If Yellowstone exploded... UF

...it'd be a bad day for everyone.

Becky Oskin has a LiveScience article about it:

Although fears of a Yellowstone volcanic blast go viral every few years, there are better things to worry about than a catastrophic super-eruption exploding from the bowels of Yellowstone National Park (photo).
Scientists at the US Geological Survey's (USGS) Yellowstone Volcano Observatory always pooh-pooh these worrisome memes, but that doesn't mean researchers are ignoring the possible consequences of a super-eruption. Along with forecasting the damage, scientists constantly monitor the region for signs of molten rock tunneling underground. Scientists scrutinize past super-eruptions, as well as smaller volcanic blasts, to predict what would happen if the Yellowstone Volcano did blow.
Here's a deeper look at whether Yellowstone's volcano would fire up a global catastrophe: 

Most of Yellowstone National Park sits inside three overlapping calderas. The shallow, bowl-shaped depressions formed when an underground magma chamber erupted at Yellowstone. Each time, so much material spewed out that the ground collapsed downward, creating a caldera. The massive blasts struck 2.1 million, 1.3 million, and 640,000 years ago. These past eruptions serve as clues to understanding what would happen if there was another Yellowstone mega-explosion.
If a future super-eruption resembles its predecessors, then flowing lava won't be much of a threat. The older Yellowstone lava flows never traveled much farther than the park boundaries, according to the USGS. For volcanologists, the biggest worry is wind-flung ash. Imagine a circle about five hundred miles across surrounding Yellowstone; studies suggest the region inside this circle might see more than four inches of ash on the ground, scientists reported on 27 August 2014, in the journal Geochemistry, Geophysics, Geosystems.
The ash would be pretty devastating for the United States, scientists predict. The fallout would include short-term destruction of Midwest agriculture, and rivers and streams would be clogged by gray muck.
People living in the Pacific Northwest might also be choking on Yellowstone's fallout.
"People who live upwind from eruptions need to be concerned about the big ones," said Larry Mastin, a USGS volcanologist and lead author of the 2014 ash study. Big eruptions often spawn giant umbrella clouds that push ash upwind across half the continent, Mastin said. These clouds get their name because the broad, flat cloud hovering over the volcano resembles an umbrella. "An umbrella cloud fundamentally changes how ash is distributed," Mastin said. But California and Florida, which grow most of the country's fruits and vegetables, would see only a dusting of ash.
Yellowstone Volcano's next supereruption is likely to spew vast quantities of gases such as sulfur dioxide, which forms a sulfur aerosol that absorbs sunlight and reflects some of it back to space. The resulting climate cooling could last up to a decade. The temporary climate shift could alter rainfall patterns, and, along with severe frosts, cause widespread crop losses and famine.
But a Yellowstone megablast would not wipe out life on Earth. There were no extinctions after its last three enormous eruptions, nor have other superoeruptions triggered extinctions in the last few million years.
"Are we all going to die if Yellowstone erupts? Almost certainly the answer is no," said Jamie Farrell, a Yellowstone expert and assistant research professor at the University of Utah. "There have been quite a few super-eruptions in the past couple of million years, and we're still around."
However, scientists agree there is still much to learn about the global effects of super-eruptions. The problem is that these massive outbursts are rare, striking somewhere on Earth only once or twice every million years, one study found. "We know from the geologic evidence that these were huge eruptions, but most of them occurred long enough in the past that we don't have much detail on what their consequences were," Mastin said. "These events have been so infrequent that our advice has been not to worry about it."
A far more likely damage scenario comes from the less predictable hazards, large earthquakes and hydrothermal blasts in the areas where tourists roam. "These pose a huge hazard and could have a huge impact on people," Farrell said.
Human civilization will surely survive a super-eruption, so let's bust another myth. There is no pool of molten rock churning beneath Yellowstone's iconic geysers and mud pots. The Earth's crust and mantle beneath Yellowstone are indeed hot, but they are mostly solid, with small pockets of molten rock scattered throughout, like water inside a sponge. About nine percent of the hot blob is molten, and the rest is solid, scientists reported on 15 May 2015, in the journal Science. This magma chamber rests between three to six miles beneath the park.
Estimates vary, but a magma chamber may need to reach about fifty percent melt before molten rock collects and forces its way out. "It doesn't look like at this point that the Yellowstone magma reservoir is ready for an eruption," said Farrell, co-author of the 2015 study in the journal Science.
How do researchers measure the magma? Seismic waves travel more slowly through hot or partially molten rock than they do through normal rock, so scientists can see where the magma is stored, and how much is there, by mapping out where seismic waves travel more slowly, Farrell said.
The magma storage region is not growing in size, either, at least for as long as scientists have monitored the park's underground. "It's always been this size, it's just we're getting better at seeing it," Farrell said.
As with magma mapping, the science of forecasting volcanic eruptions is always improving. Most scientists think that magma buildup would be detectable for weeks, even years, preceding a major Yellowstone eruption. Warning signs would include distinctive earthquake swarms, gas emissions, and rapid ground deformation.
Someone who knows about these warning signals might look at the park today and think, "Whoa, something weird is going on!" Yellowstone is a living volcano, and there are always small earthquakes causing tremors, and gas seeping from the ground. The volcano even breathes, the ground surface swells and sinks as gases and fluids move around the volcanic "plumbing" system beneath the park.
But the day-to-day shaking in the park does not portend doom. The Yellowstone Volcano Observatory has never seen warning signs of an impending eruption at the park, according to the USGS.
What are scientists looking for? For one, the distinctive earthquakes triggered by moving molten rock. Magma tunneling underground sets off seismic signals that are different from those generated by slipping fault lines. "We would see earthquakes moving in a pattern and getting shallower and shallower," Farrell said. To learn about the earthquake patterns to look for, revisit the 2014 eruption of the Bardarbunga Volcano in Iceland. Both amateurs and experts "watched" Bardarbunga's magma rise underground by tracking earthquakes. The eventual surface breakthrough was almost immediately announced on Twitter and other social media. As with Iceland, all of Yellowstone's seismic data is publicly available through the U.S. Geological Survey's Yellowstone Volcano Observatory and the University of Utah.
"We would have a good idea that magma is moving up into the shallow depths," Farrell said. "The bottom line is, we don't know when or if it will erupt again, but we would have adequate warning."

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