Geothermal Features

Geothermal features are surface manifestations of the internal heat of Earth interacting with groundwater. Earth's internal heat developed during the early evolution of the planet, and additional heat is generated by the decay of radioactive elements, primarily uranium, thorium, and potassium. However, the most important heat source is shallow magma bodies and associated volcanic activity.

Geothermal areas can be classified into high temperature and low temperature, based either directly on subsurface water temperatures or indirectly on the geological settings. High-temperature geothermal areas have water temperatures in the uppermost 1,000 m (meters) of greater than 200°C. Low-temperature geothermal areas are characterized by water temperatures ranging from close to 20 to 150°C and can be found in formerly active volcanic regions, on the margins of [Page 1271]active volcanic regions, or in areas where there is a deep component of groundwater flow, combined with geological features, such as faults or fracturing, that allow the upward movement of heated water to the surface. The primary water source for most geothermal regions is meteoric water, usually as recharge to an aquifer.

High-temperature geothermal areas form where there is a high-temperature gradient due to the presence of a shallow magma body (typically at around 15–5 km [kilometers], i.e., 9–3 mi. [miles] depth). Because groundwater at depth is subjected to greater pressures than shallow groundwater, it can remain in a liquid state while reaching temperatures greater than 100°C. Consequently, in high-temperature geothermal areas, groundwater typically reaches temperatures of around boiling point at 1 km (0.6 mi.) depth, and it can reach temperatures of more than 350°C at 3 km (2 mi.) depth. Groundwater that is heated at depth becomes less dense and rises to the surface. If the temperature exceeds the boiling point as it reaches the surface, then it is emitted as steam. Most high-temperature geothermal fields are associated with young and active rhyolitic volcanism. A faulted, fractured, or porous host rock is necessary to allow water to move from the heat source to the surface. Depending on the temperature at which the water emerges, and the surface conditions, it can form a range of geothermal features, including fumaroles, mud pots, hot springs, and geysers.

Fumaroles

A fumarole is any vent in Earth's surface from which steam is emitted. Fumaroles form in geological settings where groundwater is maintained at temperatures in excess of the boiling point close to the surface. Steam temperatures can range from 98°C to as high as 1,000°C, but commonly in the range of 200 to 300°C. As such, fumaroles are always found in association with active volcanism or in high-temperature geothermal fields associated with very shallow magma chambers. Where there is no well-defined vent, a steam field may develop, where steam is emitted from porous ground over a wide area.

Oblique aerial view of secondary steam fumarole near Spirit Lake, Kamania County, Washington, 1980. The fumarole was 300 feet in diameter. Eruptions occurred hourly and continued for 2 months after May 18, 1980.

Source: U.S. Geological Survey.

The steam from fumaroles is generally composed of a range of gases in addition to water vapor. Typically, these include carbon dioxide (CO2), sulfur dioxide, hydrochloric acid, and hydrogen sulfide. The ratio of water vapor to gas varies depending on the temperature of the steam, rising to more than 20% at temperatures of approximately 300°C but falling again at higher temperatures (>500°C). Fumaroles that emit sulfurous gases are sometimes referred to by the term solfatara, particularly in Italy. Hydrogen sulfide present in the steam (which produces the distinctive “rotten egg” smell common in geothermal areas) tends to be oxidized at the surface by atmospheric oxygen, either into elemental sulfur, [Page 1272]which is deposited around the vents, or into sulfuric acid. For this reason, surface water around steam vents can be highly acidic, with pH values as low as 1. This acidic surface water reacts easily with volcanic material to form fine-grained silica and clay mixtures.

...