How does Geothermal Energy form?
The interior of the Earth is hot. Volcanoes are a dramatic reminder that there is heat in the Earth. Miners who work in deep mines know that the deeper the level, the higher the temperatures. Oil-well drillers also know that the drill pipes are hot when they are pulled from a deep well and the oil is hot as it rises to the surface. What is the source of this heat?
An early theory was that all of this heat was primordial, or remained from the formation of the Earth. With the discovery of radioactivity, however, an additional source of heat was found. Unstable isotopes of uranium (235, 238U), Thorium (232Th), and potassium (40K) exist in sufficient quantities in most rocks to supply a significant fraction of the heat that is lost from the modern Earth. The total, present-day rate of heat loss from the Earth is estimated to be 46 TW (terawatts or million billion watts), or the equivalent of approximately 69,000 average-sized US coal-fired power plants (average power generation capacity 667 MW).
The Earth is simmering in geologic time. These heat sources are not sufficiently concentrated enough to form a volcano or a geothermal resource directly, but are like a burner on low on a range top. Given enough time, they can bring soup to a simmer. The result is movement of the tectonic plates, a solid crust on the simmering pot of the Earth, broken into pieces that move relative to each other along their boundaries. Most geologic interactions, volcanoes, earthquakes, mountain building, occur close to these boundaries, although there are some important exceptions. Isolated volcanic centers, such as Yellowstone and the Hawaiian Islands pierce the plates far from their edges, sedimentary basins continue to develop long after they have an association with a plate boundary.
Plate tectonics is an important process for geothermal resources in a number of ways. Most volcanoes are associated with plate boundaries and high-temperature geothermal resources are usually found close to active volcanoes. Mountain belts are generally formed in association with plate tectonics. Sometimes the association is obvious, such as the collision of the Indian subcontinent with Asian to form the Himalaya. Sometimes the association is less clear such as the origin of the current elevation of the Colorado Rocky Mountains. However, topographic variations and young faults often allow water to circulate deep in the earth (a few km or a couple of miles) and rise to the surface as a hot spring/thermal resource. Finally, mineral resources can become concentrated in association with melting and recycling of the crust during the plate tectonic cycle. The minerals include those that contain the heat-producing isotopes and some rocks in the continental crust, particularly granitic rocks, contain significantly more heat production than other rocks. These high heat production rocks can produce local warm spots in the crust.
The rate of increase in temperature with depth in the crust is called the geothermal gradient (see Colorado Geothermal Gradient map). In most areas the geothermal gradient is in the range of 15 to 30 °C/km (0.8 to 1.6°F per 100 feet). The average temperature in Denver is about 10°C (50°F), so with these gradients you would need to go down between 2.8 and 5.7 km (between about 5,600 and 11,150 feet) to reach a temperature of 95°C (202°F), the average temperature at which water boils at the elevation of Denver. [The boiling temperature of water decreases by about 1.1°C (2°F) for every 300 meters (1,000 feet) increase in elevation above sea level.] These depths are relatively deep to drill for such modest temperatures.
In some areas geothermal gradients are significantly higher than others. These areas are usually associated with plate boundaries, but can also be associated with high concentrations of heat producing radiogenic isotopes in the upper crust, thick sections of sedimentary rocks that conduct heat poorly, or hot spots, areas of mid-plate volcanism.
Water flow can increase the geothermal gradient at shallow depths: upward flow increases the gradient, downward flow decreases the gradient. Water flow may raise the gradient in addition to other mechanisms that increase the gradient. Where these increases in gradient occur are the locations of geothermal resources. Young volcanic activity is commonly associated with geothermal resources, but is not a requirement.