The eruption of the Indonesian supervolcano Toba is one of those things you hope will never happen again – at least not in your lifetime. Taking place around 74,000 years ago, it turned a good part of the region into a giant ashtray and resulted in a six-year long nuclear winter. That said, for all that's known about its aftermath, scientists can only speculate about the factors that triggered the eruption. However a new study suggests that tiny quartz crystals hiding amidst volcanic ash and rock might hold the answer to this enduring mystery.

In the same way tree rings record climate change, quartz crystals found in magma are a useful tool in helping researchers learn more about the changes that take place as a supervolcano primes itself for a megablast, says lead author David Budd, a researcher at Uppsala University. "When the conditions in the magma change, the crystals respond and produce distinct growth zones that record these changes."

What makes supervolcanoes like Toba unique is the dense magma reservoir that lies underneath it. Scientists believe that this reserve, which is kept under wraps by a thick crust, is the reason for the long intervals between super-eruptions as well as the sheer scale of these events. According to Budd, when the magma is subjected to a combination of high pressure and temperature, it sets off a chain reaction of dynamic interaction processes that affect the magma composition, which in turn affects the eruption style and behavior of the supervolcano.

The researchers believe that this is what happened in the case of Toba. Analysis of volcanic quartz crystals from the region revealed a distinct shift in the isotopic composition towards the outer rim of the crystals. Differences in oxygen isotope concentrations are an important indicator of temperature fluctuations, and in this case, the relatively lower proportion of the heavy isotope 18O compared to the lighter 16O (the most common isotope found in nature) indicates that something in the magmatic system changed drastically just before the big eruption.

In the study of ocean waters, lower levels of heavy oxygen (i.e. 18O) are often associated with warmer temperatures on a global scale. In this case, it indicates that the magma melted and assimilated a large volume of a local type of rock that contains a lot of water, which upon release into the magma, produced steam and resulted in an increase in gas pressure inside the magma chamber, explains co-author Frances Deegan, a geologist at the University of Uppsala. "This rapidly increased gas pressure eventually allowed the magma to rupture the overlying crust, and send thousands of cubic kilometers of magma into the atmosphere."

While this is not the first study that analyzes quartz crystals found in magma for information about super-eruptions, it corroborates findings of a study published last year where researchers from Vanderbilt University and the University of Chicago demonstrated, via the analyzation of titanium concentrations in quartz crystals, how it would take only a year for the accumulation of gasses within the chamber to reach critical mass once enough magma has built up in the reservoir. In other words, we would have just one year to prepare for a super-eruption.

That said, while these studies shed light on what might trigger a super-eruption, researchers are still unsure of what the signs of an impending super-eruption look like, a conundrum that is made more challenging by the fact that none of these have ever taken place in recorded human history. What is known, apart from the death toll and environmental consequences, is that it would have a devastating impact on the world economy. For comparison, though not a super-eruption, the 1980 Mount St. Helens catastrophe cost more than US$1 billion in property damage and economic losses. Considering that the last Yellowstone explosion, which took place 640,000 years ago, was a thousand times the size of the Mount St. Helens volcano, it isn't hard to imagine the kind of damage that an eruption would wreck on the national economy, starting with the agriculture industry.

In recent years, Italian geologists have been keeping an eye on the rumblings of Campi Flegrei in Naples, which some believe could soon be approaching its critical degassing pressure. For the half a million currently living in the vicinity of the caldera, it might be a cause of concern that the Italian government has raised the alert from green (nothing to worry about) to yellow (actively monitoring). Still, some scientists say that there's no point fretting over this as the rumblings might not amount to anything much. According to volcanologist Guilherme Gualda, it doesn't house the type of melt-rich, giant magma body needed to produce a super-eruption, though the fact that past eruptions have taken place makes it likely that another one will occur in the future. For those who take solace in numbers, it might be comforting to know that this is probably not going to happen soon: according to one set of calculations, only 1.4 super-eruptions occur every one million years.

The study was published in Scientific Reports.

Source: Uppsala University

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