Steam Explosion

The large magma reservoir beneath Yellowstone may have temperatures higher than 1,475°F (800°C), and the surrounding rocks are heated by it. Because of this, the average heat flow from the Earth’s interior at Yellowstone is about 30 times greater than that typical for areas elsewhere in the northern Rocky Mountains. As snowmelt and rainfall seep deep into the ground, they can absorb enough of this heat to raise the temperature of the ground water close to the boiling point. Geyser basins and other thermal areas in Yellowstone National Park are places where hot ground water has risen close to the surface. Research drilling at Yellowstone in the 1960s confirmed that the ground water beneath many of the park’s thermal areas is very hot. At Norris Geyser Basin, water temperatures as high as 460°F (238°C) were recorded at depths of only 1,090 feet (332 m).

Because the boiling point of water increases with increasing pressure and pressure increases with depth, deep water can be hotter than boiling water near the surface. If the pressure that confines this deep water is reduced quickly, pockets of water may suddenly boil, causing an explosion as the water is converted to steam. Such activity drives the eruptions of geysers, like Old Faithful, which are repetitive releases of plumes of steam and water. Rarely, steam explosions are more violent and can hurl water and rock thousands of feet. In Yellowstone’s geologic past, such violent events, called “hydrothermal explosions,” have occurred countless times, creating new landscapes of hills and craters.

A recent and notable hydrothermal explosion occurred in 1989 at Porkchop Geyser in Norris Geyser Basin. The remains of this explosion are still clearly visible today as an apron of rock debris 15 feet (5 m) across surrounding Porkchop’s central spring. In the 1880s and early 1890s, a series of powerful hydrothermal explosions and geyser eruptions occurred at Excelsior Geyser in the Midway Geyser Basin. Some of the explosions hurled large rocks as far as 50 feet (15 m).

Much larger hydrothermal explosions have occurred at Yellowstone in the recent geologic past. More than a dozen large hydrothermal-explosion craters formed between about 14,000 and 3,000 years ago, triggered by sudden changes in pressure of the hydrothermal system. Most of these craters are within the Yellowstone Caldera or along a north-south-trending zone between Norris and Mammoth Hot Springs.

The largest hydrothermal-explosion crater documented in the world is along the north edge of Yellowstone Lake in an embayment known as Mary Bay. This 1.5-mile (2.6 km)-diameter crater formed about 13,800 years ago and may have had several separate explosions in a short time interval. What specifically triggered these very large events is not firmly established, but earthquakes or a pressure release caused by melting glaciers or rapid changes in lake level may have been a significant factor.

These very large and violent hydrothermal explosions are independent of associated volcanism. None of the large hydrothermal events of the past 16,000 years has been followed by an eruption of magma. The deeper magma system appears to be unaffected even by spectacular steam explosions and crater excavations within the overlying hydrothermal system.

Although large hydrothermal explosions are a feature of Yellowstone’s recent geologic history, most explosions in historical times have been relatively small and have left craters at most a few yards across. For example, in early 2003, a long linear fissure appeared on a hillside above Nymph Lake, north of Norris Geyser Basin, venting steam and throwing bits of rock onto the surrounding hillside. Although most hydrothermal explosions in the park are small, their remains can be noticed by observant visitors and attest to the nearly continuous geologic activity at Yellowstone.

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