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Ashes On Our Heads

Gigantic volcanic eruptions can have global consequences, but they still cannot be forecast or prevented. Luckily for civilization, the laws of nature work in our favor.

Among many doomsday or fatalistic scenarios, “super-volcanoes” have become the recent hot topic. Why?

It’s almost certain that a super-volcanic eruption will happen, probably within 100,000 years. But the odds that it will happen in our lifetime are small. However, the probability is still perhaps 100 to 1000 times greater than the odds of earth’s collision with a large asteroid, and the consequences could be comparable. We have reason to worry.

A super-volcanic eruption delivers at least five hundred cubic kilometers of magma to the surface (the biggest ones could produce as much as ten thousand) in a matter of days. These eruptions supply ash, rapidly moving obliterating pyroclastic flows, and poisonous gas into the atmosphere. All of this happens in response to caldera collapse, when a piece of a land measuring thirty by thirty kilometers or more faults suddenly (in a matter of hours) and sinks down to several kilometers into the molten magma chamber.

As the surface rock is getting buried, it pushes more gashing magma, ash, and gas high into the air until it shuts itself down. The collapsing piece of surface and the territory around it – which might cover ten times as much area as the caldera itself – now lies buried under tens to hundreds of meters by hot rocks, ash, and debris, which soon solidifies and forms volcanic tuff. Remember Pompeii? Just like that.

A continental ash blanket

An eruption of Lava Creek Tuff of Yellowstone 640,000 years ago created a caldera measuring seventy by forty kilometers and turned the area equivalent of the size of half of the Netherlands into an unlivable desert covered by steaming ash, thereby obliterating all life. The entirety of North America was covered by fine-grained volcanic ash ranging in thickness from many tens of centimeters five hundred kilometers around the eruptive center to millimeters near the East Coast. Rains delivered ash and pumice into rivers, clogging the Mississippi for months or years. Sulfuric acid and halogen poisoning from ash likely killed most open-breeding livestock.

But the worst is yet to come. Such large eruptions last for many days and weeks, incessantly supplying ash and sulfur dioxide gas into the stratosphere, where they spread laterally with a jet stream speeds of a flying Airbus jet. Since Earth’s atmospheric circulation is from the equators to the poles, and since the boundary between the lower troposphere and the higher stratosphere is as low as eleven kilometers near the tropics, particularly nasty super-eruptions will introduce large amounts of ash into global circulation systems. Eruptions like the famous “Toba eruption” in Indonesia 70,000 years ago pose a threat to hemispheric and global climate.

We know about the effects of super-volcanoes because past eruptions of large magnitude supplied sulfuric acid aerosols to the ice caps in Greenland and Antarctica. By looking at ice cores, we can study past “unknown” events and try to determine their source, for example in the Andean highlands or in the forested wilderness of the Kamchatka peninsula. Modern civilization has never experienced the effects of such eruption, which would be a hundred times as large as the famous eruption of Krakatoa in 1883.

What can we do against such a threat? We cannot prevent it. But the good news is that such eruptions are preceded by a series of indicative events, often decades prior to an eruption. Rapid groundswells over a large territory by many meters or tens of meters are easily detectable by satellite and aircraft. The same goes for early gas escape, which can cause specific patterns of dead trees, and for a series of characteristic long-period earthquakes indicating magma movements. Then there are, of course, precursor eruptions and hydrothermal explosions that can be seismographically detected.

Current geophysical methods of seismic imaging do not provide a picture of complete certainty in order to permit the estimation of the size and state of the magma chamber and sometimes confuse hydrothermal and magmatic reservoirs. However, one would think that in the coming decades methods will improve dramatically and we will be able to “MRI” image the monstrous magma chambers when they assemble at shallow depths.

What we don’t know yet

But while we can predict that an eruption is coming within a few years, we often cannot predict its exact date, whether it is going to be a strong eruption or a series of smaller eruptions, and its character. Yet for public policy as well as regional and national economies, these are important questions. For example, when do you warn people to abandon their houses and evacuate, how long should they stay away, what are your liabilities if nothing happens despite a costly evacuation?

Some dream even bigger and ask: ‘Can we diffuse a large caldera-forming volcanic eruption?’ Presently no. We cannot even deal with tornadoes and typhoons, and we understand too little about the internal dynamics of volcanoes.

Even in the 21st century, we are rather clueless. How long does it take for magma to assemble in a shallow subsurface reservoir? Estimates range from a hundred to one million years. How much magma is getting erupted, and how much stays behind to fuel future eruptions? Estimates oscillate between ten and hundred percent. Why does magma sometimes erupt violently at once, but in many other cases oozes quietly out over hundreds of thousands of years? We don’t know.

But there’s good news as well. During an eruption, sulfur dioxide forms sulfuric acid aerosols that stay in the stratosphere for years after the eruption, causing the surface to cool (some scientists have proposed using such aerosols to prevent global climate warming). By examining the ice core record and performing atmospheric circulation modeling, we have learned that “volcanic winters” or “years without summer” can last from several years to a decade. But after a while, the climate reverts to normal. This is in contrast to the earlier doomsday scenarios of super-volcanic eruptions causing glaciations and bottlenecks in human evolution.

The danger posed by super-volcanoes is largely instantaneous: They can destroy whole regions and cause damage to the global economy. But over the long run, the earth is remarkably resilient.

Read more in this debate: Daniel Guéguen, Bill McGuire, Massimiliano Vasile.

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