A warming planet due to human-induced climate change is likely to contribute to an increase in volcanic activity, according to a recent study in the journal Geology.

While a relationship between climate and volcanism might seem counterintuitive, it turns out that pressure exerted by thick glaciers on the Earth's crust — what geologists call "surface loading" — has an impact on the flow of magma below the surface.

The correlation affects "magma flow and the voids and gaps in the Earth where magma flows to the surface as well as how much magma the crust can actually hold," study lead author Graeme T. Swindles, an associate professor of Earth system dynamics at the University of Leeds, wrote in an email to Scientific American.

In the study, published last month, Swindles' team examined the geologic record of eruptions of Icelandic volcanoes 5,500 to 4,500 years ago — a period in Earth's history when the climate was cooler, but still not a full-blown ice age. The level of volcanic activity was discerned by looking at the record of ash that fell over Europe and settled on the peat bogs and lakes, Swindles says.

Comparing the volcanic record with glacial coverage, the team found that the number of eruptions dropped significantly as the climate cooled and ice cover increased. The eruptions that did occur also tended to be smaller in magnitude.

"There's a big change in the record in the mid-Holocene [epoch], where we see no volcanic ash in Europe and very little in Iceland," says Swindles. "This seems to overlap with a time where there's cold climate conditions, which would have favored glacial advance in Iceland."

Swindles says his team found about a 600-year lag between advancing glaciers and diminished volcanic activity. "That's because it takes a long time to grow ice masses," he told the magazine.

In reverse, the team found that as the climate warmed and glaciers melted, there were more and bigger eruptions.

"After glaciers are removed the surface pressure decreases, and the magmas more easily propagate to the surface and thus erupt," Swindles says.

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