Microscopic Magic

Hydrothermal Vents

Since their astonishing discovery less than 40 years ago by scientists exploring the Galapagos Spreading Centre in the submersible ‘ALVIN’ at 2500 metres depth (Desbruyères et al., 2000), more than fifty hydrothermal vent sites have been discovered (Kelley et al., 2002; Van Dover et al., 2002). Major vent sites have been found along the East Pacific Rise, Northeast Pacific Ridges, Western Pacific Back-Arc Spreading Centres and along the Mid-Atlantic Ridge (Fig. 3) (Van Dover et al., 2002).

Fig. 2. Distribution of known hydrothermal vent sites (Van Dover et al., 2002).

Fig. 3. Distribution of known hydrothermal vent sites (Van Dover et al., 2002).

Their distribution follows a similar pattern, occurring at divergent plate boundaries where tectonic plates are moving apart due to convection currents within the Earth’s mantle, forming new crust (Tunnicliffe, 1991).

This spreading of the seafloor creates faults in the sea bed through which seawater percolates, circulating within the Earth’s crust below where it picks up volatiles and is geothermally heated due to the high temperature within the crust (Kelley et al., 2002). This fluid will eventually find its way back to the surface where it erupts at velocities of m s¯¹ as a 400°C, acidic, black toxic cloud out of chimneys standing tall on the sea floor that are aptly named ‘black smokers’ (Kelley et al., 2002; Martin & Russell, 2007; Köhler at al., 1994). The black colour of these clouds is owed to the high concentrations of sulphides within the fluid (Köhler et al., 1994). The fluid remains liquid still at temperatures of 400°C due to the high hydrostatic pressure at thousands of meters depth where hydrothermal vents occur, created by the immense weight of the water above (Rothschild & Mancinelli, 2001). The video below shows hydrothermal fluid containing sulphides from a ‘black smoker’ at 2400 metres depth in the Southern Ocean (Jon Copley, 2010).

These black smoker chimneys are formed by the deposition of sulphide and sulphate minerals derived from within the Earth’s crust that are delivered to the surface dissolved within the emitting cloud of fluid. These minerals precipitate out of solution as the super-heated fluid mixes with the cold (2°C) seawater and are deposited around the fissure, resulting in the formation and growth of these cylindrical structures upwards and outwards overtime (Kelley et al., 2002). Chimneys reach a height of up to 10 metres and a diameter of 50 centimetres (Köhler at al., 1994), however one of the most extraordinary structures discovered at the Juan de Fuca Ridge stood 45 metres off the sea floor and was called ‘Godzilla’ (Kelley et al., 2002).

Smaller chimneys that emit lighter coloured fluids, owed to the presence of high concentrations of calcium sulphates and silica, are also formed and are called ‘white smokers’ (Fig. 4). The fluid emitted from a white smoker has a lower temperature of 270°C to 350°C, but some have even been found to be lower than 100°C, and is emitted at a velocity of cm s¯¹ (Köhler at al., 1994).

Hydrothermal fluid containing calcium sulphates and silica from a 'white smoker' in the Mariana forearc region (Dr. Robert W. Embley, PMEL/NOAA).

Fig. 4. Hydrothermal fluid containing calcium sulphates and silica from a ‘white smoker’ in the Mariana forearc region (Dr. Robert W. Embley, PMEL/NOAA).

Conditions at hydrothermal vents are truly extreme as the environment is bathed in a hot, toxic fluid in the darkness of the deep sea where pressure is greatest. Yet within these environments oases of life flourish and at the base of it all are a diverse community of bacteria (Kelley et al., 2002; Van Dover et al., 2002).

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