Oceans cool, dense deep waters

Water, in liquid oceans, is what makes Earth s tectonic history different from Venus and Mars. On Earth oceanic crust and hence plate is cooled quickly by water, because the surface is close to 0 C, not 500°C as on Venus. This cooled plate thickens and becomes dense more quickly than it would on Venus. Plates fall into the astheno-sphere as a steady regular process. Andesite volcanism is fluxed by water given off by subducted oceanic crust. Mars is so cold that reintroduction of water to the interior does not occur. The only volcanism in the past billion years on Mars appears to have been from deep-rooted plumes. 

The deep waters of the oceans are cool (around 2-4°C) and dense relative to overlying seawater. This dense water percolates into fissures in the basaltic crust. Slowly it penetrates deep into the crust, gradually heating up, particularly as it approaches the heat source of the underlying magma chamber. This massive heat... 

The density of seawater thus depends both on its temperature and salinity. Highly saline waters at about 4°C are the most dense and will tend to sink, whereas warm, less saline water is less dense, and will tend to rise. This behavior has important implications for vertical motions in the sea. Descent of surface waters to the deep ocean is possible only where saline waters are cooled. This actually occurs in only a few locations, most notably in the North Atlantic off Greenland, and in the Weddell Sea, part of the Antarctic Ocean south of the Atlantic. North Atlantic deep water has a temperature of 2.5°C and a salinity of 35 g salt per kg of seawater. Deep water forming in the Weddell Sea has a temperature of — 1.0°C and a salinity of 34.6 g salt per kg seawater. In both of these locations, saline surface waters are cooled and sink, a process known as downwelling. The surface waters are made saline by evaporation nearer the equator in the case of the North Atlantic, and by the formation of sea ice in the case of the Weddell Sea. Shallower downwelling, to intermediate depths in the oceans, also occurs in areas of high evaporation. 

On a Goldilocks planet the surface temperature is just right. Where volcanism occurs, water is ejected from the interior, but is held under the cold trap until it can be returned to new crust via hydrothermal systems. Old plate is dense and falls in the water returns to the interior. Much of this water fluxes melt and is returned at andesitic volcanoes. A small amount is retained by plate entering the deep mantle. This intake is controlled by the ambient temperature and in effect the system becomes self-fluxing. If it is locally too hot, abundant melt production removes water and heat. As the planet cools, andesitic melt production falls and the cooler deep mantle accepts more water, to allow melt production to continue at mid-ocean ridges. All this depends on a surface temperature that allows rehydration and cooling of the new plate. 

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