The detection of sea-floor gas hydrates (GHs) proves that they can coexist in contact with seawater. We calculate that sea-floor hydrates undergo rapid dissolution, yet they are maintained by the constant high rate of upward migration of gas-saturated waters. If upward migration originates from a point source, then the thickness of sea-floor hydrates varies with distance from the center of the upwelling (depending on heat flow). However, nearsurface fracturing may control the actual points of exit onto the sea floor above. If gas migration rates decrease from the center of the mud volcano to its periphery, a concentric pattern in distribution of temperature, methane concentration, and GH contents can be described.
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