| The Yellowstone region has
produced three exceedingly large volcanic eruptions in the past 2.1 million
years. In each of these cataclysmic events, enormous volumes of magma
erupted at the surface and into the atmosphere as mixtures of red-hot
pumice, volcanic ash (small, jagged fragments of volcanic glass and rock),
and gas that spread as pyroclastic (“fire-broken”) flows in all directions.
Rapid withdrawal of such large volumes of magma from the subsurface then
caused the ground to collapse, swallowing overlying mountains and creating
broad cauldron-shaped volcanic depressions called “calderas.”
 The first of these
caldera-forming eruptions 2.1 million years ago created a widespread
volcanic deposit known as the Huckleberry Ridge Tuff, an outcrop of which
can be viewed at Golden Gate, south of Mammoth Hot Springs. This titanic
event, one of the five largest individual volcanic eruptions known anywhere
on the Earth, formed a caldera more than 60 miles (100 km) across.
A similar, smaller but still huge eruption occurred
1.3 million years ago. This eruption formed the Henrys Fork Caldera, located
in the area of Island Park, west of Yellowstone National Park, and produced
another widespread volcanic deposit called the Mesa Falls Tuff.
The region’s most recent caldera-forming eruption
640,000 years ago created the 35-mile-wide, 50-mile-long (55 by 80 km)
Yellowstone Caldera. Pyroclastic flows from this eruption left thick
volcanic deposits known as the Lava Creek Tuff, which can be seen in the
south-facing cliffs east of Madison, where they form the north wall of the
caldera. Huge volumes of volcanic ash were blasted high into the atmosphere,
and deposits of this ash can still be found in places as distant from
Yellowstone as Iowa, Louisiana, and California.
Each of Yellowstone’s explosive caldera-forming
eruptions occurred when large volumes of “rhyolitic” magma accumulated at
shallow levels in the Earth’s crust, as little as 3 miles (5 km) below the
surface. This highly viscous (thick and sticky) magma, charged with
dissolved gas, then moved upward, stressing the crust and generating
earthquakes. As the magma neared the surface and pressure decreased, the
expanding gas caused violent explosions. Eruptions of rhyolite have been
responsible for forming many of the world’s calderas, such as those at
Katmai National Park, Alaska, which formed in an eruption in 1912, and at
Long Valley, California.
If another large caldera-forming eruption were to
occur at Yellowstone, its effects would be worldwide. Thick ash deposits
would bury vast areas of the United States, and injection of huge volumes of
volcanic gases into the atmosphere could drastically affect global climate.
Fortunately, the Yellowstone volcanic system shows no signs that it is
headed toward such an eruption. The probability of a large caldera-forming
eruption within the next few thousand years is exceedingly low.
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Lava Flows
Volcano Main Page
Text from the United States Geological Survey website. Photo by
Bruce Gourley. |
