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Caldera chronicles: Silver Gate: The Mammoth Terraces of yesteryear

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Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Michael Poland, geophysicist with the U.S. Geological Survey and scientist-in-charge of the Yellowstone Volcano Observatory.

A few miles south of Mammoth Hot Springs in Yellowstone National Park, Highway 89 winds through the white/gray jumble of rocks known as the Hoodoos, or Silver Gate, that formed when travertine from Terrace Mountain collapsed in a landslide.

Imagine you have just arrived at the North Entrance to Yellowstone National Park. You’ve stopped at Mammoth Hot Springs and walked along the boardwalks over the terraces, and you’ve marveled at the beautiful features that have formed as hot water deposited calcium carbonate rocks at the surface. You get back into your car and head south toward the park’s interior, but only a few minutes after leaving Mammoth you pass through a jumble of rocks. Informally known as “the Hoodoos,” the official designation for the area is Silver Gate, named by an early park visitor because of the way the road wound between the closely spaced white/gray boulders. What is the story behind this strange landscape?

Having just left Mammoth Hot Springs, you’ve already seen the modern version of Silver Gate. It’s an ancient hot spring deposit.

Thermal activity in Yellowstone comes in three flavors. Acid-sulfate springs form where acid gases separate from thermal waters, creating mudpots and fumaroles at the surface, like at Mud Volcano. Alkaline-chloride springs, on the other hand, have a basic pH and carry silica that is deposited as sinter and makes up the cones of geysers like Old Faithful. Finally, some hot spring waters carry calcium carbonate because the thermal waters pass through and dissolve carbonate-rich rocks that were deposited tens to hundreds of millions of years ago, when the Yellowstone area was covered by a shallow sea. These calcium carbonate, or travertine, springs are present in only a few places in Yellowstone National Park, most notably at Mammoth Hot Springs (Terrace Spring, near Madison Junction, is another example).

What does this have to do with Silver Gate? It turns out that the rocks that make up Silver Gate are also composed of calcium carbonate, but they aren’t marine rocks. Instead, they are from ancient, now-extinct travertine hot springs that probably once resembled the terraces of Mammoth Hot Springs.

Now that we know what Silver Gate is made of, how did it get to be where it is? The jumbled nature provides a clue.

Many geology classes that visit Yellowstone make a stop at Silver Gate, and the instructors challenge the students to interpret the deposit (to those instructors, we apologize for giving away the story if you haven’t yet taken your field trip). The students generally are quick to recognize that the rocks are calcium carbonate, and then they notice the jumble. The various boulders all have layers that point in different directions. It’s almost as if a giant dumped a bunch of rocks into a big pile in that spot. That’s not too far off from what actually happened — Silver Gate is a landslide.

Although it’s hard to tell from the road, looking across the valley from nearby Bunsen Peak provides a clear view of Silver Gate’s origin. The top of the ridge above Silver Gate, known as Terrace Mountain, is composed of travertine and resembles inactive parts of Mammoth Hot Springs, with a dusty white/gray appearance and a mantling of trees. Falling away from this ridge is a chaotic jumble of white/gray rocks. We don’t know when this landslide occurred, or if it is even just one landslide event, but it must have happened within the past 15,000 years, after the most recent ice age. Otherwise, it would have been scoured and erased by the glaciers that covered Yellowstone.

The story of Silver Gate is a testament to the dynamic nature of Yellowstone, where hot springs turn on and off, and where geological forces like landslides can alter the landscape in sudden and catastrophic ways. Truly a Wonderland.

This article originally ran on billingsgazette.com.

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