Researchers at Harvard and the University of Hawaii believe they’ve resolved a long-standing controversy over the roots of islands — volcanoes in the middle of tectonic plates — showing that the islands’ lava provides a window into the early Earth’s makeup. Assistant Professor of Geochemistry Sujoy Mukhopadhyay and Helge Gonnermann at the University of Hawaii ran sophisticated computer models examining changes in gases dissolved in magma as they rise from the mantle through the Earth’s crust. The magma emerges as lava, sometimes in spectacular eruptions. As it cools, it can pile up to enormous heights, building ocean islands such as Hawaii’s Mauna Kea, the world’s largest mountain measured from its base to its summit. The controversy revolves around how one interprets apparently conflicting evidence presented by the helium in the magma of oceanic islands versus that in mid-ocean ridges — the long undersea mountain chains that run along the sea floor where tectonic plates spread apart and new oceanic crust is created. One measure — the ratio of two different types of helium called isotopes — indicates that the lava making up oceanic islands is in part derived from the Earth’s mantle and has been unchanged since the formation of the Earth. The second measure, however — the magma’s low concentration of helium — seems to indicate that the part of the mantle that melts to produce the oceanic island has been previously melted, which would let helium gas escape. This would indicate that the lavas making up oceanic islands like Hawaii have been recycled, going through a process of melting and solidifying and melting again, like lavas that erupt in the mid-ocean ridges.

In a report in the Oct. 25 issue of the journal Nature, Gonnermann and Mukhopadhyay explain that the low concentration of helium in island magma doesn’t have to mean that it has been recycled. The two showed that helium would be lost from the island magma as it moved to the surface for the first time and as the enormous pressure it was under decreased.

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