The boundary that separates the Earth's crust from the mantle is called the Mohorovičić discontinuity, often shortened to Moho.

Here's why this boundary exists:

* Compositional Change: The Moho marks a distinct change in the composition of Earth's layers. The crust is primarily composed of lighter elements like silicon and aluminum, while the mantle is mostly made up of denser minerals like olivine and pyroxene.

* Seismic Wave Velocity: The Moho is characterized by a sudden increase in the speed of seismic waves, especially P-waves, as they travel from the crust to the mantle. This increase in velocity is attributed to the denser and more rigid nature of the mantle.

Therefore, the Moho is not a physical layer, but rather a transition zone marked by a change in composition and physical properties.