This article originally appeared on Changing the Climate, a blog written by postgraduate students in Earth Sciences at Oxford. 

The suggestion that volcanic eruptions can influence the climate makes intuitive sense. Volcanoes emit carbon dioxide, an important greenhouse gas, and provide a link between deep carbon reservoirs in the Earth’s interior and the Earth’s surface. Mass volcanic activity can release large quantities of carbon dioxide, warming the climate over millions of years. On much shorter timescales of just a few years, sulphurous volcanic gasses may cause global cooling – a ‘volcanic winter’ – as sulphate aerosols in the upper atmosphere reflect the sun’s radiation.

The idea that a changing climate could actually influence volcanism is an interesting one. The key to this process (at least for terrestrial volcanoes) is ice. Consider Iceland, an extremely volcanically active area which was almost totally covered by ice sheets during the Last Glacial Maximum. Studies of the volume of lavas from various volcanic systems over Iceland have indicated that eruption rates were vastly elevated – up to 100 times higher – after deglaciation compared to during the glacial period or during recent times. There seems to be a strong temporal correlation: the climate warms, the glaciers covering Iceland melt and volcanic activity increases.

So what causes this link?

Ice caps are heavy, and when they melt the land underneath them decompresses as it’s relieved of all that weight. In volcanic systems like the ones on Iceland this is thought to cause more melting of the mantle (the layer beneath the crust which is the ultimate source of the lava erupted from volcanoes), a theory with supporting geochemical evidence from lava compositions, and hence greater eruption rates as this melt is transported rapidly to the surface.

There’s some evidence for this deglaciation-induced volcanism on a global scale. In areas where both glaciers and volcanoes co-existed, eruption frequency appears to increase by two to six times during the last deglaciation compared to glacial and recent times. Eruption frequency is measured simply by counting the number of eruptions that can be seen in the geological record within a given time frame. Unfortunately however, the geological record is far from complete: the magnitude of the climate’s effect on volcanism is disputed, as evidence of past eruptions may have been eroded and lost, skewing our record. Attempts to quantify eruptions during the glacial period are particularly difficult as glaciers are strongly erosive, making skewing even more likely.

If however you subscribe to the view that climate can influence volcanism, an interesting question is what the effects of twenty-first century warming will be on volcanic activity. In comparison to deglaciation, the current extent of ice cap melting has been fairly small – causing less than a tenth of the reduction in ice thickness – so the effect on eruption rates is likely to be much smaller too. Since melt, once produced, has to reach the surface to erupt, we may not even see the consequences of twenty-first century warming for a few thousand years.

On the other hand, ice loss due to a warming climate may also induce the eruption of magma already present in magma chambers. A decrease in the ‘confining pressure’ of a magma chamber can cause it to erupt as the gas within it expands explosively. This occurred at Mt St Helens due to a landslide, but could also occur due to the removal of ice weight as glaciers melt. As glaciers across the globe are retreating at different rates, we may have to think local when investigating how global warming will affect volcanism.

Bibliography

Huybers, P., and Langmuir, C. (2009). “Feedback between Deglaciation, Volcanism, and Atmospheric CO2.” Earth and Planetary Science Letters, 286, p. 479–91.

Maclennan, J., Jull, M., McKenzie, D., Slater, L., Grönvold, K. (2002). “The Link between Volcanism and Deglaciation in Iceland.” Geochemistry, Geophysics, Geosystems, 3, p. 1062-1087.

Tuffen, H. (2010). “How Will Melting of Ice Affect Volcanic Hazards in the Twenty-First Century?” Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, 368, p. 2535–58.

Watt, S.F.L., Pyle, D.M., Mather, T.A. (2013). “The Volcanic Response to Deglaciation: Evidence from Glaciated Arcs and a Reassessment of Global Eruption Records.” Earth-Science Reviews, 122, p. 77–102.