Yellowstone’s supervolcano is essentially a giant, lid-topped cauldron, and it’s so vast that it can only truly be seen from low-Earth orbit. Its crater is 72 kilometers (45 miles) across, and its underlying plumbing contains several tens of thousands of cubic kilometers of magmatic material.

By the latest estimate, it would take several centuries for both sides of the Niagara Falls to fill up just its shallow chamber, let alone its far more voluminous deeper reservoir.

What would happen if much of this suddenly re-emerged in a horrific supervolcanic eruption? Who would live, who would die – and would the United States of America survive? We spoke to one of the country’s most respected volcanologists to get the most up-to-date low-down on the future of the world’s most famous supervolcano.

Tick Tock

Right now, the two-step magma chamber is in a state of dormancy. According to Yellowstone Volcano Observatory’sScientist-In-Charge, Dr Michael Poland, it may not have enough energy at present to produce a supereruption. "Right now, much of Yellowstone's magma body is partially solidified, and you need a lot of magma to feed a large eruption."

Throughout most of its life, the region has featured extensive lava flows or (far more frequently) hydrothermal blasts, which suggests that any future eruption is far more likely to replicate this. Although these will cause a problem, they certainly won’t be anything apocalyptic – and even these eruption types are exceedingly rare.

The chances of a supervolcanic paroxysm are currently around one-in-730,000, which makes it less likely than a catastrophic asteroid impact.

The lava flows of Yellowstone.

However, a sudden injection of new magma from beneath, or a sudden weakening of the geological layers encasing it, as unlikely as this is, may be enough to trigger a sudden depressurization event, and the entire system would violently expunge onto the surface and up into the atmosphere.

What happens next is somewhat speculative, but Yellowstone’s frightening history gives us a clue. We’re thinking about the worst-case scenario here, so let’s assume its entire magmatic belly is emptied in a colossal supervolcanic explosion.

This has happened at Yellowstone three times on a cycle of 660,000-800,000 years: 2.1 million years ago, 1.3 million years ago, and 640,000 years ago.

The most explosive eruption was its first, which produced about 2,500 times the amount of volcanic material as the 1980 destruction of Mount St Helens. Even the most recent blast created an eruptive column so colossal that it coated about 60 percent of the contiguous United States in thick layers of ash.

So let’s say that the original record-holding blast was to happen again: What would happen to the United States and the wider world?

Zero Hour

It’s unclear how much warning organizations like the United States Geological Survey (USGS) would get, but shortly before the eruption happened, the ground around Yellowstone National Park would rise upwards somewhat. Hydrothermal system, including the geysers and geothermal pools, would rapidly heat to temperatures above boiling, and they’d likely become extremely acidic – more so than usual.

A swarm of earthquakes would be detected making their way towards a central point, indicating magma rising rapidly through the crust. Then, the roof rock would fail and the eruption would begin.

A vast column of ash and lava would shoot upward to heights of around 25 kilometers (16 miles). Sustained by both raw explosive energy and the release of heat through cooling lava blebs and bombs, it would sustain itself for days, pumping ash into jet streams that would transport it around the stratosphere.

When the eruptive column or parts of the column fail, enormous pyroclastic flows would blast their way across the park.

These mixtures of ash, lava blebs, and superheated gas exceed temperatures of 1,000°C (1,832°F) and can move at speeds of up to 482 kilometers per hour (about 300 miles per hour). If they hit anyone, they’d die within seconds; those nearby would be burned as the air heats up to around about 300°C (570°F).

Generally speaking, pyroclastic flows travel up to 15 kilometers (9.3 miles) out from their source, but they can theoretically reach up to 100 kilometers (62 miles).

This is basically the length of Yellowstone National Park, so if the vent emerged directly in the center, and the pyroclastic flows were particularly energetic, many in the park would die, either from the pyroclastic flows or the collapsing caldera roof itself.

On average, there’s about 11,000 visitors there at any one time, based on a yearly visitor count of 3.8 million. There are far more visitors in the summer months, so a summer eruption would be far more deadly.

