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Interesting facts about the mirabilite spring mounds near Great Salt Lake Marina

What are these white mounds and when did they form?

In late October 2019, as air temperatures started to cool to near freezing, unique mineral mounds began forming on the south shore of Great Salt Lake, just east of the Great Salt Lake Marina. They are not composed of common table salt (sodium chloride—NaCl), but mirabilite (hydrated sodium sulfate—Na2SO4•10H2O), also known as Glauber’s salt.

How do mirabilite mounds form?

When the sodium-sulfate-rich spring water hits the cold winter air, mirabilite crystals form and build up a collection of small terraces, similar in appearance to the travertine rimstone and dam terraces that form at Mammoth Hot Springs in Yellowstone National Park.

What conditions are needed for the mounds to form?

In the 1940’s, researchers investigated this area and reported finding a 3-6 foot thick mirabilite layer 30 inches down in the subsurface. Groundwater seems to be partially dissolving this mirabilite layer, which is then reprecipitated at the surface as the spring water emerges. Additionally, mirabilite is most stable and precipitates in sub-freezing dry environments. When temperatures rise above freezing, the impressive clear mirabilite crystals that form the mounds will dehydrate to form a white, powdery, easily eroded mineral called thenardite (Na2SO4). Furthermore, the mounds will only form if the area (at about 4194 ft) is above lake level, a rare occurrence until the past few years.

Why are there multiple mounds?

It is hypothesized that as the mounds grow, they eventually seal off their spring water source, causing the groundwater to find a new pathway to the surface, and thus, a new mound begins to form a few dozen yards away. As of January 2020, the beach has four mounds that have grown up to 3 feet tall and several yards wide.
   

How rare are mirabilite mounds?

The mineral mirabilite is quite common and found in saline lakes around the world, including the north arm of Great Salt Lake. In saline lakes, mirabilite crystals form in the water column, float to the surface, and are washed ashore to form an amorphous, slushy slurry or dune-like accumulations of crystals. However, spring-fed crystalline mirabilite mounds are rare―they have never before been scientifically documented at Great Salt Lake. Mirabilite-precipitating springs and the formation of terraced mounds have been documented in the Canadian Arctic, central Spain, and the Antarctic. These features are ephemeral; warmer spring temperatures will eventually turn the mirabilite to the powdery, easily eroded thenardite, if the rising lake level doesn’t erode the mounds first. These features may or may not reform again next winter.


Is there a Mars connection?

While Mars has conditions conducive to the formation of mirabilite (dry, cold, etc.), the mineral has not yet been documented there. Mars does have topographic mounds that some researchers believe may be related to saline groundwaters. In addition, orbital spectrometers have suggested the presence of sulfates on the surface. Because of these similarities, some researchers are interested in the growth of mirabilite mounds on Earth because they may serve as analogues for understanding geologic processes on Mars.


Don’t tread on me.

These mirabilite mounds are fragile and researchers are still actively studying them. Please do not walk on the mounds or collect samples. Besides, mirabilite is unstable at temperatures above freezing and dehydrates to form a white powder (thenardite), so the samples would not last more than a day.

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RARE MIRABILITE FORMATIONS FOUND ON GREAT SALT LAKE SHORELINE

One of the mirabilite mounds hanging over the water of the Great Salt Lake.

GREAT SALT LAKE STATE PARK — Park Rangers and Geologists have discovered four rare formations on the south shore of the Great Salt Lake – just north of the park and marina. Ranger Allison Thompson first noticed the salt formations in October 2019. As time passed, she noticed formations continue to grow and eventually she reached out to members of the Utah Geological Survey for their help.

After investigating, staff discovered that the mounds were, in fact, Glauber’s salt, also known as “mirabilite.” Researchers are particularly interested in the precipitation of mirabilite mounds because they may serve as analogues to similar features and conditions on Mars.

Mirabilite mounds are rare and have never before been documented at the Great Salt Lake. They have only been found at a few locations around the world – primarily in the Arctic.

Geologists have determined that the mirabilite is precipitating from warm, high salinity sulfate-rich springs, visible only when the lake level falls below an elevation of 4194 feet. A closer inspection of the mounds revealed that they are a built-up collection of crystallized terraces, similar in appearance to the travertine rimstone and dam terraces that form at Mammoth Hot Springs in Yellowstone National Park.

Water from the saline spring flowing through the top of a mirabilite mound.

As the mounds at the Great Salt Lake grow, they eventually seal off their spring source and a new mound will begin to form a few dozen yards away. Currently, the beach has four mounds that have grown up to 3 feet tall and several yards wide.

In addition to the requirement of saline spring waters with very specific chemical composition, mirabilite is only found in sub-freezing environments, such as those found in the polar regions of Earth.

When temperatures rise above freezing, the impressive clear mirabilite crystals that form the mounds at the Great Salt Lake will dehydrate to form a white powdery mineral call thenardite (NaSO4). As such, these mirabilite mounds will disappear with changing temperatures and the rising lake levels.

Staff at Great Salt Lake State Park will be offering guided tours of the mirabilite mounds to the public. These tours will be available this coming Saturday and Sunday (January 11-12, 2020) and will be available every half-hour each day between the 10 a.m. and 4 p.m. Those attending a guided tour are advised to wear water-resistant or waterproof boots as the lake water and mud can rise to mid-calf.

As research into these mounds continues, we are asking that members of the public do not damage these mounds or intentionally remove pieces of them.

An above view of one of the mirabilite mounds with a notebook shown for scale.

The four mirabilite mounds along the Great Salt Lake shoreline with the State Park gift shop in the background.

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