The seismic anomaly beneath Iceland extends down to the mantle transition zone and no deeper

A 3-D teleseismic tomography image of the upper mantle beneath Iceland of u nprecedented resolution reveals a subvertical low wave speed anomaly that i s cylindrical in the upper 250 km but tabular below this. Such a morphologi cal transition is expected towards the bottom of a buoyant upwelling. Our o bservations thus suggest that magmatism at the Iceland hotspot is fed by fl ow rising from the mantle transition zone. This result contributes to the o ngoing debate about whether the upper and lower mantles convect separately or as one. The image also suggests that material flows outwards from Icelan d along the Reykjanes Ridge in the upper 200 km, but is blocked in the uppe r 150 km beneath the Tjornes Fracture Zone. This provides direct observatio nal support for the theory that fracture zones dam lateral flow along ridge s.