Photograph of north and eastern rim of the 9,400-year-old Turbid Lake explosion crater showing the primary explosion ejecta rim with a secondary explosion ejecta rim inside the lake-occupied explosion crater. Many, if not most, larger explosion craters have multiple explosion histories and are long-lived hydrothermal systems/USGS, Lisa Morgan

Editor's note: Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Lisa Morgan, emeritus research geologist with the U.S. Geological Survey.

Although Yellowstone Lake itself may seem calm, the floor of the lake is littered with hydrothermal explosion craters. Studies are beginning to reveal the details of these explosions, like the one that formed Elliott’s Crater about 8000 years ago.

Hydrothermal explosions are considered one of the most serious natural hazards in Yellowstone.  On land, hydrothermal explosion craters, especially large ones, hundreds of meters in diameter, are easily identified.  

Within the Yellowstone Caldera, their bowl-shaped depressions are commonly filled with lakes surrounded by raised rims composed of explosion breccia ejecta.  Well-known explosion craters in Yellowstone include Pocket Basin in Lower Geyser Basin, and Indian Pond, Turbid Lake and Mary Bay craters north of Yellowstone Lake.  Their explosions occurred thousands of years ago and were extreme events where boiling water, steam, hot mud, and rock fragments up to several meters in diameter were ejected hundreds of meters above the crater and rained down on the landscape. Visitors can see those craters today, but what about those craters that can’t be easily seen beneath the waters of Yellowstone Lake?

Elliott’s Crater is one of multiple explosion craters on the floor of Yellowstone Lake, a beautiful, serene-looking large body of water that straddles the southeast margin of the Yellowstone Caldera. Of the many fascinating discoveries that have been made in Yellowstone Lake, identification of hydrothermal explosion craters is one of the more significant.  Until a detailed bathymetric mapping campaign coupled with collection of seismic reflection profiles in 1999-2003, the large (838-meter-diameter, 0.536 km2) Elliott’s Crater hydrothermal explosion crater in the northern part of the lake was unknown. 

The crater, named after Henry Elliott, the first to conduct soundings on Yellowstone Lake as part of the 1871 Hayden Survey, is a large feature in the northern lake. Despite the crater being less than 2 kilometers south of the northern lake shoreline, no deposits from this explosion have been mapped on land, whereas hydrothermal explosion deposits from the 13,000-year-old Mary Bay, the 9,400-year-old Turbid Lake, and the 2,900-year-old Indian Pond explosion events are well-exposed. 

Recent piston coring efforts of the lake floor, done as part of the National Science Foundation-funded, and USGS- and National Park Service-supported Hydrothermal Dynamics of Yellowstone Lake project, have identified a set of fining-upward sedimentary sequences tens of centimeters below the Mazama ash (which was deposited when Crater Lake formed in Oregon 7,600 years ago).  This fining-upward sequence is interpreted as the hydrothermal explosion breccia from Elliott’s Crater, based on the deposit’s distinct hydrothermal chemical signature and physical properties relative to other lake sediments, as well as its distinct hydrothermally altered rock composition and age.  Material from the explosion deposit is extensively altered, indicating that pervasive hydrothermal activity occurred at this site prior to the explosion.

Swath bathymetric image of the Elliott’s Crater explosion crater in Yellowstone Lake. Inset shows location of the crater and the swath image (red box) within the northern part of the lake/Public domain.

The age of the Elliott’s Crater explosion event originally was estimated to be 8,000 years based on assumed northern lake sedimentation rates (1 meter per 1,000 years) and thickness of deposits in the crater from seismic reflection profiles. In the sediment cores, stratigraphic position combined with new radiocarbon ages also support an age close to 8,000 years.

Composition, mineralogy, thickness, and distribution of the Elliott’s Crater explosion deposit prove that it originated at Elliott’s Crater, and physical sorting of the deposit indicates the explosion was deposited in water. This, plus studies of terraces north of the lake, indicate the explosion occurred under water in about 20-meter water depth.

A detailed examination of the deposit reveals that the hydrothermal explosion generated at least three pulses, separated by 40-160 years. The first event produced the thickest and most coarse-grained deposit, showing that it was the most intense explosion.

Each subsequent explosion decreased in intensity as reflected by decreasing clast size and thickness of the primary deposit. Based on the distribution and cumulative thickness of the Elliott’s Crater explosion deposit, debris from the explosion was directed south and somewhat west of the source crater. Crater rim heights vary from north to south by ~6-10 meters and suggest an ejection angle that directed most of the deposit to the south. This explains why no exposures of the Elliott’s Crater deposit are along the northern shore of Yellowstone or along the eastern or western shorelines.

Today, Elliott’s Crater remains hydrothermally active. In fact, Elliott’s Crater contains several smaller craters on the western and southern areas of the crater floor. Recent imaging and sampling of active, hot hydrothermal vents inside and along the edge of Elliott’s Crater indicate this feature has been active for over 8,000 years and will continue to be active into the future. 

Exploration and discovery in Yellowstone Lake opens our eyes to previously unknown hydrothermal activity, and the lake sediments preserve features that cannot be studied by other means, allowing us to understand the processes that create these events.