Hydrothermal explosions occur when superheated water trapped below the surface of the earth rapidly converts from liquid to steam, violently disrupting the confining rock. Boiling water, steam, mud, and rock fragments are ejected over an area of a few meters up to several kilometers in diameter. Although the energy originally comes from a deep

igneous Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...

source, this energy is transferred to the surface by circulating meteoric water or mixtures of meteoric and magmatic water rather than by

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natura ...

, as occurs in volcanic eruptions. The energy is stored as heat in hot water and rock within a few hundred feet of the surface. Hydrothermal explosions are caused by the same instability and chain reaction mechanism as geysers but are so violent that rocks and mud are expelled along with water and steam.

Cause

Hydrothermal explosions occur where shallow interconnected reservoirs of water at temperatures as high as 250° Celsius underlie thermal fields. Water usually boils at 100 °C, but under pressure its

boiling point The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding env ...

increases, causing the water to become superheated. A sudden reduction in pressure causes a rapid

phase transition In chemistry, thermodynamics, and other related fields, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states ...

from liquid to steam, resulting in an explosion of water and rock debris. During the last Ice Age, many hydrothermal explosions were triggered by the release of pressure as

glacier A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such a ...

s receded. Other causes are seismic activity,

erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is d ...

, or

hydraulic fracturing Fracking (also known as hydraulic fracturing, hydrofracturing, or hydrofracking) is a well stimulation technique involving the fracturing of bedrock formations by a pressurized liquid. The process involves the high-pressure injection of "fra ...

Yellowstone

Yellowstone National Park Yellowstone National Park is an American national park located in the western United States, largely in the northwest corner of Wyoming and extending into Montana and Idaho. It was established by the 42nd U.S. Congress with the Yellowst ...

is a thermally active area with an extensive system of hot springs, fumaroles, geysers, and

mudpot A mudpot, or mud pool, is a sort of acidic hot spring, or fumarole, with limited water. It usually takes the form of a pool of bubbling mud. The acid and microorganisms decompose surrounding rock into clay and mud. Description The mud of a m ...

s. There are also several hydrothermal explosion craters, which are not to be confused with

caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...

s, which are collapse features. Eight of these hydrothermal explosion craters are in hydrothermally cemented glacial deposits, and two are in

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...

ash-flow

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...

. Each is surrounded by a rim composed of debris derived from the crater, 30 to 100 feet high. More than 20 large hydrothermal explosions have occurred at Yellowstone, approximately one every 700 years. The temperature of the magma reservoir below Yellowstone is believed to exceed 800° Celsius causing the heating of rocks in the region. If so, the average heat flow supplied by convection currents is 30 times greater than anywhere in the

Rocky Mountains The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in straight-line distance from the northernmost part of western Canada, to New Mexico ...

Snowmelt In hydrology, snowmelt is surface runoff produced from melting snow. It can also be used to describe the period or season during which such runoff is produced. Water produced by snowmelt is an important part of the annual water cycle in many p ...

and rainfall seep into the ground at a rapid rate and can conduct enough heat to raise the temperature of ground water to almost boiling. The phenomena of geyser basins are the product of hot ground water rising close to the surface and occasionally bubbling through. Water temperatures of 238° Celsius at 332 meters have been recorded at Norris Geyser Basin. Pocket Basin was originally an ice-dammed lake over a hydrothermal system. Melting ice during the last glacial period caused the lake to rapidly drain, causing a sudden change in pressure triggering a massive hydrothermal explosion.

Geysers

A hydrothermal explosion is similar to a geyser's eruption except that it includes surrounding rock and mud and does not occur periodically. One well-known hydrothermal geyser is Old Faithful which throws up plumes of steam and water approximately every hour and a half on average. Rarely has any steam explosion violently hurled water and rock thousands of feet above the ground; however in

Yellowstone Yellowstone National Park is an American national park located in the western United States, largely in the northwest corner of Wyoming and extending into Montana and Idaho. It was established by the 42nd U.S. Congress with the Yellow ...

’s geological history these colossal events have been recorded numerous times and have been found to have created new hills and shaped parts of the landscape. The largest hydrothermal explosion ever documented was located near the northern edge of

Yellowstone Lake Yellowstone Lake is the largest body of water in Yellowstone National Park. The lake is above sea level and covers with of shoreline. While the average depth of the lake is , its greatest depth is at least . Yellowstone Lake is the largest fre ...

, on an embankment commonly known as “Mary Bay”. Now consisting of a 1.5 mile crater, it was formed relatively recently, approximately 13,800 years ago. It is believed this crater was formed by a sequence of several hydrothermal explosions in a short time. What triggered this series of events has not yet been clearly established, but

volcanologist A volcanologist, or volcano scientist, is a geologist who focuses on understanding the formation and eruptive activity of volcanoes. Volcanologists frequently visit volcanoes, sometimes active ones, to observe and monitor volcanic eruptions, col ...

s believe a large earthquake could have played a role by accelerating the melting of nearby glaciers and thus depressurizing the hydrothermal system. Alternatively, rapid changes in the level of Yellowstone Lake may have been responsible.

Recent explosions

Most of Yellowstone's recent large hydrothermal explosions have been the consequence of sudden changes of pressure deep within the hydrothermal system. Generally, these larger explosions have created craters in a north-south pattern (between Norris and

Mammoth Hot Springs Mammoth Hot Springs is a large complex of hot springs on a hill of travertine in Yellowstone National Park adjacent to Fort Yellowstone and the Mammoth Hot Springs Historic District. It was created over thousands of years as hot water from the ...

). It is estimated that all of the known hydrothermal craters were created between 14,000 and 3,000 years ago. Volcanologists believe no magma has ever broken through the fragile crust of Yellowstone Park or stirred the movement of magma in the reservoir beneath Yellowstone.

References

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External links