Two of the most powerful forces on Earth are glaciers and volcanoes. What happens when they collide?

One dampens the other. And one makes the other more dangerous.

During the most recent ice age, glaciers covered a third of Earth’s surface. During this time, volcanic activity declined …

Because the sheer weight of ice pressing down on the land increased the pressure on Earth’s mantle beneath the crust.

The higher pressure raised the mantle’s melting point, so that the fiery temperatures couldn’t melt it as easily to form volcanoes.

When the ice age ended and glaciers retreated, the surface rebounded. Pressure on the mantle decreased, and it was easier to melt into magma, which erupted on the surface as volcanoes.

Even today, when volcanoes occur at high latitudes, glaciers can moderate their activity.

On the other hand, when a volcano does erupt beneath a glacier, it can melt it from the bottom up. Steam hollows out a huge chamber inside the glacier, which fills with millions of gallons of meltwater.

As the lava continues to flow, it melts more ice, until it eventually breaches the top of the glacier or forms a lava tunnel within it. And all that water is catastrophically released.

These so-called “glacial outburst floods” are powerful and often unexpected and therefore can be very dangerous.

So who wins in this epic power struggle? Glaciers beat volcanoes. Until volcanoes beat glaciers.