# New report describes Yellowstone's volcanic and hydrothermal hazards

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A major goal of the Yellowstone Volcano Observatory, when it was created in 2001, was a first-ever assessment of the hazards of Yellowstone's massive volcanic and hydrothermal system. Two months ago, scientists completed a preliminary hazard report.

Porkchop Geyser, in the Norris Geyser Basin, formed by a hydrothermal explosion in September 1989 (Fournier and others, 1991). Blocks of silica sinter thrown from the geyser's throat form a berm around the new crater. The silica will likely continue to dry out and disaggregate over time, subduing evidence of the explosive activity. Photo by Jim Peaco, 2003 (NPS stock photo).

(Public domain.)

The 94-page report brings together a wealth of information and observations that provide a complete picture of the range of volcanic and hydrothermal activity that can occur over time.

The report is based on the history of past eruptions and hydrothermal explosions, as determined from decades of geologic studies and historical observations. These include small hydrothermal explosions and accidental exposure to high concentrations of carbon dioxide and hydrogen sulfide gas in certain areas of Yellowstone National Park.

There is a broad range of eruptive activity that can occur at Yellowstone, making it difficult to characterize future eruptions in terms of probabilities and discrete areas that can be affected.

For example, past eruptions range from three colossal explosive eruptions in the past 2.1 million years to dozens of outpourings of lava—on average, about every 12,000 to 38,000 years—in different parts of Yellowstone. No lava has erupted in the past 70,000 years, however, indicating that volcanic activity has slowed down.

The giant explosive eruptions created enormous calderas measuring 40 to 90 km (25 to 56 miles) in diameter; by comparison, Kīlauea caldera is 6 km (3.8 miles) in diameter. The eruptions buried thousands of square kilometers (miles) with thick layers of hot ash and pumice and sent a rain of ash over much of western and central North America.

The authors estimate the likelihood of a similar massive eruption, to be below the "threshold of useful calculation." Based on the eruptions of lava in the past 640,000 years, fluid basaltic lava erupts about every 16,000 years, and the highly viscous (sticky) rhyolite lava erupts every 20,000 to 38,000 years or so.

One of the most common hazards at Yellowstone, however, is shallow-rooted explosions of steam, water, and rock without any associated volcano activity. These hydrothermal explosions occur at depths less than a few hundred meters (yards) below the surface, violently breaking the overlying rocks and ejecting them from a newly formed or existing crater. The resulting craters range in size from less than a meter to a few kilometers in diameter.

Individual explosions can last a few seconds or as long as several hours, but explosive activity can continue intermittently for years.

The geologic record of hydrothermal explosions only exists for events in the past 16,000 years. The most recent glaciation erased evidence of earlier explosions. At least 18 large explosions that created craters larger than 100 m (100 yards) in diameter have been identified. Some craters were only recently discovered beneath Yellowstone Lake. Such explosions can hurl rocks and hot debris more than 2 km (1.2 miles) from the explosion site.

The report also lists the 26 small explosions that have been reported in various ways during the 126-year historical record of Yellowstone National Park. Other explosions probably occurred but were not observed.

On average, the large explosions can be expected to occur about every 200 years, whereas small rock-throwing explosions occur about every 2 years.

The authors also describe two volcanic gases of particular concern at Yellowstone: carbon dioxide and hydrogen sulfide. The concentrations of these gases are typically low and non-threatening, but they can build up in valleys, caves, and tunnels during windless conditions.

The most recent episode of toxic-gas inhalation occurred in March 2004, when five bison died along a section of the Gibbon River. The gases may have accumulated along the river during a brief time of cold and atmospheric inversion that occurred about a week before the animals were found.

This preliminary report will be followed in the next couple of years by a revised assessment, which will include a detailed evaluation of the earthquake activity of the greater Yellowstone area, one of the most seismically active in the United States.

The full report is available on-line from the home page of the Yellowstone Volcano Observatory.

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### Volcano Activity Update

This past week, activity levels at the summit of Kīlauea Volcano have remained at background levels. The summit caldera has been expanding, indicating inflation, since the beginning of 2007. The number of earthquakes located in the summit area is at low levels (usually fewer than 10 per day are large enough to locate). There have been local concentrations of earthquakes beneath the southeast edge of the summit caldera.

Eruptive activity at Puu Oo continues, though the level of activity seems somewhat subdued compared to the last few years. On clear nights, glow is visible from three vents within the eastern half of the crater. Lava is still flowing through the PKK lava tube, and unusual activity along the uppermost portion of the tube has built new hornitos over old skylights. The PKK tube may be carrying some lava to the top of Pulama pali, but most of the lava is apparently going into the Campout tube that branches off from the PKK tube about 1 km south of Puu Oo. The Campout tube carries lava to the ocean at Kamokuna located inside Hawaii Volcanoes National Park.

The flow field continued to host very little activity in the past week until Monday when surface flows were seen at the top of Pulama pali. On Thursday morning, three channelized aa flows could be identified. Only a few sluggish breakouts have been reported on the west side of the coastal plain but there has been scattered activity on the east side at the base of Royal Gardens subdivision. Surface flows were a little more than 700 m (0.4 mile) from the coast on Thursday. The laze plume created by the ocean entry at Kamokuna remains small, suggesting that little lava is entering the water.

Access to the sea cliff near the ocean entries is closed, due to significant hazards. The surrounding area, however, is open. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.

Three earthquakes beneath Hawaii Island were reported felt within the past week. A magnitude-2.9 earthquake occurred at 0:31 a.m. (H.s.t.) on Saturday, April 28, and was located 6 km (4 miles) northeast of Holualoa at a depth of 22 km (14 miles). A magnitude-2.2 earthquake occurred at 8:23 p.m. on Monday, April 30, and was located 6 km (4 miles) east of Kawaihae at a depth of 11 km (7 miles). A magnitude-3.5 earthquake occurred at 5:35 a.m. on Wednesday, May 2, and was located 3 km (2 miles) west of Ookala at a depth of 10 km (6 miles).

Mauna Loa is not erupting. One earthquake was located beneath the summit. Extension of distances between locations spanning the summit, indicating inflation, continues at steady, slow rates.