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Over the past 3 billion years, at least six supercontinents have assembled and split apart in what some earth scientists refer to as the supercontinent cycle. About that much there is general agreement; just how this process works, however, has been a source of considerable controversy. One camp would have the continents spread apart and reunite much like an accordion. Another thinks that land masses break up and circumnavigate the globe before getting back together again. The resolution lies in establishing the crystallization dates of oceanic crust that was generated or consumed during continental movement, but "seeing" back further than one cycle has proved difficult. The authors have used a unique radioactive-isotope ratio to determine that both tectonic processes have likely been at play.

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