To investigate the history of plate subduction-when one of Earth's
tectonic plates slides beneath another -Sarah M. Aarons and colleagues
compared ancient rocks from the Acasta Gneiss Complex in Canada to
modern rocks. Using isotope analysis, the researchers found that Acasta
rocks dating to about 4.02 billion years ago (bya) most strongly resemble
modern rocks formed in a plume setting (an area in which hot rocks from
Earth's mantle flow upward into the crust). By contrast, they found that
Acasta rocks dating to about 3.75 bya and 3.6 bya have an isotope
composition that is similar to that of modern rocks formed in a subduction
setting. Aarons's team therefore concluded that
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