Most occur in the Western-Pacific, along active plate-margins associated with island arcs in areas of subduction (Fig.3), whereby oceanic-crust and continental plates, that are moving in the same direction, impact (Van Dover 2000).
In the subduction zone, the oceanic-crust sinks and creates a force on the overlying-continental plate and eventually causes it to split, creating what is known as an ‘extension zone’, or spreading centre, whereby an upwelling of heated rock, or magma, can occur (Van Dover 2000).
Back-Arc Basins that occur outside of the western-Pacific region are situated in the Southern-Atlantic-Ocean and NW Indian-Ocean Ridges (Van Dover 2000).
Amongst the boundaries of tectonic-plates, on the sea-floor, oceanic ‘mountain-ranges’ occur; these are known as ocean ridges. The total global Mid-Oceanic-Ridge system exceeds 55,000km in length (Van Dover 2000; Van Dover et al. 2002).
Ocean ridges are formed when plates on the ocean floor are separated by tectonic movements such as earthquakes, and as this occurs; magma from the Earth’s core rises and occupies the space between the plates (Van Dover 2000). This area of crust-production is relatively narrow and possesses a trough, or ‘axial-valley’ along the plate’s spreading zone (Van Dover 2000; Van Dover et al. 2002).
Major occurrences of oceanic ridges are in the mid-Atlantic, north-east Pacific, east Pacific and Galapagos (Van Dover 2000).
Distributions of vents and the effects of this on communities:
Many areas of oceanic ridges and back-arc basins still need to be explored for indications of hydrothermal vents; however, in all previous systematic-explorations of the oceanic ridges and back-arc basins, indications of hydrothermal vent systems have been found, and with these, often rich communities of life (Gage & Tyler 1991; Tait & Dipper 1998; Van Dover et al. 2002).
Communities that occur around vent sites in the Mid-Atlantic Ocean Ridges are usually much less diverse than communities in the East-Pacific Rise (Fig.1), with abundant species in the Pacific regions such as bivalves and tube worms, much less predominant in Atlantic hydrothermal vent ecosystems (Gage & Tyler 1991; Tunnicliffe 1988).
Vents sites are often separated by hundreds of kilometres and the question of how vent sites are initially colonized is still being studied (Tait & Dipper 1998). Despite this distance, however, many of the common species, such as the tube worms, occur over all of the studied sites in the Eastern Pacific Ridge, suggesting some populations may be capable of sufficient larval dispersal mechanisms (Gage & Tyler 1991).