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Geologists say they have found the mysterious missing layers of Earth's crust, solving one of the great geological puzzles and laying to rest the question of why the crust is so much thinner than comparable layers of Mars and the moon.

Don Anderson, professor of geophysics at the California Institute of Technology, said Sunday he and his team found the crust 250 to 400 miles beneath Earth's surface in a region of the planet's middle mantle, also known as the transition zone.Anderson said the missing crust was found using a highly sophisticated technique that allowed the scientists to determine the composition of the planet's middle region by studying Earth's deep vibrations or seismic waves.

The crusts of the moon and Mars comprise 10 percent of those planetary bodies, but Earth's crust is only four-tenths of 1 percent of the planet's terrestrial mass.

The team of geologists said if this newly found crust was returned to Earth's surface where it originated millions of years ago, the outer crust would be about 125 miles thick instead of the estimated average thickness of only 12.5 miles.

Discovery of the crust material is evidence of the violence of the earthquakes that have helped create the mountains, islands and the shapes and contours of all the continents by forcing vast plates of Earth's crust into the planet's deep inner layers, Anderson said.

"The middle mantle appears to be a dumping ground for the huge slabs of crust that dive into Earth's interior at subduction zones," he said, "such as the `ring of fire' (volcanic region) around the Pacific."

Earth consists of concentric layers of varying chemistry and density. The outer shell or crust forms the continents and basins of the oceans. Beneath that is the mantle, followed by a liquid core that surrounds the planet's solid iron center, located thousands of miles below the surface.

Anderson and his team speculate that because Earth's crust is broken into massive plates that slide around under the surface, this activity permitted portions of the crust to sink far into the mantle.

The mantle, which contains most of Earth's mass, is approximately 1,800 miles deep, but scientists know less about it than they know about the composition of the sun, Anderson said.