Here's why:

* Moho (Mohorovičić discontinuity): This is not a distinct layer, but rather a boundary. It represents the transition zone between the Earth's crust and the upper mantle.

* Crust: The Earth's outermost layer, composed primarily of lighter, less dense rocks like granite and basalt.

* Mantle: The layer below the crust, extending to the Earth's core. It is primarily composed of denser, heavier rocks like peridotite.

The Moho is characterized by a sudden increase in seismic wave velocity, which is detected by seismologists. This increase in velocity is attributed to the change in composition and density between the crust and mantle.

Therefore, the Moho layer isn't a separate layer that exists above the mantle; it is the very boundary where the crust transitions into the mantle.