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Earth's crust wobbles like dessert

A new, long-term look at the earth's crust suggests that the continental plates are not completely rigid after all.

Rather, when viewed on very long timescales, they can behave with surprising fluidity, report scientists.

"As usual, things are more complicated," says earthquake researcher Professor Roland Bürgmann of the University of California at Berkeley, of the classic view of stiff plates over a more fluid mantle.

It may be that sometimes the crust behaves as if its plates are rigid and strong all the way down, with only the underlying mantle showing any jelly-like elasticity.

But on longer timescales, the same area of crust can act more like pudding, with only the top few kilometres, the rocky lithosphere, staying consistently strong and rigid.

"We're tying to take a new look at an old problem," says Dr Wayne Thatcher, a geologist with the US Geological Survey (USGS).

He and his USGS colleague Dr Fred Pollitz's provocative paper on the changing ground under our feet is the cover story of the April/May issue of GSA Today, published by the Geological Society of America.

"I think of the lithosphere as ice on a pond," says Thatcher. The thicker a patch of ice, the wider and gentler it will dip in response to weight, such as a truck driving over it, he says.

Of course, thinner ice will dip more dramatically under the same weight.

Replace the ice in that analogy with a lithosphere and the truck for mountain ranges and you get a basic picture of the structure underground.

"We tried to use different kinds of loads to probe different properties of the lithosphere," Thatcher says.

Mountains, glaciers, earthquakes

Mountains are one load that the crust responds to on the scale of tens of millions of years.

Other loads are glaciers, which grow and shrink on a mid-range timescale of hundreds to tens of thousands of years.

Briefest of all loads are earthquakes, which trigger adjustments in the crust on the scale of just days, weeks and months.

On the earthquake timescale, a patch of crust behaves rigidly, as does the mantle under it. Using the ice/truck analogy, the ice is thick and strong.

But at the glacial timescale, the mantle behaves more like jelly, effectively limiting the more rigid 'ice' to just the crust.

Finally, on the mountain-load timescales of millions of years, only the upper crust really behaves rigidly, making for the thinnest 'ice', which bows the most under the weight of a truck.

"It's a little different way of trying to put all of the pieces together than has been done before," Thatcher says.

"It turns out that what happens on the short term and what happens on the million-year timescale is different."