Very few areas on land lie on such a rift zone. In other areas on land, it is necessary to drill much deeper to access such high temperatures. The difficulty and cost of drilling through a large amount of rock can be avoided by drilling offshore. According to the USGS, the Earth’s crust in continental landmasses averages approximately 30,000 meters in thickness, and can be as thick as 100,000 meters, but the thickness of the Earth’s crust under the oceans averages about 5,000 meters and is less in certain areas. The most promising areas on the ocean floor are in the oceanic rift zone, which wraps around the world “like the seams on a baseball,” as described in a recent National Geographic production about the oceans. The geothermal resources in the ocean floor are vast enough to supply all future energy requirements. The question is how to access those resources.
The west coast of the US and Mexico has excellent geothermal resources that lie close to the shore. Features have been recently discovered that demonstrate these resources clearly. The Juan de Fuca tectonic plate is a (comparatively) small tectonic plate that forms the ocean floor along the coast of Northern California and the Pacific Northwest. A number of “black smokers” have been discovered on the plate approximately 100 miles from shore. (A “black smoker” is a submarine geothermal vent that spews hot water, generally at a temperature of approximately 400°C, into the ocean. Black smokers are found at an average depth of approximately 2,100 meters below sea level.) The Juan de Fuca plate also has a “spreading zone,” where the sea floor splits and magma rises to form new crust, along the Gorda Rise and the Juan de Fuca Rise, which parallel the coast of Northern California, Oregon and Washington. A more graphic indication of the available resource under the ocean floor, the Geothermal Map of North America (see http://www.smu.edu/geothermal/2004NAMap/2004Namap.htm) shows the surface heat flows in many of the most favorable areas of the Western United States, such as the Geysers and the Salton Sea, in shades of orange and medium red. Most of the area of the ocean floor off the coast of the Western United States, however, is shown in shades ranging from red to hot pink, indicating that huge areas adjacent to the coast produce significantly more heat than the most productive areas on land.
A significant advantage to drilling offshore is the much higher temperatures at which the geothermal resources can be accessed. Most of the geothermal reservoirs under development in the West provide heat at a temperature of 250°C or less. Based on submarine geothermal discoveries, offshore geothermal resources can exceed 500°C. The higher temperature provides a disproportionately large advantage in efficiency. Another major advantage is that the reservoirs are more sustainable, because the heat flow through the ocean floor is much higher, as reflected in the Geothermal Map described above. This approach will also access much more extensive geothermal resources than the conventional geothermal resources currently used. Also, there is a virtually unlimited supply of seawater with which to create or enlarge geothermal reservoirs, if enhanced geothermal systems are necessary, or to help to recharge existing reservoirs.