The Underwater Castles of Yellowstone

Two national park rangers wore insulated diving suits and carried flashlights while swimming through frigid and murky Yellowstone Lake last summer. They were in search of an apparition that belonged on another planet.

And then they were upon it: a forest of stone spires, some tall and narrow like tree trunks and others wide enough to resemble the turrets of a castle, all rising straight out of the otherwise monotonous lake floor.

It was a submerged province of Yellowstone National Park that University of Wisconsin scientists using sophisticated sonar and a remote-controlled submarine had happened onto for the first time earlier in the year.

The rangers took to the water to inspect the discovery, glimpsing 10 to 15 spires in the 40 minutes they spent in the water that warms to no more than 39 degrees in midsummer.

“You’re swimming along and all of a sudden, boom, there are the spires coming out of the muddy bottom,” said ranger Paul Anzalone. “It’s amazing that no one ever found them before.”

The eerie legion of towers as tall as 30 feet has stretched the theories of researchers who have spent decades understanding how Yellowstone’s steaming hot springs and geysers deposit minerals to form geyser cones and terraces on land.

It makes sense that mineral-laden hot water flowing out of the ground will leave some of its contents when it contacts cooler air, just as crusty scale forms around a bathroom faucet. But it’s more puzzling to see how such minerals, if emerging dissolved from the lake bottom, could have amassed into stone columns underwater instead of dissipating in the lake like cream in coffee.

Such large formations have never before been found in fresh water and are taller than most similar formations on dry land. Some scientists compare the Yellowstone spires in form to much larger, dark chimneys, known as “black smokers,” that develop where hot springs discharge into the bottom of the sea.

“These things are really a mystery right now,” said Pat Shanks of the U.S. Geological Survey, who has long studied the ocean floor vents. “They are just a whole new breed of formation.”

In examining a crumbly sample of the spires collected by the park rangers, researchers found that it so closely resembled geyser deposits on dry land that they initially wondered whether the spires could have formed on land only to later sink under water.

But even if they ever did stand dry, geologist Kenneth Pierce of the U.S. Geological Survey doubts that the formations could have gradually dropped beneath lake waters at a slow geological pace without waves and thick winter ice eroding and breaking them to bits.

Although evidence trickling in appears to indicate that the spires did form underwater, scientists still have a hard time explaining exactly how. The ocean’s gushing “black smokers” operate under extreme pressures that prevent water from boiling even when its temperature rises to 500 degrees Fahrenheit or more, far hotter than any temperatures in Yellowstone. The intense heat keeps lots of minerals dissolved only until the scalding discharges hit icy cold water at the ocean bottom. Just as Jell-O jells more rapidly the more quickly it’s cooled, the minerals emerging into the ocean turn solid in a flash, forming dark chimneys standing 100 feet high or taller.

Because temperatures in Yellowstone do not rise much higher than the local boiling point of 199.4 degrees, the sharp division of temperatures found at the sea bottom does not exist. In other words, it doesn’t seem like the Jell-O would cool fast enough to jell before it floats off in the lake like your steamy breath on a cold winter day.

The Mystery of the Spires

The same University of Wisconsin team that discovered the spires has found small mineral pipes no wider than household plumbing and less than a foot long, reaching like bony fingers out of hot spots on the lake bottom. They believe that the pipes formed as hot water flowed through channels in the lake sediments; minerals in the water apparently came to line the channels and changing currents later washed the sediments away, leaving only the mineral lining exposed. Because the pipes have sediments cemented into them, so should the spires if they had formed the same way.

But they do not, again suggesting that the spires formed freely within the lake water.

“We’re having to rethink our basic understanding of these kinds of structures and how they develop,” said Val Klump of the University of Wisconsin at Milwaukee’s Center for Great Lakes Studies, one of the teams that has been surveying Yellowstone Lake with a remotely operated submarine for more than five years.

Using X-rays and scanning electron microscopes, scientists have found that the spires are made mainly of silica--the same mineral deposited around active geysers and springs--but in places loosely connected, like popcorn coated in caramel. They hold high concentrations of elements such as arsenic, which is common in many Yellowstone springs.

Certain isotopes of oxygen are present at levels suggesting that the minerals forming the spires solidified at temperatures of 149 to 194 degrees Fahrenheit. Traditional carbon dating works only with materials containing carbon; scientists are instead trying to discern the age of the spires through more complicated uranium dating methods.

And they are using their imaginations to piece together how the seemingly unimaginable underwater spires came to be. New calculations by Shanks suggest that once a small mineral nozzle forms around a lake bottom vent, it might control warm, seeping outflows enough that in calm conditions they could cool and deposit silica before mixing with lake water.

Although the formations do not appear to be actively spewing water, the flow might be too slow to be obvious. Over hundreds or thousands of years, spires might slowly appear. Supported by the surrounding water, they could grow higher than similar formations on land.

Scientist Dave Lovalvo, who piloted the submarine, said guiding the craft among the spires was like flying a helicopter among skyscrapers. The submarine’s camera eye revealed strange living sponges and worms hanging onto the towers.

“It may be something like a coral reef, where it supports its own special ecosystem,” Lovalvo said.

One spire has toppled over and geologists are now developing plans to hoist it to the surface next summer for study. Rangers are laying out “no anchor zones” to protect the pinnacles.

That the spires, lying only a stone’s throw from a busy marina on Yellowstone Lake’s northern shore, have remained hidden until now is a tantalizing hint that the largest high-altitude lake on the continent may yet have other secrets.

“It would be nice to be able to tell park visitors how these things came to be there and how special this place really is,” Anzalone said. “For all we know, they may be the only ones like this in the world.”