Breaking plates.

ome of the consequences of the motion of Earth's tectonic plates are quite evident: various mountain chains for instance. Others, such as events occurring deep in subduction zones where one plate plunges beneath another, can be seen only indirectly through seismic imaging techniques. The approach allows the structure of Earth's interior to be reconstructed from observed seismograms, and it continues to pay dividends. An example appears on page 763 of this issue, where Levin et al. 1 argue that they have detected two instances of break-up of a sub-ducting slab at the junction of two oceanic plates. There are indications 2,3 that slab disintegration has occurred where oceanic and continental plates collide. But because earthquake activity is uniformly distributed with depth in subducting oceanic plates, it had seemed that purely oceanic plates maintain their integrity. Levin et al. imaged the structure of the upper 200 km of Earth's mantle in the northwest Pacific, in the complex setting where the Aleutian and Kamchatka subduction zones meet (Fig. 1). South of the junction beneath Kamchatka, they identified a high-velocity feature co-located with earthquakes. This they interpret as cool, rigid subducting oceanic plate. North of the junction, velocities were slower than average, with no evidence of deep earthquakes, the implication being that there is no subducting slab there. This is an unexpected finding. In the past, subduction also occurred further north, the plate concerned being generated by ocean-floor spreading in the Komandorsky basin (Fig. 1a). If oceanic plates are rigid, the previously subducted plate north of the junction should still be attached to the Komandorsky basin, which has remained at the surface. Levin and colleagues' conclusion is that the missing plate has broken off and descended deep into the mantle (Fig. 1b). Subduction zones are regions where Earth's surface returns to cool the hot interior. Counterintuitively, they are also the site of volcanism. The melting of rocks in Earth's interior that is required for this volcanism is thought to result from the entry of hydrous fluids into the hot overlying mantle from the descending oceanic crust 4–6. North of the junction lie volcanoes that are now extinct, and the inferred absence of oceanic crust there is taken by Levin et al. as an explanation: there is now no source of water to promote magmatism. The volcanoes have been extinct for at least five million years, so the plate must have become detached before …