The primary heat source for the Earth is the radioactive decay of uranium, thorium and potassium, found in the mantle and crust. The Earth produces more heat through phase changes, such as the freezing of liquid iron in the inner core and minerals in other parts of its core. In addition, the slow cooling process of the Earth gives off heat that melts magma.
At any location on the Earth's crust, drilling deep enough gives access to heat from the core. In certain areas, the magma comes closer to the crust, making the energy more accessible. The most useful type of geothermal energy comes from steam fields. Dry steam fields found in New Zealand, Italy and California power turbines to generate electricity. Wet steam fields, while less efficient, also power generators. One method of capturing geothermal energy is by drilling deep wells and pumping water into the hot rock to operate power plants. In areas where there is not sufficient energy readily available to produce electricity, hot water from the ground provides heat for buildings and energy for industrial processes. Geothermal heat pumps use shallow wells or trenches in the Earth to heat homes and businesses with greater efficiency than air source heat pumps.