A sharp lithosphere–asthenosphere boundary imaged beneath eastern North America

Plate tectonics on the rocks Although a rigid lithosphere (about 100 km of crust and uppermost mantle) moving over a weaker asthenosphere (a few hundred kilometres of more deformable rock) is a widely accepted aspect of plate tectonics, the properties of the boundary between them are poorly understood. When compressional waves (P-waves) from an earthquake encounter an interface at an angle, some of the energy is converted to shear waves (or S-waves). These ‘converted’ waves can be used to construct an image of the interface. Based on converted waves recorded in eastern North America, the lithosphere–asthenosphere boundary has been resolved as a sharp gradient that cannot be explained by thermal gradients alone, indicating that the boundary is controlled by melt or high water content in the asthenosphere.