Yellowstone was set aside as the world’s first national park because of its hydrothermal wonders. The park contains more than 10,000 thermal features, including the world’s greatest concentration of geysers as well as hot springs, mudpots, and steam vents. Research on heat-resistant microbes in the park’s thermal areas has led to medical, forensic, and commercial uses. Oil, gas, and groundwater development near the park and drilling in “Known Geothermal Resources Areas” identified by the US Geological Survey in Island Park, Idaho, and Corwin Springs, Montana, could alter the functioning of hydrothermal systems in the park.
Under the Surface
The park’s hydrothermal system is the visible expression of the immense Yellowstone volcano; they would not exist without the underlying partially molten magma body that releases tremendous heat. They also depend on sources of water, such as the mountains surrounding the Yellowstone Plateau. There, snow and rain slowly percolate through layers of permeable rock riddled with cracks. Some of this cold water meets hot brine directly heated by the shallow magma body. The water’s temperature rises well above the boiling point but the water remains in a liquid state due to the great pressure and weight of the overlying water. The result is superheated water with temperatures exceeding 400°F.
The superheated water is less dense than the colder, heavier water sinking around it. This creates convection currents that allow the lighter, more buoyant, superheated water to begin its journey back to the surface following the cracks and weak areas through rhyolitic lava flows. This upward path is the natural “plumbing” system of the park’s hydrothermal features. As hot water travels through this rock, it dissolves some silica in the rhyolite. This silica can precipitate in the cracks, increasing the system’s ability to withstand the great pressure needed to produce a geyser.
The silica coating the walls of Old Faithful’s geyser tube did not form a pressure-tight seal for the channel of upflow. Lots of water pours through the “silica-lined” walls after an eruption stops. Amorphous silica is a lot less strong than the rock it might coat. The pressure in the geyser tube is not contained by the strength of the wall, rather the water pressure in the tube is contained by the greater pressure of colder water outside of the tube.
At the surface, silica precipitates to form siliceous sinter, creating the scalloped edges of hot springs and the seemingly barren landscape of hydrothermal basins. The siliceous sinter deposits with bulbous or cauliflower-like surfaces are known as geyserite.
No basin-wide changes in hydrothermal activity have been observed in the park in recent years. The average Old Faithful eruption interval is 88 minutes as of January 2013, but remained at 90 to 91 minutes for several years. Steamboat Geyser had a major eruption on September 3, 2014. Echinus Geyser, one of the largest acid-water geysers known, has had a period of limited activity. Work continues on the park’s hydrothermal monitoring program, with progress made in documenting the status of the hydrothermal system by measuring the total amount of thermal water and the total heat output for selected geyser basins. Aircraft and helicopter thermal infrared images are being used to determine a baseline and document changes in the hydrothermal areas.
The widely dispersed locations of the park’s hydrothermal areas make a systematic monitoring program to protect the features difficult. The most visible changes in individual thermal features receive the most public attention but do not necessarily represent human influences or changes in the entire hydrothermal system. In order to distinguish human influences from natural changes, the natural variability of the hydrothermal system must be characterized by gathering reproducible data over many years. The park’s geothermal monitoring strategy therefore includes remote sensing, field studies of groundwater flow, measurement of individual hydrothermal feature temperatures and surface water flow, and collaboration with many researchers from outside the National Park Service.
National Park Service policy generally prohibits any interference with geothermal activity in Yellowstone. New road or other construction through hydrothermal areas is designed to mitigate impacts. In 1994, the National Park Service and State of Montana established a water rights compact and controlled groundwater area to protect geothermal resources in the park from groundwater or geothermal development that could occur in a designated area north and west of the park in Montana.