The Pleistocene is justly famous for the glaciations, which certainly dominated things. But the world doesn’t stop for glaciers, and plenty of other things were going on. Like supervolcanoes.
A supervolcano is a big one, conventionally taken to be a single eruption of more than 1,000 cubic kilometers, or 240 cubic miles. That’s a volume vastly greater than even many significant, damaging eruptions – for comparison, Mt. St. Helens in 1980 ejected about 1.2 cubic kilometers (or less) of material. So a supervolcano would be at least 833 times that volume.
|In this comparison of "dense rock equivalent," Toba erupted more than 11,000 times the volume of Mt. St. Helens in 1980.|
We’ve talked about a lot of volcanic events in this series, but most of them were likely to be many events over long periods of time, a million years or more, adding up to a lot. One exception might be the eruptions that created what is now the Ordovician Deicke Bentonite that we talked about March 24. That might have been a single eruption, and if it was, it might have been the largest in at least the past 600 million years. Its ejecta volume is estimated at 5,000 cubic kilometers or more.
So these things were probably happening sporadically throughout earth’s history. The favored locations would be subduction zones or hotspots, places where heat can build up and pressures can increase to the point where the crust can’t contain them, and they erupt violently.
During the Quaternary, we know of six eruptions with volumes of 1,000 cubic kilometers or more. The largest, at what is now Lake Toba in Sumatra, had a volume of 2,800 cu km and happened about 74,000 years ago. That eruption is linked to a controversial idea that the ensuing global winter lasted perhaps 10 years, and, based on genetic studies, might have reduced the existing human population of the planet to as few as 3,000 to 10,000 individuals. It is controversial, and 75,000 years ago the evidence of human life is spotty at best. Consider this to be another idea for which the jury is still out.
The second largest supervolcano eruption was at Yellowstone. In fact two of the six Quaternary supervolcano eruptions were there, one at 2.1 million years ago, and the other at 640,000 years ago, with volumes of about 2,500 and 1,000 cu km respectively. There was another large eruption there about 1.3 million years ago, only about a tenth the size of the one at 2.1 million years ago.
The fourth Quaternary supervolcano was in Argentina, about 2.5 million years ago just as the Quaternary was starting, and the other two were in New Zealand, in the Taupo Volcanic Zone on the North Island. Those two eruptions were at about 254,000 and 27,000 years ago – the latter is the most recent supervolcano eruption that has occurred. Its volume, about 1,200 cu km, is still 1,000 times that of Mt. St. Helens in 1980.
Some supervolcanoes seem to work in ways that are different from regular volcanoes. To release the vast volumes, special conditions are required. Let’s use Yellowstone as an example – and in passing, make the argument for why such an eruption cannot be imminent there.
The mouths of supervolcanoes are much larger than the craters that form at the top of a standard volcano. A caldera is a collapsed region that has fallen into a magma chamber beneath it – a magma chamber that had to evacuate its magma in many small eruptions to allow for the collapse. When the crust over the chamber collapses, all the fractures cause a rapid release of pressure, and the confined, pressurized magma that’s still down there can come out, violently. It’s like a pressure cooker – the relief valve on the top is like the geysers at Yellowstone, releasing pressure and keeping things safe. Without that, the pressure could increase and ultimately blow the cooker apart.
Or think of an apple pie. You poke holes in the crust to allow steam, the pressure inside, to escape. If you didn’t, cracks might develop and some of the filling could escape, but the crust would still be there. But if enough of the filling escapes, say around the edges of the pie, the crust on top might collapse, cracking, and the instant reduction of pressure would allow the entire contents of the pie to explode up to the ceiling.
So my point is, you can’t really have a caldera collapse, which would make a supervolcano eruption, without emptying enough of the magma chamber for the crust to collapse into it. The last time any magma was erupted at Yellowstone was 70,000 years ago – and not much came out. I think we need to have a LOT more little eruptions – magma, not just the hot water – before anything like a major collapse is likely that would produce a supervolcano eruption. I live 120 miles away and I’m definitely not losing any sleep over it. Besides, if it does happen, there’s precious little we can do about it. Yellowstone’s supervolcano eruptions have deposited ash as far away as the state of Mississippi, so the area of devastation would be huge, and in a big way dependent on the wind directions at the time.
With only three data points for the present Yellowstone caldera eruptions, it’s irrational to see any predictable regularity to them, at least not more than ball-park figures like plus or minus 200,000 years, and even that could be far, far off in such a chaotic system.
Supervolcano eruptions happen. Don’t worry about it. Regular eruptions are far more frequent and we can plan for them, though even much smaller events can be incredibly disruptive, as the unpronounceable Icelandic eruption a few years ago proved. Its erupted volume was less than 1 cubic kilometer.
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December 28, 1835, was the birthday of Archibald Geikie, in Edinburgh, Scotland. He was an eminent scholar of Scottish geology, but he expanded his work on volcanics to include western North America as well. He is probably as well known for his popular writings about science as for his technical work. Also born this day, in 1894 at White Plains, New York, was Alfred Romer. His focus from his base at Harvard was in the field of vertebrate paleontology. He classified the labyrinthodonts, and the basics of his general classification of the vertebrates is still in use today, with modifications and expansions.
Also on this date, December 28, 1908, a strong earthquake hit Messina, Sicily, the location that the Messinian stage of the Miocene was named for. Messina and other major cities were practically destroyed, and at least 70,000 people were killed. There was a tsunami as well, and the final total death toll is put at about 123,000. The quake was a result of the ongoing Alpine collision between Africa and Europe. Africa is pushing northward and small blocks – Italy, Sardinia, and the oceanic crust in the Tyrrhenian Sea, are being forced over the leading edge of Africa. It’s a really complex zone, and Mt. Etna, the active volcano near Messina, is another consequence of this tectonic activity.
—Richard I. Gibson