Historical Super Volcanoes

Again we get a quick lesson in questioning our initial assumptions.  When you think about it we know of two clear active super volcanoes today.  One is Yellowstone and the other is more convincingly Iceland.

Rather obviously the ash production can be stretched out over centuries but still leaving a concentrated geological message.  In fact the volcano may quickly lose its access to water in particular and have mostly dry eruptions with a lot less ash production.

So in its way the archaeological record is telling us to relax and not be too concerned about super volcanoes.  We can live with that.  One less remote fear is always welcome.

The real control on the size of a volcano it a pyramidal geometry about 10,000 feet high and no one has seen two very close together.  They act as caps and they are inherently weak.  A major eruption is usually heralded by the explosion or collapse of the cone followed by a long period of rebuilding.  Yet the size limits put an outer limit over how big it is possible to be.

It is still messy but we may have seen best examples.

The only truly scary volcanoes are rare basalt flows which Hawaii shows us to be largely ash free.  The only problem is that they like to cover a thousand square miles and must be obviously avoided.  That has not stopped us from living on Hawaii.  It is less threatening than a monsoon.

APRIL 19, 2010

Proper civil defense planning needs to have an accurate and precise picture of the threat and effects. If we overestimate the effect we would misallocate resources or not take proper steps to mitigate impacts. If we properly determine that fire and climate risks of nuclear bombs can be minimized by ten times or more with simple steps, then we should do so. 

Recent archaeological and geological work in India seems to support claims, suggesting the environmental impact of the Lake Tuba volcanic super-eruption was much less than previously imagined. Firstly, had there been a sudden deforestation event caused by the cooling and drying of the atmosphere, topsoil no longer anchored by trees would be expected to wash down into valleys, where it would quickly accumulate. 

"We are not saying that it wasn't difficult for humans after Toba," says Mike Petraglia at the University of Oxford, who has led the investigations. "We are just saying that we don't think it was a catastrophic change." 

Hominin life appeared to continue in the same vein immediately after the eruption, with hundreds more stone tools in the layers immediately above the ash fall. The team uncovered a similar story 1000 kilometres further north of Jwalapuram, in the Middle Son river valley. "We see very little change in tool technology across the Toba ash. They may have had to relocate for a short period of time, but within a generation or so they were back where they were before, making the same kinds of stone tools," says Chris Clarkson, a stone-tool specialist from the University of Queensland in Brisbane, Australia, who worked at the digs in India.

Background on Volcanic Winter and Supervolcanoes and Nuclear Winter

Toba is a supervolcano on the Indonesian island of Sumatra. It has blown its top many times but this eruption, 74,000 years ago, was exceptional. Releasing 2500 cubic kilometres of magma - nearly twice the volume of mount Everest - the eruption was more than 5000 times as large as the 1980 eruption of mount St Helens in the US, making it the largest eruption on Earth in the last 2 million years (see "Blown away").

The new work portrays a somewhat different view of the eruption from the most popular current theory. Previous computer models of the eruption had suggested the event was truly cataclysmic - very nearly a doomsday for early humankind. With calculations based on the assumption that Toba belched out 100 times more aerosols than the 1991 eruption of mount Pinatubo in the Philippines, and scaling the environmental effects accordingly, the models suggested global temperatures dropped by about 10 °C following the blast. This supports the idea of a decade-long "volcanic winter" and widespread catastrophe

Yet this theory has drawn some criticism since it was first put forward 17 years ago, with scholars such as Hans Graf, an atmospheric scientist at the University of Cambridge, believing that the climate change following the explosion has been wildly overestimated.

For Graf, the crux of the argument concerns the precise cooling effect of the sulphur dioxide released by the explosion. During smaller eruptions, like that of mount Pinatubo, most of the released sulphur dioxide reacts with hydroxide (OH) ions from water molecules in the atmosphere to form particles of sulphate - a highly reflective substance that bounces sunlight back into space before it can warm the Earth. 

Previous estimates had placed Toba's sulphur dioxide production at 100 times that of mount Pinatubo's output. Graf thinks this figure is misguided: recent chemical analyses of Toba's fossilised magma suggests it should be roughly half that. "We think Toba was more of a giant in ash production, not sulphur," says Graf. 

What's more, he says, the atmospheric effect of a super-eruption is incomparable to a smaller, typical eruption. Whereas most of the sulphur dioxide from mount Pinatubo would have been rapidly converted to sulphate particles, there simply would not have been enough OH ions in the atmosphere available to react with all the sulphur dioxide released by Toba, delaying the formation of these reflective particles. Even those particles that had formed would have probably clumped together and settled to the ground rather than stay in the atmosphere. 

Taking all of this into account, Graf and his colleagues suggest a new estimate of global cooling of just 2.5 °C, which lasted for just a few years. According to this model, the effects were also highly regional. In places like India the average temperatures may only have fallen by about 1 °C - not such a dramatic climate shift.

This new view is highly contentious. Alan Robock from Rutgers University in New Brunswick, New Jersey, who came up with the original simulations, stands by his original predictions. "Our model showed that extra water would be lofted into the stratosphere because of warming at the top of the troposphere [the lowest layer in the atmosphere], so water would not be a limiting factor," he says. "We simulate a decade or two of very cold, dry, dark conditions, which would have been difficult for humans to adapt to."

Yet recent archaeological and geological work in India seems to support Graf's claims, suggesting the environmental impact of the super-eruption was much less than previously imagined. Firstly, had there been a sudden deforestation event caused by the cooling and drying of the atmosphere, topsoil no longer anchored by trees would be expected to wash down into valleys, where it would quickly accumulate. "We don't find a rapid influx of soil arriving on top of the ash layers," says Peter Ditchfield of the University of Oxford

To build further evidence, Ditchfield analysed the ratio of different carbon isotopes - which are each absorbed at different rates by different plants - in ancient plant remains in the Jwalapuram region of southern India and the Middle Son river valley in central northern India, both of which are around 2000 kilometres from Toba. He saw only a slight increase in the carbon-13 isotope after the Toba eruption, which suggests there was just a small increase in grassland environments at this time. "Woodlands weren't obliterated by Toba. We see a diverse range of habitats persisting after the eruption, which would have provided a diverse range of game and hunting opportunities," he says. 

Nevertheless, hominin species living at the time of the eruption would undoubtedly have faced tough conditions. The blanket of ash, for example, would have been quickly washed into the freshwater supplies: Ditchfield found deposits up to 3 metres deep on the valley floors where rivers would once have flowed. And there is no doubt that in the years immediately following the eruption the early humans would have had to adjust to colder temperatures, probably having to economise significantly as food resources dwindled.

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