ABSTRACT
As discussed in Chapter 3, a tremendous amount of internal heat energy escapes across the surface of the Earth. This escaping heat energy is not uniformly distributed. Just like the distribution of earthquakes and volcanoes, regions of elevated heat ow are concentrated along discrete zones. Most of these zones of elevated heat ow lie along or near the margins of the Earth’s tectonic plates, as do most volcanoes and earthquakes. Tectonic plates are large slabs of the lithosphere (crust and uppermost mantle) that move continually (although movement is mainly tful on human time scales), reecting the huge amount of work done by Earth’s internal heat energy. For example, consider the energy required to move a section of the Earth’s crust about 10 m along a distance of about 1300 km. That is what happened in the 9.1-magnitude Sumatra-Andaman earthquake in Indonesia in 2004. The energy released at the surface, which is an indication of seismic potential for damage, amounted to about 20 × 1017 joules or 2.0 petajoules (PJ) (USGS, 2014a). That amount of work equals approximately 5.6 × 109 MWh. For comparison, total electricity generation in the United States from 2011 to 2014 averaged a little less than 4.1 × 109 MWh (EIA, 2016). The 2004 Sumatra earthquake released enough energy in a matter of minutes to power the current electrical needs of the United States for about 1.4 years. Understanding the distribution of geothermal energy across the surface of our planet requires examining plate tectonics. Also, as we began to explore in Chapter 3, the
roles of earth materials (thermal and other physical properties of rocks and minerals) are critical in characterizing whether or not a geothermal resource is viable for development. Finally, forces imparted on rocks, mainly through the interaction of moving tectonic plates, produce structures, such as faults, that typically affect the ow of hydrothermal uids.
