Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Pat Shanks and Lisa Morgan, Scientists Emeriti with the U.S. Geological Survey.

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Northern and western Yellowstone Lake is within the Yellowstone Caldera and overlies the magma chamber deep beneath the surface.  This results in extremely active lake-bottom hydrothermal areas with very high heat flow values, multiple large (>100 m [330 ft] diameter) hydrothermal explosion craters, dozens of hydrothermal domes, and hundreds of active and currently inactive hot spring vents on the lake floor.  Hydrothermally altered muds on the bottom of the lake provide information on the types of fluids that are circulating beneath the lake floor, and 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).  Diatoms interact with hot-spring fluids and commonly show evidence that while 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.

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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 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.

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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.

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!