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The 2003 bulge beneath Yellowstone Lake

High-resolution bathymetric relief map of Yellowstone Lake, acquire...
High-resolution bathymetric relief map of Yellowstone Lake, acquired by multibeam sonar imaging and seismic mapping, surrounded by colored geologic map of the area around Yellowstone Lake. (Public domain.)

Articles in the press in 2003 reported a "bulge" beneath Yellowstone Lake and have generated some concern about possible dangers for residents or visitors to the area. Below, we answer some questions we've been asked by the public and press.


Is there a bulge beneath the lake?

Mapping of the lake bottom has revealed a variety of faults, hot springs and craters beneath Yellowstone Lake. In a scientific report by Morgan et al., 2003, one feature was informally named the "inflated plain" by USGS researcher Lisa Morgan, who organized surveys of the lake beginning in 1999. In mapping the entire lake, she and her colleagues identified a region about 2,000 feet long that rises about 100 feet above the lake floor. The area is in the northern part of Yellowstone Lake, south-southwest of Storm Point. The area is home to many hot springs and the nearby sediments have undergone chemical changes (alteration) due to the flow of thermal water.

Why was it called the "inflated plain"?

Seismic images of the lake sediments in this area show that they were tilted, hinting that the region may have been pushed up or "inflated." The amount of inflation would be much less than the 100-foot height of the feature, but is currently unknown. The images appear to indicate that the uplift is associated with accumulation of gas from Yellowstone's hydrothermal (hot water) system. Similar inferred gas accumulations were also noted elsewhere within the lake. Future research will assess the amount of uplift and its origin, whether by gas buildup or other potential mechanisms.

Has the "inflated plain" been growing?

At present, there is no evidence of recent growth of any features beneath the lake, and there is no indication that residents or visitors are in any danger. Temperature measurements from hydrothermal vents taken this year indicate no change in temperatures compared to those taken last year. The feature may have been there for decades or much longer.

So what's the big deal?

There may be none. This region has active hydrothermal features, and possibly some uplift. It's possible that the area could host future hydrothermal explosions, but so could other areas beneath the lake and other areas within the Park.

What's a hydrothermal explosion?

Hydrothermal explosions occur when water that feeds Yellowstone's geysers and hot springs is explosively flashed to steam, breaking rocks and throwing them into the air. Small hydrothermal explosion events occur every few years at Yellowstone, mostly in the geyser basins, and usually pose little hazard. There is geologic evidence for a few large hydrothermal explosions, some leaving craters thousands of feet across near Yellowstone Lake and in other areas of the park. Such large explosions have not occurred within the last several thousand years. Two classic papers discuss evidence for hydrothermal explosions at Yellowstone, both in the geyser basins (Muffler et al., 1971) and beneath Mary Bay in Yellowstone Lake (Wold et al., 1977).

Do any of the features beneath the lake relate to possible volcanic eruptions?

It is very unlikely. All active features are related to faults and hot water (hydrothermal) vents. Identified craters were formed by collapse or as a result of old hydrothermal explosions. Many of the rocks beneath the lake are lava flows more than 100,000 years old.

Is there any possibility of a toxic gas emission from Lake Yellowstone as occurred in Africa some years ago?

No. What occurred in Cameroon (Lake Nyos in 1986) resulted from CO2 buildup in the bottom waters of a tropical lake. In warm climates, lakes easily become stratified. In cold climates, however, the waters of lakes "turn over" once or twice per year. Cold water from melting ice sinks to the bottom of the lake. This creates a well-mixed lake with minimal potential for gas buildup. In addition, the CO2 at Lake Nyos was cold and not associated with thermal water. Waters and gases from the hot vents beneath Lake Yellowstone will tend to rise through the lake due to their low density.

Is anybody assessing the hazards from new hydrothermal explosions?

A geologic hazard assessment of Yellowstone National Park is underway by YVO. The assessment will evaluate the known and potential hazards at Yellowstone from earthquakes, volcanic eruptions and hydrothermal explosions. Any new information on Yellowstone Lake will be made available to YNP officials and the general public.