THE OCEANS CHIMNEYS - HYDROTHERMAL VENTS.


Contents

What are they?

Where are they found?

How do they work?

Why are they important?



WHAT ARE THEY?

Hydrothermal vents are fissures in the ocean floor that leak hot, acidic water. The size of the opemings range from less than half an inch to more than six feet in diameter. The largest vent is called TAG (Trans Atlantic Geotraverse) and is about the size and shape of a football stadium.

The openings spew out jets of hot brine (up to 350C) at velocities of up to five metres a second. These brines are complex solutions containing lots of elements leached from the rock formations that they have transversed. As they are often black in colour these vents are sometimes also referred to as 'Black Smokers'.

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WHERE ARE THEY FOUND?

These vents are normally found along the crest a of mid ocean ridge, a 46,000 mile long chain of mountains that wraps around the earth. They tend to form in linear zones a few kilometres long by a few hundred metres wide. A few vents have also been found at seamounts (Seamounts are isolated submarine elevations of at least 700m of volcanic origins). Hydrothermal vents are normally found at depths of greater than one mile.

Lots have been found on the East Pacific Rise, an underwater mountain range about 500 miles southwest of Alcapulco and particularly close to the Galapagos Islands.


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HOW DO THEY WORK?

Vents are formed where the planets crustal plates are slowly spreading apart and magma is welling up from below to form mountain ranges known as mid ocean ridges.

Cold seawater percolates down through jointed and permeable rocks and descends to the lower reaches of the sheeted complexes, two to three miles deep. (Magma collects in magma chambers underground and finds its way through a systems of dykes to the surface where it forms a volcano. These sheeted dykes are known as sheeted complexes). The heated brines ascend and produce pervasive alteration (metamorphism) of the rocks with which they come in contact. The heated brines then shoot out as jets (up to 350C) and boiling is prevented by the high hydrostatic pressures of the overlying sea water.

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WHY ARE THEY IMPORTANT?

These vents deposit ore deposits on the sea floor due to the minerals dissolved in the brines as they find there way to the surface of the oceanic crust. The solutions cool and chemical conditions change as the metal bearing hydrothermal solutions rise through the upper levels of oceanic crust and so deposit ore.

Large scale precipitations of sulphides, particularly those of zinc, iron and copper are common. These precipitates build up mounds and chimneys like tubes around the vent, these vents can 'grow' up to 10 metres tall.

It has been estimated that for each 71cm3 of magma solidified the heat loss due to the circulating brines leads to the eventual deposition of 1 million tonnes of sulphide ore.

In fact at some time in the future it maybe viable to mine these deposits, particularly as mineral deposits on the land are being used up rapidly. In a few years time these deposits could become vital.

It has been hypothesised that ancient ophiolite bodies such as those that comprise the Troodod mountains in Cyprus - (Huge copper sulphide deposits of up to 20 million tonnes) were probably formed by black smoker type sea floor springs.

The ore deposits associated with these are not the only reason why these vents are important. Studies have found that the areas around the hydrothermal vents support a huge variety of life in a sunlight free environment. the life found in these places is unique. Itr was once thought that nothing could survive the harsh combinations of toxic chemicals, high pressures, high temperatures, and total darkness at these vents. Scientists have discovered huge tube worms up to 4 feet tall anchored to the sea floor as well as mussels, shrimps, clams and crabs. The fact that these animals are unique and we also know very little about them makes them so important. Around the vents enormous amounts of bacteria live and breed, they can withstand higher temperatures than any other organisms. This makes them attractive to researchers who are developing heat-satble enzymes for genetic engineering, and culturing bacteria designed to break down toxic waste.

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Other related sites


Ocean Planet
Washington Aquarium
Microsoft Encarta
Stuttgart University
Omphacite
Vents


Last updated 23rd April 1997

Written by Alexandra Gulliver

Sources

Ocean Planet
Wood's Hole Oceanographic Institute