A fragment of oceanic crust from the Tethys Ocean on Cyprus in the eastern Mediterranean was the first to be recognised as such. Situated in the heart of Cyprus is Mt Olympus that the ancient Greeks believed was the home of the gods, is the core of the Troodos Ophiolite, dark heavy mantle rock that had been greatly altered, covering an area of about 10 km², surrounded by several hundred km² of crustal rocks. It is one of the few places on Earth where it is possible to cross the Moho discontinuity, the junction of the mantle and the crust, normally 10-20 km beneath the ocean floor.

There are whole hillsides of pillow lava basalt that were formed on the ocean floor on the other side of the island. There is a very lightweight, light-coloured rock, umber, that is present in some large depressions in some of these flows on other parts of the island. Thousands of tons of dissolved metals that were leached from the crust, such as iron and manganese among others, were brought to the surface of the seafloor and into the cold bottom water of the Tethys Ocean by hot fluids, and are now present as umbers. The metals precipitated out as a variety of oxides and hydroxides instantly on meeting the cold water, to form metal-rich sediments in natural hollows astride the mid-ocean ridge. Since Roman times they have been quarried extensively to be used in dyes and pigment, and later as fluxes in a number of chemical processes. Geologists found the metal ‘chimney stacks’ that were the vents of the hot fluids and the metals that precipitated out, much more recently.

In the late 1970s 2 thriving communities were discovered on the mid-ocean ridge of the Pacific Ocean and, close to the Galapagos Islands, a fracture zone. Since then many other vent sites have been discovered, as well as fossilised vent communities on land. It has been suggested that these vent communities hold the key to the origin of life on Earth.

Hydrothermal vents (submarine hot springs) are centres of life in the depths of the ocean where light never reaches and have  been found along the mid-ocean ridges where they are vents for hot water rich in minerals that accumulate on the sea floor. The water emitted at these vents is normal seawater that has been superheated by contact with basaltic magma at temperatures of 1,000^o C, having percolated down many hundreds of metres from the seafloor. After contact with the magma they are enriched with minerals such as sulphur, iron, copper, zinc and other metals they have gained through exchange with  the basalt. These minerals precipitate out when the hot vent water encounters the extremely cold seawater. Tall chimney-like structures are built up above the seabed by the minerals that precipitate instantaneously on contact with the near-freezing water that surrounds the vents. The vents have been termed black or white smokers because of the clouds of different mineral species that precipitate, and where these clouds settle over much broader areas they form umbers such as those found on Cyprus.

The water coming out of these vents is mostly at temperatures over a range of 300-450^o C and are rich in substances that are normally highly toxic to life forms of all kinds, though unexpectedly there are rich, thriving communities that thrive within an area of a few metres around the vents. Among the life forms in these communities are giant mussels, fast-growing clams, sea anemones, barnacles, limpets, amphipod crabs, worms, white shrimps and fish, and nearly all these highly adapted species are unique to vent environments, as well as many being new to science. There are several new species of tube worm as thick as a human arm that grow to 3 m in length and have a blood-red structure resembling gills protruding from their white tubes. There are also Pompeii worms that grow in clusters that resemble cabbages that grow closest to the emerging vent water.

The primary producers of these communities without sunlight are bacteria, growing symbiotically in and on the tissues of many different organisms, providing energy by oxidising hydrogen sulphide that is normally toxic, for the manufacture by the host organism of organic compounds from CO₂. The entire community is dependent on the chemicals and heat from within the Earth that replaces sunlight used by photosynthesising organisms. Some of the vent animals feed directly on the bacteria. Among these are self-grazing shrimps, clams and mussels, and others simply absorbing organic molecules from dead bacteria. Symbiotic bacteria comprise about 50 % of the body weight of the tubeworms that have no mouth, anus or digestive system. Bacteria comprise up to 75 % of the body weight of clams and mussels, though in their case they have retained the ability to filter feed and have a digestive system that is rudimentary. Eelpout fish feed by nibbling at the worms and clams.

Sources & Further reading

Stow, Dorrik, 2010, Vanished Ocean; How Tethys Reshaped the World, Oxford University Press.