Scaling up from smaller eruptions is all the experts can do, too, since no one in recorded history has ever seen a supereruption. (The last one, depending on whose definition of supereruption you use, was either 26,500 or 74,000 years ago.) "It's very difficult to forecast quite what would happen, because it would be of a scale we just have no experience of," says Stephen Sparks, a volcanologist at Bristol University in England.
Nevertheless, by studying deposits from earlier such cataclysms, experts like Sparks can make educated guesses about what would transpire if a supervolcano went off. And, as I learned, the eruption I saw resembles a supereruption like the detonation of a firecracker resembles a nuclear explosion.
When it will blow
Knowing when Semeru ("suh-MAY-roo") will erupt again is a piece of cake: It explodes about every 20 minutes, like clockwork. I had been watching this progression of blasts for days before I ascended the mountain, admiring the mushroom-shaped plume that rose above the peak's ashen slopes for a few minutes before its source was choked off.
No such luck with supervolcanoes. Supereruptions occur so infrequently that even with the best-studied supervolcanic hot spots, experts only know about two or three at most that have occurred there, which makes it hard to nail down frequency. Moreover, since any early humans who saw and survived the eruption of Taupo in New Zealand 26,500 years ago or Toba in Sumatra 74,000 years ago—the most recent known megablasts—did not record any details, experts today have no idea what the precursor signals to a supereruption might be.
"Some of us think you wouldn't be able to miss the signs and that they'd probably go on for quite a long time beforehand," says Stephen Self, a volcanologist at the Open University in England. "Others doubt this. They think there might be cases where there could be a very short period of unrest before a big eruption."
"Probably something like a third of the United States would be more or less uninhabitable."
Add to this uncertainty the fact that scientists are still trying to figure out where all the supervolcanoes are. The first authoritative list of these giants was published only in 2004, and though considered a good start, it's incomplete. "There are considerable numbers [of supervolcanoes] missing from the list," says Self. "It's not anybody's fault. We just don't know about them."
On Semeru, I was able to sit on top and enjoy the periodic eruption because the crater is actually a quarter mile away and a few hundred feet below the summit. Happily the wind carried the ash cloud to the south, away from me. And of the six or eight eruptions I observed while up there, the most intense coughed up rocks at most the size of basketballs, and these landed not far from the crater rim.
A supereruption would not only take out the summit but the entire mountain and much else besides. The caldera that underlies Yellowstone National Park—"caldera" essentially means humongous crater—is over 50 miles long and nearly 30 miles wide. You could fit four Manhattans placed end to end inside. The amount of magma, or molten rock, thrown out by its most recent supereruption 640,000 years ago was a staggering 240 cubic miles, with an ash volume two to three times that.
The intensity of such convulsions matches the magnitude. In A.D. 79, Vesuvius belched out an astounding 100,000 cubic yards of magma per second over a 24-hour period. Yet this is chicken feed compared to the output of a supereruption, which can emit volcanic debris at up to 100 million cubic yards per second. You might have thought such violence was reserved for astronomical events like supernovas and gamma-ray bursts, but no—our own Earth can generate it.
All that erupted material wouldn't just fall nearby or waft harmlessly away like Semeru's ash clouds, either. If something the size of that 640,000-year-old Yellowstone eruption occurred under New York City, it would not only obliterate all five boroughs but bury what was left as well as large portions of Long Island, New Jersey, and Connecticut under half a mile of pyroclastic flow deposits. Pyroclastic flows comprise all the heavy stuff that collapses out of an ash cloud, and in supereruptions they can travel up to 60 miles away at speeds of 100 yards per second—again, unimaginable fury. (A 2005 Geological Society of London report, which is the best single source for the layman on supereruptions that I've seen, puts the danger succinctly: "No living beings caught by a pyroclastic flow survive.")
A planetary peril
The devastation would spread much farther than the surrounding region. A stone monument atop Semeru memorialized several people who had died there after breathing volcanic fumes. But a supereruption would kill millions. "If you're close enough to the eruption—and in a supereruption that can mean thousands of miles away—if you breathe in the ash in an unprotected way, you're breathing in tiny glass needles," says geologist Michael Rampino of New York University. "They cause the blood vessels in your lungs to pop. Water in your lungs combines with this volcanic ash, and essentially you drown in a kind of soup or cement of wet volcanic ash."
Even if you survived long enough for the ash to settle, you could still succumb to longer-term effects. A supereruption would smother many millions of square miles under an inch or more of ash. Less than an inch can disrupt most forms of agriculture, so a single supereruption could lead to the starvation of millions of people. (Wildlife and natural habitats, needless to say, would suffer just as grievously.) Ash would collapse roofs, poison water supplies, and clog machinery such as vehicle and aircraft engines, causing transportation to grind to a halt.
"It would be pretty dire," says Bristol University's Sparks, who chaired the working group that prepared the Geological Society of London report, referring to another Yellowstone supereruption. "Probably something like a third of the United States would be more or less uninhabitable maybe for a few months, even a year or two."
"At some stage we will face one of these eruptions. They are inevitable."
A big enough supereruption—the biggest yet identified unloaded 1,200 cubic miles of volcanic debris—could even have a global impact, threatening everyone on Earth. Aerosols shot into the atmosphere could create a worldwide haze that blocks sunlight sufficiently to change the climate for several years, with potentially disastrous effects on global agricultural yields. Refugees pouring into surrounding areas, disrupted satellite communications, reeling world financial markets—the fallout could be "sufficiently severe," notes the Geological Society of London report, "to threaten the fabric of civilization."
Before you pack up and move to Mars, know that supereruptions do have one encouraging characteristic—they don't happen very often. "Something on the order of every 100,000 years," says Jake Lowenstern, head of the Yellowstone Volcano Observatory (which keeps a close eye out for any unrest there).
"We could merrily go on for another 10,000 or 20,000 years without anything of this size happening," Sparks says. "On the other hand, at some stage, assuming that humanity outlasts all its other problems, we will face one of these eruptions. They are inevitable. So I think they have to be taken seriously."
Sounds like a good idea. In the meantime, if you want to catch an eruption—and survive it—go and watch Semeru from a safe distance.