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How thick is a continent? Seismic waves and diamonds hold clues


Seismologists studying the reflection of seismic waves inside the Earth have measured the thickness of cratons, the ancient stable cores of continental tectonic plates. Richard A. Lovett reports.


Almannagja tectonic fault in Iceland.
Almannagja tectonic fault in Iceland.
Dhwee / Getty

Seismologists studying the reflections of seismic waves from the thickest parts the Earth’s lithosphere (the layer that includes the crust) have found a layer of partially melted rock at depths of 130 to 190 kilometres.

These thick parts of the lithosphere, known as cratons, form the oldest regions of Earth’s continents, dating back billions of years. Previous measurements, based on the depths at which diamonds form, have indicated that some may extend to at least 175 kilometres, but some scientists thought they might extend even deeper than that.

A study published in Science, however, used seismic readings from thousands of earthquakes to probe the thickness of nine cratons, scattered from Western Australia to Canada.

The work took advantage of the fact that seismic waves from distant earthquakes reach seismometers by multiple routes. Some travel directly through the Earth’s interior. Others reflect off the underside of the Earth’s surface. Still others reflect off structures within the lithosphere.

“The reflection point is midway between the source and the receiver,” says study author Saikiran Tharimena, a seismologist from the University of Southampton, UK. That means that when observations are pooled from numerous earthquakes, the result is a “powerful tool” for imaging structures deep with the lithosphere.

In this case, what the imaging revealed was a layer of squishy rock that appears to mark the base of the cratons.

Not that it is an ocean of magma. It’s probably only about 1% melted, says Brian Savage, a seismologist at the University of Rhode Island, USA, and author of a commentary in Science. “Think of very slightly melted ice cream.”

Being able to measure the thickness of the cratons has wide implications for our understanding of plate tectonics, Tharimena says, since it helps us understand how they interact with the underlying mantle.

Savage adds that it also may help lead to answers to other questions about cratons, including how they have managed to survive intact for billions of years. “All of these questions come back to how the cratons formed and their stability,” Savage says.

Contrib ricklovett.jpg?ixlib=rails 2.1
Richard A. Lovett is a Portland, Oregon-based science writer and science fiction author. He is a frequent contributor to COSMOS.
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