When the pyroclastic flows and ash deposits settle and cool, they may seem harmless, but they’re not. If it rains heavily after the eruption, especially on any slopes, then these could mix with mud and turn into rapidly-moving, cement-like slurries called lahars. If you get stuck in one, there’s a good chance you’ll die.

Shadowy Skies

The most dangerous aspect of the eruption, however, is the ash fallout, both locally and globally.

Breathe this in and it’ll lacerate your lungs and form a glassy cement. It’s also about six times denser than water, which means plenty of architecture would collapse under its weight as it accumulates on rooftops. Poland points out that “even a few tens of centimeters of wet ash could cause weak buildings to buckle.”

Roads and sewer systems would clog and break down, water supplies would be contaminated, and electrical grids would short out. Millions of homes could become uninhabitable.

In this sense, those taking shelter in Montana, Idaho, and Wyoming would be at the highest risk of harm. They would be so for up to a month, which is a fairly solid bet as to how long the eruption would ultimately be.

An area about 80 kilometers (50 miles) around the vent would be covered in 3 meters (about 10 feet) of ash in just a few days. Simulations have also shown that a supereruption could bury Salt Lake City and its surroundings beneath a meter (3.3 feet) of ash.

The approximate ash coverage after one month of a supereruption.

Assuming there’s no strongly prevailing winds, Denver would get about 30 centimeters (about a foot), whereas Calgary would get about 10 centimeters (3.9 inches). The Federal Emergency Management Agency (FEMA) would help with the cleanup/relocation for many months or even years.

Elsewhere – say San Francisco, Los Angeles, Seattle, Minneapolis, and Chicago – would receive about 3 centimeters (1.2 inches). A fine layer would make it as far as Miami, New York, and Toronto within a few days, still enough to cause vehicles to break down and water to become unpotable.

Flights would be grounded or diverted away from the United States – at least for a few weeks – and it’s almost certain that the National Guard and perhaps the military would be drafted in to help evacuate many tens of millions of people from the affected region.

The death toll is extremely difficult to predict, but Poland suggest that "if people were present in the vicinity of the eruption – say, within a few tens to perhaps a few hundred kilometers – they would be in peril."

Goodbye, Cruel World?

This is bad enough as it is, but worse is yet to come.

The ash’s injection into the stratosphere would cause it to darken the sky and cool regional, if not planetwide temperatures. If the eruption is particularly sulfur-rich – an efficient blocker of sunlight – then temperatures would plummet several degrees, to the point where the next few years will lack a summer.

“It’s likely there would be significant cooling for many years,” Poland explains. “But how long it would last, and how much cooling would occur, I can't say. I'm not sure anyone can.”

If the far smaller but highly sulfur-rich eruption of Tambora back in 1815 is any example, a caldera-forming blast at Yellowstone would “alter global weather patterns and have enormous effects on human activity” for many years, according to the USGS.

The paths and timings of monsoons would change. Tropical cycle formation would become far more unpredictable for a while and the spread of waterborne-diseases could take highly erratic paths.

Agriculture would also suffer, which could severely disrupt food supplies. This would add to the overall economic damage, which would be severe: A recent estimate by FEMA of a Yellowstone supereruption put the total US damage at $3 trillion, about 16 percent of the nation’s total GDP. To put that in perspective, that’s $400 million more than was lost during the recent global recession.

A winter eruption would be preferable.

The USGS is keen to point out that “scientists at this time do not have the predictive ability to determine specific consequences or durations of possible global impacts from such large eruptions.” Whatever happens, though, it won’t cause civilization to come crashing down.

“It would not mean the end of life on Earth,” Poland tells us. “In fact, this experiment has already been run, yet few people realize it.”

He points to the Toba eruption, one that occurred 74,000 years ago, and one that “was larger than anything that Yellowstone has ever produced.” Evidently, humanity survived that, and “they didn't have the benefit of technology back then!”

Make no mistake though: Another full-on Yellowstone supereruption would be a devastating natural disaster, the type that would cost both livelihoods and lives. However, it cannot be emphasized enough that it’s extremely unlikely to happen in the near-future, if ever.

If it did, it wouldn’t be a civilization-ending event either. It would, however, be one that changes the world for the worse.