One clue to the new thinking is that seismic waves traveling through the planet have long been measured to travel at inexplicably different speeds. Sharp speed changes suggest differing materials. On each side of the planet there are two big, chemically distinct, dense piles or blobs of material that are hundreds of kilometers thick – one beneath the Pacific and the other below the Atlantic and Africa, the researchers say.
"You can picture these piles like peanut butter," McNamara said. "It is solid rock, but rock under very high pressures and temperatures becomes soft like peanut butter, so any stresses will cause it to flow."
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