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The ocean tides are the latest tools scientists are using to probe the internal structure of the Earth.

As the gravitational pull of the moon and the sun tug on the Earth to create tides, the rise and fall of the oceans flex the rock underneath them by as much as about an inch (2 centimeters). Until now, researchers had not used this flexing to pin down details on the planet's interior.

Now scientists have employed a dense network of more than 700 global positioning system (GPS) receivers installed across the western United States to monitor the crust's response to ocean tides. They were able to observe the way tides flexed the Earth's surface down to as little as 1 millimeter.

Density at depth

This approach provides new key details about the planet, including estimates of both the density and elasticity of the crust, the solid, rocky top layer of the planet. Density measures how much mass there is in a given space, while elasticity is a measure of how a material responds to stress, such as what builds up between Earth's plates.

"It is difficult using seismology alone to separate the effects that variations in density have from those associated with variations in elastic properties," said researcher Mark Simons, a geophysicist at the California Institute of Technology.

Their data provided a look at the variations in density from Earth's surface down to a depth of about 250 miles (400 kilometers). This helped determine the structure of the asthenosphere -- the planet's weak and viscous upper mantle layer -- in an area beneath the western United States and nearby offshore regions.

"The asthenosphere plays an important role in plate tectonics, as it lies directly under the plates," said researcher Takeo Ito at Nagoya University in Japan. "The results of our study give us a better understanding of the asthenosphere, which in turn can help us understand how the plates move."

Lower than average

The researchers found that the density of the asthenosphere under the western United States and the eastern Pacific Ocean is abnormally low, suggesting that it might be about 570 degrees Fahrenheit (300 degrees Celsius) hotter than the global average at those depths. This might be linked with the volcanic history seen in the western United States at places such as Yellowstone as well as the uplift of the Colorado Plateau, but "any direct causal relationship is conjectural at this point," Simons said.

This kind of data provides keys to understanding the chemical and mechanical dynamics of the planet, such as how heat flows through the mantle and how tectonic plates on Earth's surface are evolving. Now the researchers hope to use dense GPS arrays across the globe, Simons told OurAmazingPlanet.

"The method we developed for gathering data from GPS devices has significant potential for improving 3-D images of Earth's internal structure," Ito said.

The scientists detailed their findings online April 14 in the journal Science.