NONSOLAR ENERGY SOURCES: GEOTHERMAL AND TIDES
We have already discussed one form of energy which is not sun- derived, namely nuclear energy. Geothermal energy and tides are two other forms.
An enormous amount of heat is trapped in the interior of Earth. We are reminded of this by the eruption of volcanoes, or by geysers such as Old Faithful in Wyoming, and by the hot springs that many use to cure ailments. Indeed some estimates indicate that the interior energy is 35 billion times the amount consumed annually on Earth. About 60 percent of this is linked to the planet's formation and 40 percent is heat generated by the radioactive elements still in Earth's mantle and crust.
Only a small fraction of this energy could become usefully available. Economic and technological reasons limit us to exploring only about eight kilometers below the surface. Even so, there is enough energy in this region to deliver a useful source likely to last thousands of years provided it could be tapped.
Lest this estimate seem an exaggeration, we can refer to an article by Stephen J. Gould, a Harvard University professor, in the March, 1996 issue of Natural History. He describes the recently discovered realm of living things which subsist on energy not derived