Seismic discontinuities are boundaries within the Earth where seismic wave velocities change abruptly. These changes indicate differences in composition, phase, or state within the Earth’s interior.

1. Mohorovičić Discontinuity (Moho): Located at an average depth of 35 km beneath continents and 7 km beneath oceanic crust, the Moho marks the boundary between the Earth’s crust and the mantle. It is characterized by a rapid increase in seismic wave velocities due to the transition from less dense rocks in the crust to denser rocks in the mantle.
2. Gutenberg Discontinuity: Found at a depth of about 2900 km, this boundary separates the mantle from the outer core. It is identified by a significant drop in P-wave velocities and the complete cessation of S-waves, indicating the outer core is liquid.
3. Lehmann Discontinuity: Situated at approximately 5100 km depth, this boundary lies between the outer core and the inner core. It is marked by an increase in P-wave velocities and the reappearance of S-waves, suggesting a transition from the liquid outer core to the solid inner core.

These discontinuities help scientists understand the internal structure and composition of the Earth, providing crucial insights into geological processes and the dynamics of Earth’s interior.