Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Lisa Morgan, emeritus research geologist with the U.S. Geological Survey.
One of the most common questions asked of the Yellowstone Volcano Observatory is "when is the next big one?" This is an interesting question given the multiple potential natural hazards that exist at Yellowstone. Big earthquake? Big lava flow? Big explosion? Most people mean the big explosion, but that's not a particularly likely event. In fact, most people do not know about the potential hazard associated with a hydrothermal (hot water) explosion, which is far more common than any eruption of lava or volcanic ash.
Hydrothermal explosions are violent and dramatic events resulting in the rapid ejection of boiling water, steam, mud, and rock fragments. The explosions can reach heights of 1.2 miles and leave craters that are from a few yards up to more than 1.2 miles in diameter. Ejected material, mostly breccia (angular rocks cemented by clay), can be found as far as 1.8 to 2.5 miles from the largest craters.
Hydrothermal explosions occur where shallow interconnected reservoirs of fluids with temperatures at or near the boiling point underlie thermal fields. These fluids can rapidly transition to steam if the pressure suddenly drops. Since vapor molecules take up much more space than liquid molecules, the transition to steam results in significant expansion and blows apart surrounding rocks and ejects debris. Hydrothermal explosions are a potentially significant local hazard and can damage or even destroy thermal features. In Yellowstone, hydrothermal explosions occur within the Yellowstone Caldera and along the active Norris-Mammoth tectonic corridor.
Large hydrothermal explosions occur on average every 700 years, and at least 25 explosion craters greater than 328 feet wide have been identified. The scale of these craters dwarfs similar features in geothermal areas elsewhere in the world. Large hydrothermal explosions in Yellowstone occurred after an icecap greater than .6 miles thick receded from the Yellowstone Plateau around 14,000-16,000 years ago.
Studies of large hydrothermal explosion events in Yellowstone indicate: 1) none are directly associated with magma; 2) several smaller historic explosions have been triggered by seismic events, like the 1959 Hebgen Lake earthquake; 3) rocks ejected by hydrothermal explosions show significant mineral alteration, indicating that explosions occur in areas subjected to intense hydrothermal processes; and 4) many large hydrothermal explosion craters in Yellowstone are similar in area to active geyser basins and thermal areas.
Critical components for development of large hydrothermal systems require high heat flow, abundant water (Yellowstone Plateau receives about 70 inches of precipitation annually), and seismicity (Yellowstone experiences 1,000-3,000 earthquakes a year) to maintain open fractures. Active deformation of the Yellowstone Caldera and seasonal changes also contribute.
Hydrothermal systems with explosive potential have a water-saturated system at or near boiling temperatures and an interconnected system of well-developed joints and fractures along which hydrothermal fluids flow. Ascending hydrothermal fluids flow along fractures that have developed due to repeated inflation and deflation of the caldera, which causes rocks to break, and along edges of low-permeability rhyolitic lava flows. The size and location of hydrothermal fields may be limited by excessive alteration of rocks and development of clay minerals that can create caprocks and seal the system. If a portion of the system is sealed, any sudden or abrupt drop in pressure causes water to flash to steam, which is rapidly transmitted through interconnected fractures. The result is a series of multiple explosions and the excavation of a crater. Similarities between the size and dimensions of large hydrothermal explosion craters and thermal fields in Yellowstone indicate that this type of event may be an end stage in geyser basin evolution.
Although large hydrothermal explosions are rare events on a human time scale, the potential for additional future events of the sort in Yellowstone National Park is not insignificant. Based on the occurrence of large hydrothermal explosion events over the past 16,000 years, an explosion large enough to create a 328-foot wide crater might be expected every few hundred years.