Yellowstone’s Supervolcano
A supervolcano is a volcano that, at one point in time, erupted more than 1,000 cubic kilometers
of deposits such as lava, ash, and pumice. Volcanoes that produced exceedingly large pyroclastic
eruptions and formed large calderas in the past 2 million years would include Yellowstone, Long
Valley in eastern California, Toba in Indonesia, and Taupo in New Zealand. 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. Other "supervolcanoes"
would likely include the large caldera volcanoes of Japan, Indonesia, and South America. The most
recent supervolcanic eruption on Earth occurred 27,000 years ago at Taupo located at the center of
New Zealand's north island.
Bob Christiansen is the USGS scientist who described the three Yellowstone calderas and told the
world about the great eruptions that formed them. While studying the volcanic history of Yellowstone
National Park, Bob became puzzled about something that, oddly, had not troubled anyone before. He
couldn’t find the volcano in Yellowstone. It had been known for a long time that Yellowstone was
volcanic in nature, that’s what accounted for all its geysers and hot springs, and the one thing about
volcanoes is that they are usually easy to find. But Christiansen couldn’t find the Yellowstone volcano
anywhere. In particular what he couldn’t find the caldera. Around the time Christiansen was
searching for the Yellowstone caldera in the 1960s, NASA had taken high-altitude photos of
Yellowstone National Park. As soon as Christiansen saw the NASA photos he realized why he hadn’t
seen the caldera; the photos showed that almost the entire national park was the caldera. The
explosion 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. Once Bob made that realization, he traced out the caldera boundaries through old
fashioned field work... walking around with a hammer and hand lens and looking carefully at the
rocks and their distributions in order to create a geologic map. His map was then used to verify the
NASA images.
Yellowstone, it turns out, is a supervolcano. It sits on top of a huge hot spot in the continental
crust. This is fairly unusual since most hot spots occur in ocean crust. The heat from the hot spot
feeds a magma chamber that powers all of Yellowstone’s geysers and hot springs. 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. The fact that this hot spot is located under 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.
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.
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. 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. There is no doubt that when large explosive eruptions occur the whole planet can
be impacted through climate change and the cooling of the planet. 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.
Although it is possible, scientists are not convinced that there will ever be another catastrophic
eruption at Yellowstone. Given Yellowstone's past history, the yearly probability of another calderaforming eruption can be approximated as 1 in 730,000 or 0.00014%. However, this number is based
simply on averaging the two intervals between the three major past eruptions at Yellowstone — this
is hardly enough to make a critical judgment. This probability is roughly similar to that of a large (1
kilometer) asteroid hitting the Earth. Moreover, catastrophic geologic events are neither regular nor
predictable. If Yellowstone ever had another giant eruption, there would be regional effects such as
falling ash and short- term (years to decades) changes to global climate. The surrounding states of
Montana, Idaho, and Wyoming would be affected, as well as other places in the United States and
the world. Such eruptions usually form calderas, broad volcanic depressions created as the ground
surface collapses as a result of withdrawal of magma below. Fortunately, the chances of this sort of
eruption at Yellowstone are exceedingly small in the next few thousands of years.
Scientists are monitoring volcanic activity at Yellowstone and if it were to erupt again, there would
be signs. Most scientists think that the buildup preceding a catastrophic eruption would be
detectable for weeks and perhaps months to years. Precursors to volcanic eruptions include
strong earthquakes and rapid ground deformation and typically take place days to weeks before an
actual eruption. Scientists at the Yellowstone Volcano Observatory (YVO) closely monitor the
Yellowstone region for such precursors. They expect that the buildup to larger eruptions would
include intense precursory activity (far exceeding background levels) at multiple spots within the
Yellowstone volcano. As at many caldera systems around the world, small earthquakes, ground uplift
and subsidence, and gas releases at Yellowstone are commonplace events and do not reflect
impending eruptions.