The vertical temperature gradient in the earth’s crust has an average of 30˚C/km. It varies from 10-20˚C/km in the Precambrian shield areas to 30-50˚C beneath tectonically active areas. There are areas where the gradient is as high as 150˚C/km. Areas of high heat flow (more than 2 HFU – Heat Flow Units) on the continents are characterized by hot springs and products of Tertiary volcanic activity. Lardarello (Italy), Geysers, Casa Diablo, Niland (USA), Wairakei and Waistapu (New Zealand) Hvergardi (Iceland), Pauzhetsk (Russia), Otake and Matsukawa (Japan) are some of the areas where geothermal power is being tapped economically.