In recent years, documentary makers have got new mileage out of old classics by taking familiar natural disasters, such as tsunamis and volcanoes, and revealing they can happen on exponentially larger scales. The Indian Ocean tsunami of 2004 was a dreadful event with an enormous death toll, but pales compared to what might happen should half of Gran Canaria fall into the sea causing a megatsunami. In the same way, volcanic eruptions such as those at Pompeii, Pinatubo and even Krakatoa are mere geysers compared to the scale of a supervolcano, such as the one lurking beneath Yellowstone National Park.
Yellowstone might be the best known supervolcano, but it's not the only one we have to worry about. The Long Valley Caldera in California is the site of another supervolcano, one that erupted around 760,000 years ago, blanketing much of western America under volcanic ash. The caldera is a crater, 15 km by 30 km, that was formed by the massive explosion, and the popular ski resort Mammoth Mountain is on its western edge. This month's Geology has a report from David Wark and his colleagues at the Rensselaer Polytechnic Institute that sheds new light on the events that led up to the massive eruption.
By looking at the concentration of titanium present in quartz crystals that were created during the collapse of the caldera, Wark was able to determine that shortly before the explosion—within 100 years—the magma chamber of the supervolcano was recharged with fresh magma, much higher in CO2 and lower in water than before. This recharging evidently destabilized the chamber, and the result was a huge eruption. It is hoped that this research might prove useful in understanding how and when supervolcanoes chose to let go.