Gas hydrates are solids where gas molecules are locked inside cage-like structures of hydrogen-bonded water molecules. The physical properties of hydrates are remarkably close to those of pure ice. According to Helgerud (2001), the P- and S-wave velocity in methane hydrate may reach 3.60 and 1.90 km/s, respectively, while its density is 0.910 g/cm3. The corresponding values for ice are 3.89 and 1.97 km/s, and 0.917 g/cm3, respectively. As a result, sediment with hydrate in the pore space, similar to frozen earth, is much more rigid than sediment filled solely by water.
However, unlike ice, methane hydrate can be ignited. A unit volume of hydrate releases about 160 unit volumes of methane (under normal conditions). Also, unlike ice, hydrate can exist at temperatures above 0° C, but not at room conditions – it requires high pore pressure to form and remain stable.
Such stability conditions are abundant on the deep shelf: high pressure is supported by the thick water column, while the temperature remains fairly low (but above 0° C) at depths of several hundred feet below the sealoor because temperature increase with depth starts at a low level, just a few degrees Celsius at the bottom of the ocean. Hydrates also exist onshore below the permafrost which acts to lower temperature at a depth where the hydrostatic pressure is already high. Favorable pressure and temperature are necessary but not suficient for hydrate generation; its molecular components, water and gas, also have to be available at the same place and time.