Montana Earth Science Picture of the Week

Rhinos in Nebraska? Geysers in Billings?

12.5 million years ago: A "supervolcano" erupts . . .
This photo shows one of the fossil rhinoceroses that can be found at an extraordinary fossil site in north-central Nebraska. The site, called "Ashfall," features about 100 rhinos, along with horses and other animals that died as result of a volcanic eruption in Idaho about 12.5 million years ago. The eruption produced a tremendous cloud of ash that descended onto a savanah that was located in the prehistoric Midwest. As the lungs of animals filled with ash, they sought relief at a waterhole where they eventually died and were covered with several feet of ash.

Geologists have recently determined that the age and chemical composition of the ash in Nebraska matches those of an ancient volcano (caldera) in southwest Idaho called Bruneau-Jarbridge (see map below). As it turns out, a huge plume of magma from Earth's mantle, called a "hot spot," caused the volcano. The hot spot has been erupting every 600,000 years or so as the North American plate moves over it, leaving a trail of calderas across southern Idaho. Since its first known eruption 16.5 million years ago, the hot spot has blown about 100 times, including an eruption 2 million years ago that put out enough ash to bury New York state to a depth of 65 feet.

Today: Yellowstone Park . . .
The hot spot that caused the cataclysmic eruption in southwest Idaho 12.5 million years ago, now sits beneath Yellowstone Park. Heat from the hot spot causes all of the Park's geysers, mud pots, and hot springs. Geologists estimate that the magma chamber beneath Yellowstone is about 45 miles across and 8 miles thick.

The future: Geysers in Billings? . . .
Look at the map below, and consider this . . . If the Earth's crust continues to move over the hot spot at the same rate and direction that it has been for the past several million years, then a few million years from now folks from throughout the world may be traveling to the Billings area to see spectacular geysers, hot springs, and mud pots.

More about Ahsfall . . .
To learn more about the Ashfall Fossil site, chick on the Hot Link below, or check out the January 1981 issue of National Geographic Magazine. That issue featured an article that includes a couple neat paintings.

More about Yellowstone from A Short History of Nearly Everything by Steve Bryson . . .
If you're interested in learning about the history of scientific discovery, you'll like this book. You don't need to be a scientist to understand it.

1. Yellowstone and other "supervolcanoes" do not have cone-shaped peaks like Mt. St. Helens or Mt. Rainer. Instead, all that's left after a supervolcano explodes is a crater, called a caldera, which is often covered with rubble, ashflows, and/or lava. The Yellowstone Caldera is more that 40 miles across, much too big to be noticed from anywhere on the ground.

2. Pressure from the magma beneath the surface, causes Yellowstone Park and the surrounding area to be raised about 1,700 feet higher that it would otherwise be.

3. The ash from the last eruption (about 600,000 years ago) covered all, or parts, of nineteen states plus parts of Canada and Mexico.

4. Between 1924 and 1984, and area of several dozen square miles in the central part of the Park has bulged over 3 feet.

Image Courtesy of the U. S. G. S.

Term: tephra


*More about the Yellowstone Hot Spot
*Ashfall Fossil Site in Nebraska
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By Rod Benson
Earth Science Teacher at Helena High School

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