Visit Department of Energy, Energy Explained [3]..
Under “Renewable Sources," read "Geothermal" and all subpages.
Links
[1] http://www.power-technology.com/projects/the-geysers-geothermal-california/
[2] http://www.nrel.gov/data/pix/searchpix.php?getrec=16814&display_type=verbose&search_reverse=1
[3] http://tonto.eia.doe.gov/energyexplained/
[4] https://www.youtube.com/watch?v=y_ZGBhy48YI&feature=player_embedded