About 4.5 billion years ago, a Mars-sized object smashed into the young Earth, spraying debris that coalesced to form the moon, many scientists think. Some remnants of that object, called Theia, exist today as large amounts of dense material sitting atop Earth’s core, researchers propose November 1 in Nature.
In recent years, geophysicists have discovered continent-sized zones of rock at the base of Earth’s mantle where seismic waves travel abnormally slowly, suggesting the rock there is denser than the rest of the mantle rock. One of these blobs, known as large low-velocity provinces, lies beneath Africa. The other lies half a world away beneath the Pacific Ocean, says Qian
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