Numerous springs in Alaska discharge geothermal water, or water with a temperature appreciably warmer than the local average annual air temperature. The location of known geothermal springs is shown in figure 30. The temperatures for the mapped springs are those where the water emerges at the land surface; temperatures within the geothermal reservoir that houses the water at depth are greater than those mapped.
Most of the geothermal springs in Alaska issue from consolidated bedrock. The springs are most common in two areas - a belt across central Alaska that is underlain largely by intrusive igneous rocks and an arcuate area in the mountain ranges in the southern part of the State which is underlain largely by volcanic rocks. Many of the springs issue from faults and fractures at the contacts of granitic plutons.
One theory of the origin of geothermal water is that precipitation falling in upland areas circulates to great depths in the consolidated rocks, mainly along faults. At some depth, the water is warmed by the natural increase in temperature with depth in the Earth's crust (the average increase is about 1 degree Fahrenheit for each 60 to 100 feet of depth) until it becomes lighter than the overlying water. The warm water then moves upward along faults and fractures and discharges to springs.
Geothermal water is a potential source of energy but has been developed only locally in Alaska. Some of the geothermal springs on Baranof Island in southeast Alaska are used to heat buildings and to supply water for a bathhouse. Other springs in scattered mainland areas are used for similar purposes.