Sesia, The Fossil Supervolcano

In Italy’s Sesia Valley, earth scientists have found the remains of an ancient supervolcano, exposed in cross-section through surface rocks.

To you and me, it just looks like a nice picnic spot, but geologists are very excited by the discovery.


An important discovery

The Sesia Valley cross-section gives volcanologists a detailed look at a supervolcano’s plumbing all the way down to its roots in the mantle. It’s something no scientist has ever been able to see intact and in one piece before, not even at Yellowstone, the world’s most thoroughly studied supervolcano.

Very little is known about what goes on deep underneath any volcano. Earth scientists, for example, are having to set off explosions all around Mount St. Helens in order to study its plumbing (yes, that’s very ironic). Almost nothing is known about the subterranean processes that lead to a supereruption – a much more violent event. There just isn’t enough data.

Earth has had some huge and devastating volcanic eruptions while we humans have been recording history, but it hasn’t yet had a supereruption during historic times.

Perhaps that’s why there is such a thing as recorded history. The scale of such an event is mind-boggling. In order to keep the basics of our advanced civilization intact during one, we need to know a lot more about supervolcanoes than we do today.

Supereruptions compared to Mount St. Helens 1980 eruption
No real volcanic eruption you have ever seen on the screen comes close to a supereruption.

Mount St. Helens is what we’re used to. Okay, check out the USGS graphic on the right that compares the 1980 eruption of Mount St. Helens with other recent and historic eruptions (none of them are supereruptions).

MSH 1980 is up there, second from the top. You can hardly see it!

The Novarupta (Katmai), Krakatoa (Krakatau), and Tambora eruptions weren’t too shabby, but look at the size of the huge Mount Mazama eruption, down at the bottom. That’s the one that formed Crater Lake, in Oregon, some 7,000 years ago.

What impressed us so much in 1980 …

…is hardly a blip on the screen compared with Mount Mazama’s blast in 5,000 BC. (Click on the YouTube insignia, lower right, to watch the whole video.)

Now, to help you visualize the enormity of a supereruption, there is a graph below showing how the Crater Lake event at Mount Mazama measures up against the M8.4 Aira Caldera eruption we talked about last week, as well as four other supereruptions:

The scale is arranged by eruption size, not time. In other words, it’s not showing that eruptions increase in size over time. It’s just comparing different eruptions, from smallest to largest known. Guess which one is the smallest…

Mazama and supereruptions
That is what we’re talking about when we say “supereruption” (or, in formal scientific terms, something like “a catastrophic caldera-forming eruption” or “a silicic large igneous province eruption” [PDF]).

Frankly, I can’t even imagine what it would be like. It makes things like Mount St. Helens and Pinatubo seem irrelevant.

Yet supereruptions have happened repeatedly in the past, and they will happen again in the future.

A supereruption at Aira could cover almost all of Japan's major islands.  Source

A supereruption at Aira could cover almost all of Japan’s major islands. Source


Learning about supervolcanoes

We need to know as much as we can about supereruptions, so that we can make plans to deal with one. Thus far, we’ve only just begun to learn.

No one even realized that supervolcanoes were a thing until the last couple of decades. The size of some of Yellowstone’s biggest eruptions shocked everybody at first, but while the public imagination was filled with docudramas and sensational stories, experts started doing science. A lot of it.

What they have learned so far isn’t as visually stunning or easily accessible as the apocalyptic tales we use for entertainment, but it’s probably a lot closer to the actual truth about supervolcanoes.

Geologists now know for sure that a few other volcanoes besides Yellowstone, Long Valley, and Aira have had very big eruptions. A-a-a-nd…that’s about it.

Expert theories about supervolcanoes abound, but it’s only theory. To be absolutely sure of anything – especially crucial things like precursors or whether supereruptions happen at known megacalderas or if they just go off unexpectedly – scientists will need to monitor a supereruption as it happens.


That’s not an appealing alternative. Source


Oh well.

The only definitive textbook on supervolcanoes right now is the one written in the Earth. And in northern Italy’s Sesia Valley, geologists have just turned a page and are starting to read a very helpful infographic.

Global volcanic effects, not from a supereruption.  (Source)

Global volcanic effects, not from a supereruption. (Source)

Did you know that not all supervolcanoes are explosive? Or that there are supervolcanoes throughout the Solar System? Next week, we’ll take a look at large igneous provinces. After that, we’ll take a look at extraterrestrial megacalderas and how they might further our understanding of Earth’s explosive supervolcanoes.

More information:

Categories: Sunday morning volcano

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