Deep in the Earth, the hot rock of the mantle stretches and deforms like a convecting fluid. Mantle convection is blocked by the overriding lithospheric plates and causes essentially all volcanism on Earth, in particular, the volcanism on islands that overlie unusually hot spots in the mantle. This study addresses two major problems in geodynamics: the origin of the base of the lithosphere and the nature of mantle convection at mantle ?hotspots?. Iceland is a huge volcanic island that is formed by a hotspot where hot mantle is rising beneath, and impinging on the lithospheric plates. The structure of the mantle beneath Iceland is revealed in unprecedented detail by seismic waves that traveled from distant earthquakes to be recorded on Iceland. The speed that such waves traverse the mantle is slowed by excess temperature and the presence of magma, and is different for different travel directions in regions where convection preferentially aligns the minerals of the mantle. On the one hand, the records on Iceland show evidence for a thick (>150 km) and broad (>600 km) layer of hot and partially molten mantle beneath Iceland, which suggests that the hot mantle upwelling is being deflected at great depths by a lithospheric base formed by a stratification in composition. On the other hand, contradictory evidence is revealed by a clear directional dependence of seismic wave speeds, which suggest that the upwelling mantle is being deflected at much shallower depths by a lithospheric base formed by a stratification in temperature. This study aims to resolve this contradiction by using numerical models to simulate, in 3D, the mantle convection, magma generation, and crystallographic alignment beneath Iceland. The investigators will then compute seismic wave propagation through the model mantle structure and quantitatively compare the predicted seismic records with the observed records. Tests of many of such calculations with different mantle properties will be used to identify the least and most likely conditions beneath Iceland. Determining the cause of the lithosphere and the shallowest depths of mantle convection beneath Iceland will advance our basic understanding of this system as well as other settings on Earth and other planets with active convection and magmatism.
