Gas hydrate require cold temperatures at moderat pressures or warm temperature but at higher pressure. They are stable at water depths below ̃500m, where the temperature and pressure of the ocean moves into the gas hydrate stability field. Gas hydrates are known to occur at least to depths of 4400m within the ocean, and theoretically could occur at even greater depths, but usually there is not enough organic matter to generate the methane. The gas hydrate is generally confined to the upper sediment layers, as at depth the geothermal heat from below tends to melts the bas hydrate. The sediment host may be 100m thick in shallow water and up to 1000m thick in deep water. Because the bacterial decay of organic particles in sediment is required to produce the mrthane in gas hydrates (see below), they are most abundant on the continental slopes the organic poor sediment cover on the abyssal plain restricts their formation (see figure below). Gas hydrates are also hosted below tundra at high latitudes where the frozen earth stops the gas hydrate from melting and releasing the entrapped mrthane.
Where gas hydrate is stabel in an ocean basin. The temperature drops with depth in the ocean, but rises again with depth in the crust due to heating by radioactive decay from below. The depth (preassure) and temperature at which methane hydrate melts and release methane and water is also shown. Methane hydrate is stable when the geotherm is on the left of this line.
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