See a slideshow of stunning images of the volcano and the after effects of the acidic flood
ALL the fish and birds were gone, the trees were defoliated and their mosses dead, and the local lakes and rivers were lined with yellow scum. This was the aftermath of a mysterious catastrophe in July 2005, as reported by a lodge-owner near the remote Chiginagak volcano in Alaska.
A group of geologists has now pieced together what happened. In May 2005, they say, a torrent of sulphurous liquid and mist gushed from the volcano鈥檚 icy crater, leaving a trail of sulphur deposits and turning salmon spawning-grounds as acidic as lemon juice.
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鈥淲e haven鈥檛 seen an event like this in the historical record,鈥� says Johan Varekamp of the Wesleyan University in Middletown, Connecticut, who reviewed the group鈥檚 work.
Satellite pictures show that Chiginagak鈥檚 crater was filled with snow and ice in November 2004. Sometime after that, heat from the underlying magma melted the bottom of the ice, says Janet Schaefer of the Alaska Volcano Observatory in Fairbanks. Alerted to the flood by locals, she and her team visited the site four times between August 2005 and August 2006, taking samples from the waterways and vegetation, and reconstructed the events with the help of satellite pictures.
They say that as the ice melted, sulphur dioxide and other gasses bubbled up, acidifying the meltwater. By early May 2005, the water had carved a tunnel through the base of the glacier, releasing almost 4 million cubic metres of mud, rocks and highly acidic water (Geochemistry, Geophysics and Geosystems, in press).
鈥淎s the crater鈥檚 ice melted, sulphur dioxide and other gasses bubbled up, acidifying the meltwater鈥�
The floodwater reached Mother Goose lake, 27 kilometres from the volcano, wiping out life in the lake and preventing salmon from making their annual journey to their spawning grounds. Schaefer鈥檚 team sampled the lake鈥檚 water and found its pH to be 3, similar to lemon juice.
About a tenth of active volcanoes house crater lakes, says Varekamp, and about 15 of these are known to be highly acidic. Some of them, such as the one on Poas volcano in Costa Rica, leak continually into surrounding waterways. But Schaefer鈥檚 colleague William Scott at the US Geological Survey in Vancouver, Canada, says the unusual aspect of the Chiginagak event was the dense and lethal acidic mist. Schaefer says the gasses may have burst from the meltwater when it was suddenly released from the crater, or that the violent flow created a mist like that around a waterfall. The cloud killed plants over 30 square kilometres and 150 metres above the valley floor.
Acidic liquid is still oozing from the Chiginagak crater, but surrounding ecosystems are recovering: mosses are growing back and much of the vegetation that was defoliated by the acid is growing leaves again. Salmon have not yet returned to the lake, however, probably because acid levels remain too high, but the salmon have returned to the waters downstream.
Although this is the first official record of this type of catastrophe, it may have happened at Chiginagak before on a smaller scale: lodge owners reported seeing similar yellow scum in the 1970s, Scott says.
