|MadSci Network: Earth Sciences|
Geothermal energy may be either in the form of heat or hot water. Deep in the planet's core, radioactive atoms such a uranium, potassium, and thorium have decayed for many billions of years. This has generated heat intense enough to melt solid rock. Because the earth's crust is a poor conductor of heat, this heat has largely remained deep underground near where it was produced. Like heat from a wood stove in a large room, the heat produced in the core gradually diminishes with distance. If you could stick a thermometer in the earth's core, the temperature may be as high as 6,600 degrees Celsius. If you stuck it into the mantle, the temperature might register between 3,700 and 1000 degrees Celsius. A few kilometers under the earth's surface the temperature ranges from a few dozen to several hundred degrees Celsius. The fault lines throughout the world create weak points in the crust that permits this geothermal energy to escape. The pressure that this energy creates causes movement in the magma, or molten rock, which tries to push the crust apart. However, the strength and weight of the crust does not allow the heat to emerge, except where the crust is weakest along the faults. For example, along the Pacific Rim there is a great concentration of geothermal activity. That's because they lie along the boundary of large, tectonic plates. Along the fault boundaries magma rises to the surface and forms geothermal vents such as volcanoes and lava flows. This heat and pressure also causes geothermal vents created by water. Over millions of years, water has trickled through the crust and collected in layers of porous rock called aquifers. The aquifers can be heated by geothermal energy to hundreds of degrees. At sea level the water would boil and turn to steam. Because of the enormous pressures several kilometers down, it cannot. Instead, it is pushed upward through channels and fissures, or cracks. It may bubble steadily out onto the surface; or it may rise into the air through geothermal vents as curling clouds of steam and condensed water vapor; or it may jet out of the ground as a geyser.
Try the links in the MadSci Library for more information on Earth Sciences.