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The seismic evidence shows that he earth's internal structure is in the form of shells of different composition, with density increasing with depth as also the velocities of different types of waves such as P, S, Pdp', PSP', SSP' and so on. The innermost is the core at a depth of 2900 km a surface of discontinuity called Gutenberg discontinuity. So is anothercalled Lehmann at a depth of 5600 km. Most striking change from the point of view of Plate tectonics is the boundary between the lithosphere and the asthenosphere and the shadow zone, a body of earth through which S waves do not pass.