Replying at Watt’s Up With That? to an article at the Scientific American web site claiming that the disappearance of glaciers due to global warming would lead to an increase in volcanic eruptions, geologist Steven Goddard provides a rebuttal and declares that theory to be bovine excrement:
The geothermal gradient of the earth is typically about 40°C per km, so a 0.5°C change in temperature is equivalent to a depth change of about 20 metres. Near mid-ocean ridges this gradient is steeper, so the equivalent depth change in Iceland would be less than 20 metres. Is it credible that a 0.5°C decrease in the melting point could stimulate excess volcanic activity? Short answer – no. Volcanic activity is caused by magma rising to the surface, not glaciers melting. However, the loss of the glaciers would reduce the amount of steam and ash generated. Ash is formed when magma is cooled and fractured by steam. So the loss of the glaciers would reduce the size of the steam/ash cloud and make the Iceland volcanoes behave more like Hawaii volcanoes.
In short, the loss of all ice in Iceland would make the volcanoes less destructive.
Read the whole thing for a good lesson in igneous petrology and volcanology.
UPDATE: More from Goddard, who explains that it’s not that climate change causes volcanoes, it’s that volcanoes can cause climate change.