Gas Hydrates

Gas hydrates, or clathrates, are solid, icelike, watergas mixtures that form at cold temperatures (40-43°F, or 4-6°C) and pressures above 50 atmospheres. They form on deep marine continental margins and in polar continental regions, often below the sea floor. The gas component is typically methane but may also contain ethane, propane, butane, carbon dioxide, or hydrogen sulfide, with the gas occurring inside rigid cages of water molecules. Anaerobic bacteria produce the methane by the biodegradation of organic material.

Estimates suggest that gas hydrates contain twice the amount of carbon present in all of the planet's known fossil fuel deposits, and as such they represent a huge, virtually untapped potential source of energy. However, the gases expand by more than 150 times the volume of the hydrates, they are located deep in the ocean, and methane is a significant greenhouse gas. These obstacles present significant technical problems to overcome before gas hydrates are widely mined as an energy source.

See also Arabian geology; Asian geology; basin, sedimentary basin; carbon cycle; Carboniferous; economic geology; ocean basin; passive margin; sequence stratigraphy.

FURTHER READING

North, F. K. Petroleum Geology. Dordrecht, Germany:

Kluwer Academic Publishers, 1986. Seeley, Richard. Elements of Petroleum Geology. New

York: Academic Press, 1998.

hydrosphere The hydrosphere is one of the Earth's external layers, consisting of the oceans, lakes, streams, glaciers, groundwater, and part of the atmosphere. The Earth is a water-rich planet, and the hydrosphere is a dynamic mass of liquid continuously on the move. The term hydrologic cycle describes changes, both long- and short-term, in the Earth's hydrosphere. Also known as the water cycle, the hydrologic cycle is powered by heat from the Sun, which causes evaporation, transpiration from plants, and accumulation of water into clouds. This water then moves in the atmosphere and precipitates as rain or snow, which then drains off in streams, evaporates, or moves as groundwater, eventually to begin the cycle over and over again. The time required for individual molecules of water to complete the cycle varies greatly; it may range from a few weeks for some molecules to many thousands of years for others.

Hydrology is the study of water in liquid, solid, and vapor form, on local to global scales. Studies performed by hydrologists include analysis of the properties of water, its circulation, and distribution on and below the surface in reservoirs, streams, lakes, oceans, and the groundwater system. Many hydrologists assess the movement of water through different parts of the hydrologic system and evaluate the influence of human activities on the system in attempts to maximize the benefits to society. Hydrology may also involve environmental and economic aspects of water use.

A view of the Earth from space reveals that water covers most of the surface, in stark contrast to every other planet. The water on the planet is responsible for many things that allow humans to habitate the Earth. Water lubricates the upper layers of the planet, allows plate tectonics to operate, controls climate and weathering, and is part of life itself, found in the bodies of complex fauna and flora to the interior of cells of the simplest single-celled organisms. The surface of the Earth is covered by about 70 percent water, and the human bodies are also composed of about 70 percent water.

Water is the most precious resource on the planet, needed for sustaining all life, yet has also become the most threatened natural resource because of pollution and overuse. It can pose risks for catastrophic floods with increased urbanization of areas that used to store water on floodplains. Wars and political conflicts have always been fought over the ability to obtain freshwater, navigate rivers, irrigate farmland, and develop floodplains. Water rights pose difficult political issues in places where water is scarce, as in the American West and the Middle East. Since this is a finite world with a finite amount of freshwater, and the global population is growing rapidly, the management of freshwater will likely become an increasingly important issue for generations to come, yet public and political understanding of the science behind decisions on water use lags far behind political decision making and land use.

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