Glossary Earthquake – The abrupt shaking of the ground caused by a shift of rock along a fracture in the Earth. Geyser – Hot springs that periodically erupt, shooting scalding water and steam into the air. Though hot springs can be common, the special requirements needed for a geyser to erupt make them rare. In addition to hot water, a unique conduit system or natural “plumbing” system is needed for pressure to build and yet withstand the force of eruptions. A narrow constriction above a larger reservoir chamber allows water to become superheated above the boiling point without boiling. In Yellowstone, the cracks and fissures underground that make up the plumbing system of geysers are lined with silica that the hot water has dissolved out of rhyolite rock and redeposited along the inner walls of fissures. This glass-like lining creates a watertight seal that allows pressure to build within the conduit and produce geyser eruptions. Ground deformation – Change of shape of the ground surface due to an applied force, such as pressure caused by the movement of magma, volcanic gases, or volcanic fluids. Hot spring – A spring that is produced by the emergence of heated groundwater from within the Earth. Mount St. Helens – An active volcano in the Cascade Range in southwestern Washington in the United States. Its last major eruption was in 1980 and it has had periods of some additional volcanic activity since. Mudpot – An acidic hot spring with a limited supply of water and large amount of hydrogen sulfide gas. Microorganisms convert gas into acid that dissolves nearby rock into clay. This clay then mixes with the hot water to create mud. Escaping gases cause the mud to bubble. Steam vent – A vent from which volcanic gas escapes into the atmosphere. They can be some of the hottest hydrothermal features in the park. They have so little water that it boils and flashes to steam before reaching the surface. They often hiss loudly. Also called a fumarole. Supervolcano – A volcano that has an eruption of magnitude 8 on the Volcano Explosivity Index, meaning that more than 1,000 cubic kilometers (240 cubic miles) of magma (partially molten rock) are erupted. Supervolcanoes are the largest volcanoes on Earth. Two of Yellowstone’s last three major eruptions meet this criteria. Volcano – A volcano is a vent at the Earth's surface through which magma (molten rock) and associated gases erupt. It is also the name for the cone built by effusive and explosive eruptions. Yellowstone is such a large volcano that it does not have a distinct cone but rather has raised the elevation of the region approximately 1700 feet higher than surrounding areas. Yellowstone Volcano Observatory – A partnership among the U.S. Geological Survey, Yellowstone National Park, and University of Utah to strengthen the long-term monitoring of volcanic and earthquake unrest in the Yellowstone National Park region. The Yellowstone Volcano Observatory is one of the five U.S. Geological Survey volcano observatories that monitor volcanoes within the United States for science and public safety.
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Last updated: September 3, 2019