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Small hydrothermal explosions—steam blasts—are common at Yellowstone, occurring every year or two.  Most happen in the backcountry and are not observed by people.  In 1989, however, Porkchop Geyser blew up right in front of several observers on an otherwise sunny September afternoon.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Michael Poland, geophysicist with the U.S. Geological Survey and Scientist-in-Charge of the Yellowstone Volcano Observatory.

Porkchop Geyser erupting in August 1989
Porkchop Geyser erupting in August 1989.

Norris Geyser Basin is one of the most unique parts of Yellowstone National Park.  The basin hosts both acidic and neutral hot springs—a somewhat unusual combination—is the location of some of the hottest temperatures recorded in Yellowstone, and is home to both the largest acidic geyser in the world (Echinus) and the tallest geyser in the world (Steamboat).  And in 1989, it was the site of a very well-documented hydrothermal explosion.

Before 1985, Porkchop Geyser was a small pool, about 3 m (10 ft) across and shaped, not surprisingly, like a porkchop.  Eruptive activity was irregular, sometimes with years of quiescence.  When small eruptions did occur, they were generally a few meters (yards) in height and caused the pool to empty.  The eruptions came from a triangular-shaped opening only a few centimeters (1-2 inches) in size.  This vent acted like a throttle, keeping the geyser eruptions from becoming too large.

Ice cone at Porkchop Geyser in March 1989
Ice cone at Porkchop Geyser in March 1989.

Geochemists first sampled the waters of Porkchop Geyser in 1920s, and then again in 1951.  Starting in 1960, after the M7.3 Hebgen Lake earthquake of the previous year, the waters were sampled every few years to look for changes that might have resulted from shaking of the fragile geyser plumbing by the earthquake.  The chemistry of the water can reveal variations in subsurface conditions, including the temperature of the water source. 

The samples from the 1920s and 1951 revealed a steady temperature over that time, but this declined in the years following the Hebgen Lake earthquake, and the geyser eruptions stopped.  Starting in 1962, however, the source temperature for Porkchop Geyser began to increase, and by 1971 sufficient thermal energy was available to sustain small, irregular eruptions once again.  These small eruptions increased in frequency over time, and by the late 1970s the source temperature had risen above that measured in the 1920s.  And it was still increasing.

In late March 1985, the activity at Porkchop Geyser changed markedly.  Instead of irregular eruptions through a pool, Porkchop Geyser became a perpetual spouter with eruptions 6-9 m (20-30 ft) high from a mostly dry crater.  This phase of activity lasted for more than 4 years.  Sometimes, the velocity of the geyser discharge was great enough to produce a roaring sound that could be heard 2 km (over 1 mile) away!  In winter, the erupted water was dispersed into a fine spray that formed ice cones more than 7 m (23 ft) high.

Comparison of Porkchop Geyser in September 1984 (left) and August 1986 (right)
Comparison of Porkchop Geyser in September 1984 (left) and August 1986 (right).  The Geyser entered a period of near-constant eruption in 1985, and the pool disappeared; it exploded on September 5, 1989.

On September 5, 1989, at 2:40 PM local time, eight visitors to Yellowstone National Park watched as the eruptions of Porkchop Geyser suddenly increased to heights of 20–30 m (65–100 ft).  Immediately thereafter, the silica sinter surrounding the geyser was thrown into the air as the geyser exploded.  It was over in seconds, but during that period rocks over 1 m (3 ft) in size were uprooted, and some smaller material was thrown a distance of over 60 m (200 ft) from the vent.  The explosion left a crater over 10 m (30 ft) across that was surrounded by a rim of jumbled blocks ejected during the explosion.  And the porkchop shape was no more.

Porkchop Geyser erupted explosively in 1989, forming a 5-m-diameter...
Porkchop Geyser (light blue pool) as seen from a radio-controlled camera that was attached to a helium-filled balloon.  The pool is surrounded by debris that was ejected during a small explosion on September 5, 1989.  USGS photo by Brita Graham Wall, September 2005.

The sudden increase in the height of the spouting before the explosion probably indicated that the vent “throttle” failed, which allowed for sudden decompression and rapid expansion of the thermal waters just beneath the surface.  This expansion is what caused the explosion.  After, the crater filled with near-boiling water and has remained mostly quiet, except for some minor roiling eruptions that occurred in 2003 at the same time as some major changes to Norris Geyser Basin.

Fortunately, no one was injured by the blast from Porkchop Geyser, but the event illustrates one of the most significant yet least appreciated hazards in Yellowstone: hydrothermal explosions.  Even small explosions can be harmful to anyone standing nearby.  These sorts of events usually happen in the backcountry and are not witnessed by people, but occasionally they occur in developed areas—like the 2018 explosion of Ear Spring, near Old Faithful, which, in addition to rocks, ejected several decades worth of garbage that had fallen or was thrown into the pool.  This is one of the reasons visitors are advised to stay on boardwalks and keep a safe distance from thermal features.

Today, geochemists regularly sample and collect data from many pools in Yellowstone National Park, including Porkchop Geyser.  Yellowstone’s hydrothermal system is incredibly dynamic, and tracking changes over time is an important window to understanding just what is happening with all that hot water just beneath the surface!

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