Commentary

Sediment clues reveal Yellowstone Lake experienced hydrothermal explosions over last 13,000 years

Thanks to the tell-tale mineralogy of the altered sediments associated with these explosions, we know the conditions under which they occurred, write guest columnists Pat Shanks and Lisa Morgan.

August 10, 2022 4:10 am
Yellowstone Lake

The geology below Yellowstone Lake is shaped by lava flows where the lake lies within the caldera and is also shaped by glacial and other processes in the southern half of the lake that lies outside of the caldera. (Neal Herbert/National Park Service)

Northern and western Yellowstone Lake is within the Yellowstone Caldera, sometimes dubbed Yellowstone’s supervolcano, and covers the magma chamber deep beneath the surface.

This results in extremely active lake-bottom hydrothermal areas with very high heat flow values, multiple large (more than 330 feet in diameter) hydrothermal explosion craters, dozens of hydrothermal domes and hundreds of active and inactive hot spring vents on the lake floor. Hydrothermally altered mud on the bottom of the lake provides information on the types of fluids that are circulating beneath the lake floor and on the conditions in which these fluids cause chemical alteration of the lake sediment.

Recent research that examined sediment cores collected from in and around Yellowstone Lake provides new insight into the dynamics of hydrothermal explosions in Yellowstone Lake, as well as the relation between types of hot springs on the lake floor, characteristic hydrothermal alteration and the magnitude of hydrothermal explosions.

Lake-floor hydrothermal systems are different than those exposed on land in some important ways.

Hot springs ascend from depth and vent on the lake floor through sediments that are composed of diatoms (single-celled algae that have a cell wall of non-crystalline silica-SiO2) and fine-grained sediment transported by streams to the lake (mostly rock fragments, including obsidian, and also mineral grains of quartz and feldspar). The diatoms interact with hot spring fluids and commonly show evidence that while some portions have been dissolved, other minerals have formed by reactions with the hot water. Resulting hydrothermal alteration mineralogy varies depending on the type of hydrothermal fluid doing the alteration.

Yellowstone sediment sample
This is a section from the piston core YL92-1C, collected in south-central Yellowstone Lake. The core is viewed horizontally, with the top of core to the left. The core section shown is from 17–18.4 feet deep. The 0.2 inch-thick white Mazama ash, from the eruption that resulted in the formation of Crater Lake in Oregon, is clearly identifiable and shows it’s about 7,700 years old. About 11 inches below the Mazama ash is a 2-inch thick deposit of the Elliott’s Crater hydrothermal explosion (about 8,000 years old) where hydrothermally altered minerals including quartz and clay minerals chlorite and smectite are found. (Courtesy of the Yellowstone Caldera Chronicles)

 

Two main types of hydrothermal vent fluids are known in Yellowstone Lake: liquid hot springs, which discharge neutral-to-alkaline (non-acidic) waters, are called alkaline-chloride fluids, while vapor-dominated hot springs are acidic and gaseous, containing steam, carbon dioxide and hydrogen sulfide.

Alkaline-chloride fluids carry significant amounts of dissolved chemicals, including chloride and silica; these are similar to the hydrothermal fluids that feed iconic features on land, like Old Faithful and Grand Prismatic Spring. Vapor-dominated fluids carry substantial heat but no dissolved chemicals. As these two types of fluids rise to the lake floor through the sediments, the alteration they cause in that sediment is quite different. Vapor-dominated fluids dissolve diatoms and alter sediments to clay minerals, especially kaolinite, and an aluminum oxide mineral called boehmite. Alkaline-chloride fluids also can dissolve diatoms but eventually deposit silica as quartz or chalcedony and produce clay minerals like smectites or chlorites, but not kaolinite or boehmite.

Thus, the different types of hydrothermal systems on the lake floor leave mineral signatures that allow us to tell them apart — even if the hydrothermal activity has long since ceased.

This is important because studies of sediments that were altered in a pre-explosion hydrothermal system can be related back to the type of fluids that were present in the past.

Recent studies of lake sediments found multiple hydrothermal explosion deposits dating from about 160 to 13,000 years ago.

The two largest and most extensive deposits are related to the 8,000-year-old Elliott’s Crater and 13,000-year-old Mary Bay explosions. These formed from alkaline-chloride hydrothermal systems based on the alteration mineralogy found in the associated lake sediment, which included silica, smectites and chlorites, but not kaolinite.

Other explosion deposits are younger, thinner, less extensive and contain kaolinite, indicating that they were related to vapor-dominated hydrothermal systems.

This mineral stability diagram shows minerals that are stable under changing temperature and dissolved silica concentrations that are found at and just beneath the floor of Yellowstone Lake. Two important points illustrated by this diagram are: (1) the minerals that are stable when reacted with vapor-dominated fluids (kaolinite, boehmite) differ substantially from those that are stable when reacted with alkaline-chloride fluids (quartz, chlorite, smectite), and (2) pre-explosion alteration by alkaline-chloride fluids occurs in a temperature range from 257–572 degrees and at silica concentrations higher than those in the vapor-dominated systems. (Courtesy of the Yellowstone Caldera Chronicles)

 

Laboratory studies of the energy budget of hydrothermal explosions indicate that different fluid types produce explosions of different strength. Alkaline-chloride fluids flashing from liquid to steam in the Elliott’s Crater and Mary Bay systems produced two of the largest hydrothermal explosion craters and deposits known on Earth. Vapor-dominated systems in Yellowstone Lake explode by simple expansion of the steam phase, which releases about 10 times less energy than the liquid-to-steam explosions and therefore produces less voluminous and less extensive deposits.

There are no recorded observations of large explosions from hydrothermal systems beneath and around Yellowstone Lake. But thanks to the tell-tale mineralogy of the altered sediments associated with these explosions, we know the conditions under which they occurred — important information for understanding this underappreciated geologic hazard in Yellowstone.

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Pat Shanks
Pat Shanks

W. C. Pat Shanks is a emeritus research geochemist at the U.S. Geological Survey, Yellowstone Volcano Observatory. He uses fluid geochemical analyses, geochemical reaction modeling, stable isotope studies and mineralogical/chemical studies to understand the evolution of hydrothermal solutions, hydrothermal alteration of host rocks and deposition of mineral deposits.

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Lisa Morgan
Lisa Morgan

Lisa Morgan is a scientist emeritus, and former senior research geologist, for the U.S. Geological Survey. Her focus is on the geology and geophysics of volcanic terrains. She has studied the Yellowstone hot spot and Yellowstone Lake for more than 20 years.

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