Author: Russell McLendon, Mother Nature Network

Mount Rainier, Washington (Photo: USGS)

When a volcano in Iceland began pumping plumes
of ash over Europe this week, airlines across the continent went on high alert,
canceling
tens of thousands of flights and bringing air travel to a crawl. (See a
gallery of amazing
photos from the eruption.)

While the ash wasn’t always visible from the ground, the cancellations weren’t an overreaction: A volcano’s ash
clouds can be one of its most dangerous features.

That was made especially clear in December 1989,
when a Boeing 747 flew into an ash cloud unleashed by Mount Rebout in Alaska, causing all four of
its engines to lose power. The plane briefly plummeted toward the ground until
the crew finally managed to restart its engines, averting a disaster. But the
event served as a harrowing reminder – not only that the sky is no refuge from
volcanoes, but also that many parts of the Western United States are within
range of powerful volcanoes that could explode to life with little or no
warning.

As the Fimmvorduhals
volcano in Iceland continues raging – and experts point out its last major
eruption went on for two years, from 1821 to 1823 – the threat of volcanic
eruptions has suddenly become a red-hot issue for people around the world.

There are three main sections of the United States that tend to experience
volcanic activity, and scientists believe many of the volcanoes there may be
about due for a major eruption. Below is a brief look at six specific U.S. volcanoes that pose some of the
highest risks.

Mount Rainier,
Washington

The
highest peak in the Cascade Range is also a volcano loaded with the most glacier ice of any mountain in
the contiguous United States, which
will complicate things whenever it does erupt because erupting through ice tends
to create lahars (volcanic mud flows that form when hot
gas, rocks, and lava melt ice and churn up a superheated slurry).

The
U.S. Geological Survey calls Mount Rainier “potentially the most dangerous”
mainland U.S. volcano because in addition to all that ice, it looms over the Seattle-Tacoma metro area
and its 3.2 million inhabitants.

Mount Rainier’s potential volatility and its
proximity to large cities helped make it one of two U.S. Decade Volcanoes, a group of 16 volcanoes
worldwide that U.N. delegates deemed especially dangerous to human populations.
It last erupted in the 1840s, and larger
eruptions occurred as recently as 1,000 and 2,300 years ago. It’s now
considered active but dormant.

Still, it’s one of the most intensely monitored
volcanoes in the United States due to the havoc it could wreak.

Mauna Loa,
Hawaii

The
other U.S. Decade Volcano is also the largest volcano on the
planet: Hawaii’s Mauna Loa. Its last eruption was in 1984, when the lava
flow reached to within four miles of Hilo, a city of more
than 40,000.

It’s
an especially active volcano, having erupted 33 times in recorded history — the two largest
were in 1950 and 1859, and one
in 1880-81 covered land now in Hilo’s city
limits. Like Mount Rainier, it’s also closely monitored, and one theory suggests it’s currently at the
end of a 2,000-year cycle, with its summit lava flows poised to increase toward
the northwest and southeast.

Mount Saint
Helens, Washington

About
50 miles south of Mount Rainier sits the shell of Mount St. Helens, the scene of one of the
worst volcanic eruptions in U.S. history, which took place on May 18, 1980. Fifty-seven people and thousands of
animals were killed in all, and damages topped $1 billion. 

Mount
St. Helens reawakened in 2004, when four explosions
blasted steam and ash 10,000 feet above the crater. Lava continued gurgling out
and forming a dome on the crater floor until late January 2008.

Although it’s calmed
down now, this remains an “active and dangerous” volcano, according to
the USGS, and history shows it’s been relatively active since the Middle Ages,
including a blast in 1480 that was five times stronger than the 1980
eruption.

Mount Baker, Washington

After
Mount Rainier, Mount Baker is the most glaciated mountain
in the Cascades, supporting more ice than all the range’s other peaks combined,
aside from Rainier. This means it presents many of the same mudslide dangers as
Rainier, although 14,000 years of sediments show Baker to be less explosive and
less active than some other Cascade mountains.

Baker
gave locals a scare in 1975 when it began emitting large amounts
of volcanic gases, and heat flows around the mountain increased tenfold, but the feared eruption never
happened. The fumarolic activity still continues, but
there’s no evidence it’s tied to the movement of magma, which signals an
eruption may be imminent.

Lassen Peak,
California

The
southernmost active volcano in the Cascades, Lassen Peak has one of the most massive
lava domes on earth, totaling half a cubic mile. It’s the largest of more than
30 volcanic domes in Lassen Volcanic National Park that
have erupted in the last 300,000 years, and it’s part of a region that’s been volcanically active for more than 3
million years.

Lassen Peak is now dormant but remains active, posing a distant
threat to some nearby cities such as Redding and Chico.

Mount Hood,
Oregon

At
more than 500,000 years old, Mount Hood is a moody volcano, following
centuries of frequent eruptions with quiet periods that have lasted a few
centuries to more than 10,000 years. It last erupted in the 1790s, a few years
before Lewis and Clark reached the Pacific
Northwest.

Oregon’s tallest peak has produced many debris
avalanches of various sizes throughout its history, the largest of which
removed the mountain’s summit and big chunks of its flanks.

The USGS identifies two past eruptions at Mount Hood that
offer perspective on future eruptions. Mount Hood dominates the Cascade skyline from
Portland, OR, and while it’s probably not close enough to douse Portland with
a volcanic mudflow, it could dust it with tephra or ash, as Mount St. Helens did in 1980.

Russell McLendon is an associate editor at the Mother Nature Network, where a version of this post originally appeared.

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