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How Melting Glaciers Alter Earth's Surface, Spur Quakes, Volcanoes

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6/09/06

By SHARON BEGLEY

Imagine the surface of Earth as a giant trampoline that accumulated a slab of ice over the winter, and you can get a sense of what a growing number of scientists say is in store for the planet as glaciers keep melting.Alaska Sunrise

Once the trampoline's ice turns to water that drips over the edges in the warm days of spring, the concave elastic slowly rebounds to its original flat shape. That's how Earth responds as glaciers retreat, and the consequences promise to be ... interesting.

The reason is that one cubic meter of ice weighs just over a ton, and glaciers can be hundreds of meters thick. When they melt and the water runs off, it is literally a weight off Earth's crust. The crust and mantle therefore bounce back, immediately as well as over thousands of years. That "isostatic rebound," according to studies of prehistoric and recent earthquakes and volcanoes, can make the planet's seismic plates slip catastrophically, and cause magma chambers that feed volcanoes to act like bottles of shaken seltzer.

"It's unavoidable that glacial retreat will induce tectonic activity," says geoscientist Allen Glazner of the University of North Carolina, Chapel Hill.

The connection between melting glaciers and earthquakes isn't to be confused with a myth that zipped through cyberspace after the 2004 Asian tsunami. It claimed that global warming (which is not even two degrees above historical averages so far) heated magma, causing seismic plates to shift.

Instead, the world-wide melting of glaciers portends a seismically active future because of isostatic rebound and also because the meltwater from liquefying glaciers adds mass atop oceanic plates. That creates a teeter-totter effect, further destabilizing the planet's crust. "Recent findings reinforce the idea that the solid earth and the climate are inextricably linked," says Prof. Glazner.

That link has reared its ugly head in the past, especially during periods of rapid climate change such as the end of ice ages. When ice sheets retreated 10,000 years ago, for instance, Iceland experienced a surge in volcanic eruptions. Volcanoes in the Mediterranean, Antarctica and eastern California also seem to have been awakened by retreating ice.

When he analyzed 800,000 years of activity from about 50 volcanoes in eastern California (the age of rocks formed from volcanic ash can be determined by radioactive dating), Prof. Glazner found that "the peaks of volcanic activity occurred when ice was retreating globally. At first I thought it was crazy, but other scientists also found evidence that climate affects volcanism." The likely mechanism: glacial retreat lifts pressure that had kept the magma conduit closed.

The retreat of ice sheets 10,000 years ago also triggered a wave of powerful earthquakes in Scandinavia. Since isostatic rebound continues for thousands of years, it may still be contributing to quakes in eastern Canada, says geoscientist Patrick Wu of the University of Calgary.

That area has no plate boundaries, the usual site of quakes. But 9,000 years ago, when glaciers retreated, the region began experiencing frequent "intraplate" quakes up to magnitude 7, he finds. They continue to this day, with quakes such as the 6.3 Ungava event that struck northern Quebec on Christmas Day 1989.

"The pressure of the ice sheet suppresses earthquakes, so removing that load triggers them," says Prof. Wu. That creates weakened zones that remain vulnerable to seismic activity to this day, including in northern Europe. "Present-day earthquakes may have their origin in postglacial rebound," he says.

In southwest Alaska, where the Pacific plate thrusts under the continental plate, the immense mass of glacial ice counters the tendency of the plates to slip catastrophically. As global warming melts the glaciers, however, the ice load is diminishing. As a result, Earth is springing back there, too, removing the check on seismic activity. The magnitude 7.2 temblor that shook the area in 1979 is linked to a bounce-back of the crust, conclude geoscientists Jeanne Sauber of NASA's Goddard Space Flight Center and Bruce Molina of the U.S. Geological Survey.

"In southwest Alaska, glaciers have been thinning and retreating by hundreds of meters for 100 years," says Dr. Sauber. "Huge ice loads suppress earthquakes for a while, but when you remove the ice it is easier for them to occur." She therefore calls them "promoted" earthquakes. Glacial retreat, she says, "is another factor that has to be looked at when we assess seismic hazard."

Alaska isn't the only place where glacial retreat due to the current warming coincides with active faults. It does so in the Andes, the Swiss Alps, the New Zealand southern Alps, the Rocky Mountains, the Himalayas and the edges of Greenland (where scientists recently reported that the melting of the ice sheet has accelerated), says geologist Bill McGuire of University College London.

He wrote in the magazine New Scientist that "it shouldn't come as a surprise that the loading and unloading of the Earth's crust by ice or water can trigger seismic and volcanic activity." He told me that "no one knows how much unloading there can be before you trigger certain faults."

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