Q:   What happens when a caldera has become a lake or some other large body of water and then collapses? Aside from a large amount of steam, what is the reaction when the water and magma interact?
— JT
A:   This is a good question as we are not too sure. I know of supereruptions that occurred from a large caldera lake formed in previous eruptions, so I can give a partial answer. About 22,000 years ago, the Taupo volcano in New Zealand erupted and produced about 500 cubic kilometers (120 cubic miles) of magma. The deposits are all unusually fine-grained and cold (we can tell this from the magnetic properties). The fine-grained character suggests very strong interaction of cold lake water with hot magma, which pulverized the magma. The deposits also contain large balls of ash that are a bit like hailstones; we interpret this as due to the very large amounts of lake water that were evaporated and blown into the atmosphere. There is also evidence of huge floods, so we think that this water then rained out.
— Prof. Steve Sparks

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Q:   If the Yellowstone supervolcano were to erupt, what climate changes would occur? How long would the climate changes last?
— LB
A:   The latest models suggest several years and that the effects would be a severe cooling, particularly in the summer. The largest eruption of the last few hundred years was Tambora in 1815, in Indonesia. This caused the "Year Without a Summer" in Europe and frosts in New England in 1816. Benjamin Franklin wrote about these effects. A supereruption, however, is 10 to 30 times greater than the 1815 Tambora eruption, and we might expect even more severe climate effects. The climate effects might last much longer due to a polluted environment, but this idea is not yet established in the scientific community. Some scientists have speculated that the effects of having a continental area covered by ash would result in decades of unusual weather.
— Prof. Steve Sparks

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Q:   Would the Yellowstone "supereruption" be a normal eruption like Mount St. Helens, or would it be a flood eruption like the one that occurred during the Permian Era? Also, would the effect of global warming be enough to release significant amounts of methane hydrate, causing mass extinction?
— Sean
A:   A supereruption has many similarities to Mount St. Helens, just on a huge scale. The eruptions are much more powerful and produce hundreds of times more volcanic ash. However, the basic processes are similar. The eruptions are explosive, whereas the flood eruptions produce mostly lava. There have been hundreds of supereruptions from volcanoes like Yellowstone over the past 60 million years on Earth, but none of them as far as we know have led to mass extinctions. Some scientists have speculated that the giant eruption of Toba caused a bottleneck in human evolution but there is not yet any strong evidence for this idea.
— Prof. Steve Sparks

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Q:   Since no one in recorded history has ever seen a supervolcano, how can scientists possibly predict what will happen if and when a caldera erupts?
— Tom in Texas
A:   In fact, what happens is relatively easy to deduce from geological studies. The characteristics of the deposits of ash and volcanic rocks from different kinds of volcanic eruptions are now well-known from historic eruptions like Mount St. Helens. Geologists learn to recognize them in the same way that a biologist learns to recognize different plants and animals. We can, for example, deduce that in the large eruptions of Yellowstone, much of the ash formed very fast-moving pyroclastic flows (hot flows of ash and gas). The deposits are similar to those at Mount St. Helens from 1980, but simply cover a much, much larger area and are much thicker deposits. In 1980, the pyroclastic flows covered an area of 600 square kilometers (232 square miles), whereas in Yellowstone, the area was over 20,000 square kilometers (7,722 square miles). We also use computer models to estimate the speed and power of the eruptions. We find that these models work well for explaining historic eruptions where we have direct observations and also work well in predicting, for example, the area that a pyroclastic flow covers in a supereruption. From geology we can reconstruct what happens quite well. The problem is that we do not have good methods yet to forecast when such eruptions will happen.
— Prof. Steve Sparks

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