Volcanoes are the planet’s way of recycling materials back into use from the surface. Thanks to plate tectonics, there is a constant burial of continental plate material as different plates plunge under one another. Once melted down and mixed, that material can come back as lava. And that lava – volcanoes in their many different formats all share one thing in common – they are the one way we can sample the Earth’s mantle.

The local composition of the mantle should be related to what kinds of crustal material are getting recycled, but apparently, sometimes the messy surface materials don’t make it down to the lava sources. In Indonesia, researchers have been sampling materials from the Sunda Arc. This arc of volcanic islands traces out the region where the Indo-Australian Plate plunges under the Burma plate at a whooping 63-70 millimeters per year.

No one ever said geology was fast.

Researchers expected that the volcanoes along this arc would have a chemical distribution that indicated a rich mix of minerals from the crust, but they didn’t find that. What they found were tiny crystals in the lava that formed at depth had chemical abundances representative of the mantle that wasn’t polluted by crust. Exactly how that happened is unknown, but the scientists were able to measure a variation along the volcanic arc, with the purist materials being volcanoes on Bali and the chemistry getting more mixed across Eastern Java and into Central Java.

This research, which appears in Nature Communications and is led by Frances Deegan, was made possible by new technology capable of measuring the smallest crystals in the lava and the gases they had trapped in them.

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Lava from Bali Volcanoes Offers Window into Earth’s Mantle (Eos)

“Sunda arc mantle source δ18O value revealed by intracrystal isotope analysis,” Frances M. Deegan et al., 2021 June 24, Nature Communications