Download In the 1960s, while studying the volcanic history of Yellowstone

Yellowstone Super Volcano
(Excerpts from Bill Bryson’s book)
(1) When we think of volcanoes, most of us think of the classic composite volcanoes,
also known as stratovolcanoes like Mt. Fuji or Mt. Etna, which are a collection of lava
flows and pyroclastic material layered together to form a large mountain. There are
1,000’s of these stratovolcanoes visible on Earth, but only a few hundred of them are still
active, the rest are extinct.
(2) However, there is another type of explosive volcano that doesn’t make a mountain,
but instead forms a caldera. A caldera volcano is so explosive that their single powerful
eruption causes the crust to collapse into the partially emptied magma chamber, leaving a
caldera, or very large, basin-shaped crater. Yellowstone was a caldera type volcano.
(3) In the 1960s, NASA had taken high-altitude photos of Yellowstone National Park.
These photos allowed scientists to see the Yellowstone caldera for the first time. It turns
out that almost the entire national park was the caldera. The last eruption had left a crater
more than 40 miles across, way too large to be seen from the ground. At some time in the
past Yellowstone must have blown up with a violence far greater than anything known to
humans.
(4) Yellowstone, it turns out, is a supervolcano. It sits on top of a huge hot spot.
(5) The heat from the hot spot feeds a magma chamber that powers all of Yellowstone’s
geysers and hot springs. (6) This magma chamber is about 5 miles below the surface
of the crust and is about 45 miles across and has a maximum thickness of about 8 miles.
Imagine a pile of TNT about twice the size of Bucks County and reaching into the sky well
above the highest mountain peak in the world, and you have some idea of what visitors of
Yellowstone are walking around on top of. This magma chamber puts so much pressure
on the crust above, that Yellowstone and the surrounding area are about 1,700 feet higher
in elevation than they should be.
(7) Hot spots are not that rare. There are about 30 active hot spots on Earth, and they are
responsible for many of the world’s best known islands and island chains, such as Iceland
and Hawaii. What makes Yellowstone different from all of these other hot spots is that it
is located under continental crust instead of oceanic crust. No one is sure how or why the
hot spot of Yellowstone ended up under a continental plate, but the continental crust
makes a huge difference to its eruptions. Where the other hot spots, like Hawaii, tend to
have nonexplosive eruptions with runny lava flows, Yellowstone blows explosively. It
doesn’t happen often, but when it does it is a historic event.
(8) Evidence shows that the Yellowstone supervolcano has erupted about 100 times in
the past 16.5 million years, but the three most recent eruptions are the ones that get the
attention. The last eruption, 640,000 years ago, was 1,000 times greater than the Mount
St. Helens eruption; the one before that, 1.3 million years ago, was 280 times bigger, and
the one before that, 2 million years ago, was so big that we have nothing to compare it to.
It was at least 2,500 times greater than St. Helens, but possibly much, much greater.
(9) The biggest blast in recent times was Krakatau eruption in Indonesia in 1883. But if
you imagine the volume of material ejected from Krakatau as being about the size of a golf
ball, then the biggest of the Yellowstone blasts would be the size of a ball you could just
about hide behind. On this scale, Mount St. Helens’ would be no more than a pea.
(10) The Yellowstone eruption of 2 million years ago put out enough ash to bury the
entire state of Pennsylvania to a depth of almost 80 feet. The ash fall from the most recent
Yellowstone eruption, 640,000 years ago, covered all or parts of 19 western states, plus
parts of Canada and Mexico, nearly all of the United States west of the Mississippi River.
Keep in mind this is the breadbasket of America, an area that produces roughly half the
world’s cereals. And ash is not like a big snowfall that will melt in the spring. If you
wanted to grow crops again, you would have to find some place to put all the ash. Imagine
what it would take to clear the state of Kansas.
(11) Now imagine the impact this type of eruption could have on Earth’s climate from
the amount of volcanic dust and gases shot into the atmosphere. The last supervolcano
eruption on Earth was Toba, in Sumatra, 74,000 years ago. No one recorded how big the
eruption was, since humans were still hunter-gatherers back then, but there is evidence to
show it was a whopper.
(12) Ice cores taken from the very old Greenland ice sheet show that the Toba eruption
was followed by at least 6 years of “volcanic winter”. There is evidence to suggest this
super eruption may have reduced the human population to a few thousand people. No one
knows for sure how long the volcanic winters lasted from the past three Yellowstone super
eruptions, but they likely had a significant impact on life on Earth at that time.
(13) So, the big question is, will the Yellowstone supervolcano erupt again? Many
scientists suspect that some kind of smaller eruption compared to the last three
Yellowstone eruptions is highly likely at some point in the future. Some scientists suspect
the supervolcano magma chamber may be safely bottled up. For most of its history, the
Yellowstone hot spot has formed caldera volcanoes in a thinner part of the continental
crust along the western United States. As the North American plate has continued to move
west the hot spot magma chamber is now lodged beneath a much thicker part of the
continental crust among the Rocky Mountains possibly bottling up the hotpot from further
super eruptions.