Geothermal energy is a renewable energy source generated from heat within the earth. The term comes from the Greek words geo, meaning earth, and thermos, meaning heat.
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Through the decay of radioactive particles, a process happening in all rocks, temperatures a few thousand miles below the Earth's crust are hotter than those on the sun's surface. The Earth's continental plates thus float on this sea of magma - volcanoes and earthquakes are the most visible signs of this furnace.
There are various ways of using geothermal energy. Where magma comes close to the surface, it can heat groundwater locked in porous rocks. Hot water or steam then moves upwards to the surface where it can be used to heat buildings or generate electricity, technologies that provide a large portion of Iceland's energy needs while creating virtually no greenhouse gas emissions.
Such direct geothermal activity mostly occurs in an area called the 'Ring of Fire' that rims the Pacific Ocean. Hot springs and geysers are examples of this geothermal energy, but the capacity to harness this energy is limited. As of 2008, geothermal energy supplied less than one percent of the world's energy.
Another way to use geothermal energy is by exploiting the small temperature differences that exist between the earth's surface and the layers beneath it. Temperatures a few meters underground remain fairly constant throughout the year and heat pumps exploit this to heat houses in winter and cool them in summer. Such heat pumps are among the most energy-efficient, clean, and cost-effective ways to regulate temperatures.
