of mineral elements and thermal energy (up to 34% of all heat added to the oceans) into the deep sea worldwide.
Few of us will have the chance to visit these cities under the sea, but it is easy enough to see a hydrothermal vent on land. Just visit the geyser fields at Yellowstone National Park—this is somewhat the same process that takes place in the abyss.
Beneath our feet Mother Earth continues her process of movement, creation, and destruction. The continents move but we hardly notice; even the fastest spreading zone moves at less than seven inches a year. While this is supersonic speed in a geological time frame, over the brief span of a human life we hardly notice it. That is, unless you live near the active plate margins. Our planet is indeed "a wondrous work in progress."