Geothermal energy is technically an ‘alternative’ energy source rather than a renewable one, although for research and funding purposes, it is frequently classed as a renewable-energy form. It is, however, the only one that is not directly or indirectly derived from the sun. Geothermal heat is present throughout the earth’s crust in the form of hot dry rocks and is also available in certain regions as hot water or steam from underground reservoirs. The heat is obtained largely from the magma — a mixture of high-temperature gases and molten rock which lies underneath the earth’s crust; approximately 30–45 km below the surface of the oceans and land masses. Most natural geothermal activity occurs in places where the tectonic plates which comprise the earth’s crust meet and the magma approaches the surface through fissures as volcanic activity or hot-water springs. Global plate boundaries are shown in figure 9.1. which also indicates the main areas of known geothermal potential. Geologists currently believe that the continental crust itself possesses an internal heat or energy source due to radioactive decay of potassium, thorium and uranium. It is also possible that appreciable quantities of heat come from the heat released by exothermic chemical reactions within the crust, from the friction generated in faults by the sliding action of huge rock masses caused by gravitational and tectonic processes, and from the latent heat released by the crystallisation or solidification of molten rocks on cooling.
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- Renewable Land Energy Sources
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