The Moho discontinuity isn't a solid layer with a defined thickness. It's a transition zone, not a sharp boundary.

Here's why:

* Varying Composition: The Moho represents the change from the primarily felsic (silica-rich) composition of the Earth's crust to the more mafic (magnesium and iron-rich) composition of the upper mantle. This transition isn't abrupt, and the exact composition changes gradually.

* Depth Variation: The depth of the Moho varies significantly across the globe, from about 5 km beneath the ocean floor to 70 km beneath mountain ranges. This variation is due to factors like tectonic activity, crustal thickness, and density differences.

Therefore, it's more accurate to consider the Moho as a zone of transition, rather than a distinct layer with a precise thickness.