100 lltf 130 130 140 ISO 100 170 1M 170 ISO 150 140 130 130 110 100 30 BO' TO 00 50 40

Fig. 26.4 Worldwide average ocean temperature differences between 20 and f ,000 m water depths (colour palette is from f 5° - mauve - to 25° C-red)

ffi c o pr

Fig. 26.5 Volcanic activity at mid-ocean spreading ridges. (Gaba 2009)

see Alcocer and Hiriart 2008). The circulated seawater emerges from the volcanic rocks containing a range of minerals, which may contain gold, silver, copper, lead, zinc and other precious and rare metals. They also contain substantial quantities of heat, often reaching the seafloor at temperatures in excess of 350°C. It is this heat energy and the unusual mix of minerals that lead to the spectacular and unusual faunal assemblages, which are common at black smoker sites.

Since the first discoveries, most mid-ocean ridges have been found to contain black smoker sites, some at remarkably shallow water depths. Some mid-ocean ridges come close to shore as, for example, does the Tonga-Kermadec Arc to the north coast of the North Island of New Zealand or the spreading ridge at the northern edge of the Gulf of California (Fig. 26.5).

Like onshore geothermal energy, submarine geothermal energy should be fore-castable and produce baseload electricity. Although individual submarine geother-mal vents are ephemeral, regional production of geothermal fluids is usually predictable and forecastable.

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