Melting glaciers at the end of the Ice Age may have sped up continental drift, fueled volcanic eruptions

In the new research, geophysicists Tao Yuan and Shijie Zhong used computer simulations, or models, to travel back about 26,000 years into the planet's past. At the time, the massive Laurentide Ice Sheet, which stretched over North America as far south as Pennsylvania, started to recede. Melting ice flooded into the oceans, and sea levels worldwide rose by an average of around 1 centimeter per year.

The scientists discovered that this global thaw may have also had unexpected consequences -- including for plate tectonics, or the internal clockwork that has, for billions of years, torn Earth's continents apart and crushed them together.

According to the team's calculations, the motion of the North American continental plate may have sped up by 25% as the ice melted. Between about 12,000 to 6,000 years ago, spreading at the Mid-Atlantic Ocean Ridge, which sits between the North American and Eurasian plates, may have increased by as much as 40%.

"As ice volume was greatly reduced, it caused a huge motion in Earth's crust," said Yuan, a graduate student in the Department of Physics at CU boulder. "Scientists knew that the ice melting caused the plates to uplift. But we show that they also moved a lot horizontally due to the ice melting."

The researchers published their findings April 23 in the journal Nature.

Their results may have implications for the planet today. Ice sheets over Greenland are once again melting at a rapid rate, which, in a strange twist, could drive an increase in volcanic eruptions in Iceland not far away.

"That story that we've been telling for a long, long time -- that processes like seafloor spreading and continental drift operate at timescales of millions of years driven by Earth's internal engine, thermal convection," said Zhong, a professor of physics. "That's still true, but we show that glacial forcing can also cause significant motion on relatively short timescales of 10,000 years."

Moving gears

The research, which was funded by the U.S. National Science Foundation, takes a deep dive into the Mid-Atlantic Ocean Ridge. This feature runs for thousands of miles down the center of the Atlantic Ocean and cuts through the island of Iceland. It's a turbulent place: There, magma from deep within the planet bubbles up through the crust, cooling into solid rock and helping to force the continents of North America and Europe away from each other.

For generations, scientists believed that this process was largely steady -- with the ridge spreading by a consistent 2 centimeters every year for the past several million years.

"That's a fairly well-known, textbook number," Zhong said.

But could the textbooks be wrong?

To find out, Zhong and Yuan used computer models to recreate the Earth as it was thousands of years ago. The researchers simulated what might happen as glaciers that were kilometers thick disappeared from modern-day Canada and Greenland -- shifting that weight off dry land and into the ocean.

It helps to picture the globe as a memory foam mattress. If you're lying on a mattress and get up, the foam will slowly bounce back to its original shape. Something similar happened on Earth as ice sheets melt, Zhong and Yuan said.

As the weight of the Laurentide Ice Sheet was redistributed around the planet, parts of North America began to bounce back up. (Today, land around Canada's Hudson Bay is still rising by around 1 centimeter per year because of that rebound). According to the new study, the melting may have also affected the horizontal motion of North America and the Mid-Atlantic Ocean Ridge.

Volcanic eruptions

The thaw may also have had explosive consequences for Iceland, which sits not far away from Greenland, Yuan and Zhong said.

Geological evidence, for example, suggests that the island underwent a period of intense volcanic activity at the end of the last Ice Age, which has since quieted down. Enhanced spreading at the Mid-Atlantic Ocean Ridge due to ice melting from Greenland may have contributed to that fiery past -- allowing more magma to rise to the surface, fueling the eruption of volcanoes and geysers.

"This pattern of volcanism may have been partly due to the glacial melting that we studied," Zhong said.

Today, ice over Greenland isn't melting fast enough to have much of an impact on the planet's continental drift. But it could still have a major influence on Iceland over the next several hundred years, especially if glaciers begin to disappear at an accelerating rate.

"Ice sheets in Greenland and West Antarctica are still melting," Yuan said. "We think the ice melting could enhance seafloor spreading and volcanism at nearby mid-ocean ridges in the future."