WHEN Mount Pinatubo erupted in 1991 it killed hundreds of people and displaced around a million from their homes. But that may not have been the end of it. Fine ash from the Philippine volcano could still be killing people today.
Geologists believed that when Mount Pinatubo erupted 11 years ago, it launched debris toward the stratosphere in a compact fountain, forming an enormous ash cloud high above the ground. But now an international team of volcanologists says at least part of Pinatubo鈥檚 fountain collapsed, sending debris racing down the flanks of the mountain in what is known as a pyroclastic flow (Geology, vol 30, p 663). This could mean that this and other large eruptions will have much more severe long-term impacts on health than anyone thought.
Till now, nobody thought that pyroclastic flows could spawn a stratospheric cloud as big as Pinatubo鈥檚. But the team, headed by Sebastian Dartevelle of Michigan Technological University, has found evidence that this is what happened. By studying the sizes of ash particles in a 50-kilometre circle around the crater, Dartevelle鈥檚 team discovered a much greater proportion of very fine ash near the volcano than could have come from a central fountain. The fountain must have collapsed, says Dartevelle.
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As the pyroclastic flows sped over the Philippine countryside at more than 120 kilometres per hour, the turbulent flows of hot debris drew in air. As that heated, it rose to high altitude, taking the finest ash with it and forming a vast cloud. Huge amounts of ash filled the air around the volcano close to ground level, instead of shooting directly high into the atmosphere.
This ash still contaminates the area and poses a serious health threat to people living there. Inhaling it could cause a host of respiratory problems, including pneumonia, bronchitis, asthma, emphysema, and even lung cancer. After the eruption, the death rate among those living in Pinatubo鈥檚 shadow rose to 16 deaths a week per 10,000 people, and the leading cause was respiratory infection. The deaths were attributed to poor health and nutrition, but Dartevelle and others suspect ash is to blame.
The problem could persist for years because the tiny particles are light enough to be picked up by the wind again and again. It鈥檚 especially bad news for local communities still coping with massive volcanic mud flows during the rainy seasons, and the threat of flooding from Pinatubo鈥檚 crater lake (快猫短视频, 15 September 2001, p 6).
At present, evacuation plans don鈥檛 take these long-term dangers into account. Hazard teams tend to focus on evacuating people before big eruptions like Pinatubo鈥檚, but don鈥檛 generally consider the aftermath. 鈥淣ow there鈥檚 also this long-term health issue,鈥� says Mark Stusiak of the Geological Survey of Canada in Vancouver.
But while the collapse of the ash cloud was bad for locals, it was better overall for the planet. Clouds that arise from pyroclastic flows have less effect on global climate because they inject far less ozone-damaging hydrochloric acid into the stratosphere. These clouds tend to be much wetter than those formed by fountains because they spend more time in the moist lower atmosphere, where the water scrubs the hydrochloric acid from the air. The same process also scavenges some sulphur dioxide, a potent greenhouse gas.
Dartevelle suspects this type of eruption is much more common than anyone thought. 鈥淚 think we have misinterpreted past eruptions,鈥� he says. 鈥淭he consequences for people living around the volcano will be completely different.鈥�
