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
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
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.
Courtesy of the U. S. G. S.
Term: hot spot (What does this term mean to a geologist?)