Mohorovicic Discontinuity: Defining Earth's Layers

The Mohorovicic discontinuity, often called the Moho, represents a significant boundary within the Earth's structure.

It is defined as the seismic velocity discontinuity that separates:

• Earth's crust (both continental and oceanic)
• From the upper mantle directly beneath it.

This boundary is identified by a sudden increase in the speed of seismic waves as they pass from the crust into the denser mantle material.

Understanding Earth's Layered Structure

To clarify why other options are incorrect:

• Continental shelf and Ocean crust: Both are parts of the Earth's crust, not separated by the Moho.
• Asthenosphere and lithosphere: The boundary between these is the lithosphere-asthenosphere boundary, located deeper within the upper mantle. The lithosphere includes the crust and the uppermost rigid part of the mantle.
• Core and the mantle: This boundary is known as the Gutenberg discontinuity.

Therefore, the Mohorovicic discontinuity specifically marks the base of the crust and the top of the upper mantle.