The hydrangeas of plate tectonics

The blooms of hydrangeas change colour according to the aluminium content of the ground, and a look at the flowers helps understand the soil below. Similarly, volcanic arcs, which form on the edge of an overriding plate in a subduction system, can expose material that originates from the mantle. Brought up to the surface through volcanic eruptions, the resulting rocks can reveal the characteristics of the downgoing oceanic crust. On page 286 of this issue, Straub and colleagues1 report a lead isotope ratio in volcanic rocks collected from the Izu-Bonin island arc at the northwest edge of the Pacific Ocean that unexpectedly links them to the mantle in the Indian Ocean domain.