Question

What separates the crust to the mantle?

Answer 1

Mohorovicic Discontinuity or Moho

Explanation:

This was discovered by Andrija Mohorovicic, a seismologist, who noticed that a seismic wave changed it speed at some point. Meaning, there is a composition in a rock that is different and also a different density from the crust.

https://en.wikipedia.org/wiki/Mohorovi%C4%8Di%C4%87_discontinuity