Yellowstone National Park: Ear Spring SPEWS hot water
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While the geology of Yellowstone National Park is most notorious for its potential to produce large “supereruptions”, far more common are smaller, violent hydrothermal explosions. Caused when near-boiling water suddenly flashed into steam, these events release large amounts of energy, fracturing the rock downwards and often forming craters. The same hydrothermal systems that cause these explosions are responsible for producing Yellowstone’s iconic fumaroles, hot springs and geysers — like Old Faithful. Unlike these features, however, hydrothermal explosion craters have been far less studied, despite the Yellowstone Lake area hosting at least eight large craters, including three of the largest of their type known on the Earth.
The new investigation was undertaken by geologist Dr Lisa Morgan of the US Geological Survey and her colleagues.
Dr Morgan said: “The hydrothermal system in Yellowstone is the largest in the world. Over 10,000 hydrothermal features are present.”
Yellowstone’s hydrothermal system is driven, she explained, “by high heat flow over a large area, by high precipitation rates, and by active seismicity and deformation.
“For this study, we wanted to know more about the recent geologic history of Yellowstone Lake and what role hydrothermal activity has had in the lake, especially the role of hydrothermal explosions and their triggering mechanisms.”
In their study, the team collected new sediment cores from across the northern portion of Yellowstone Lake and studied these alongside cores previously taken in the area.
The researchers correlated the deposits across the various cores and analysed their chemical and physical attributes, identifying those formed by hydrothermal explosions.
Dr Morgan said: “Hydrothermal explosion sediments deposited underwater have never been described in published literature.
“In analysing the cores, we made a lot of discoveries and had several surprises. Number one was how different the explosion deposits found in the cores looked from explosion deposits on land.
“That was to be expected since one was deposited through a water column and one was deposited on land.”
In total, the researchers found evidence within the cores for at least 16 deposits formed as a result of hydrothermal explosions.
Fourteen of these were small-scale, localised events, but the remainder were associated with two of Yellowstone’s largest hydrothermal explosion craters — that of the Mary Bay and Elliott’s craters.
The Mary Bay hydrothermal explosion occurred 13,000 years ago and resulted in a crater some 1.5 miles wide, part of which is submerged under the lake.
Land-based deposits from the Mary Bay explosion have been studied previously, the team explained — but the freshly analysed sediment cores from the lake show both that the extent of the explosion deposits are larger than previously thought and that the lake level must have been lower at the time of the explosion.
Based on their analysis, the researchers have concluded that the Mary Bay explosion was triggered by a sudden, 46 feet drop in the lake level caused by a seismic event.
This tremor, they added, also caused a tsunami that eroded Yellowstone Lake’s outlet waterway.
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The explosion that formed Elliot’s Crater, the team said, occurred some 8,000 years ago and left a 2,300-feet-wide hole in the ground.
Unlike that of Mary Bay, Elliot’s crater is fully submerged by the lake, although the team concluded that its deposits are also more widespread than was previously thought.
The researchers believe that Elliot’s crater formed when a seismic event fractured a sediment dome cap in the hydrothermal system — releasing the pockets of gas or gas-charged fluids trapped underneath, triggering a hydrothermal explosion.
The majority of the smaller deposits, the team noted, were formed by previously unknown and more recent hydrothermal explosions.
Consistent with the findings of previous studies, the team found no relationship between them and volcanic activity at Yellowstone.
Dr Morgan concluded: “Given what we see from Yellowstone Lake and elsewhere in Yellowstone, hydrothermal explosions of various scales will continue to occur.”
The full findings of the study were published in the journal GSA Bulletin.
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