No Core In Volcanoes

Bristol University

A hot debate in the Earth Sciences is finally resolved in this week’s issue of Nature. Researchers from the Department of Earth Sciences at Bristol University show that large volcanoes do not contain material from the Earth’s core. This overturns previous theories that conflicted with models of how the Earth’s magnetic field is sustained.

The magnetic field results from the movement of liquid iron in the core and affects everything from bird migration to the navigation of aircraft, so it is crucial to understand how it has been sustained over geological time.

Large volcanoes, like those on Hawaii, form in response to the up-welling of hot, molten lava in so-called ‘plumes’. These plumes have long been thought to originate from depths of almost 3,000 km within Earth, right on the edge of the Earth’s core. If this were the case, the plumes would be expected to contain small amounts of material from the core, which would tell scientists a great deal about how the interior of the Earth works. Up until recently analysing such material was extremely difficult because the necessary tools were not available.

The Earth’s core contains trace amounts of the rare element tungsten. The core has different tungsten isotope ratios to the rest of the Earth and so tungsten isotopes provide a very clear way to detect any core material. The scientists at Bristol reasoned that if the Hawaiian lavas contained small amounts of the Earth’s core, then they should be able to see the core’s characteristic tungsten signature.

The Bristol team developed a new, high-precision method for this very challenging test, and then analysed samples of Hawaiian lavas and South African kimberlites – rocks that bring diamonds to the surface from great depths. Dr Anders Scherstén, principal investigator, said: ‘To everyone’s surprise, we found no evidence in these rocks of any contribution from the Earth’s core. While it is disappointing that we do not have a highway to the innermost Earth beneath Hawaii, these results are extremely important in our understanding of the history of the core and the Earth’s magnetic field.’

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