Complex sentence: The crust is separated from the mantle by the Mohorovičić discontinuity, and the thickness of the crust varies: averaging (kilometers) under the oceans and 30--50 km on the continents.
Split into simpler sentences: The crust is separated from the mantle by the Mohorovičić discontinuity. The thickness of the crust varies from about (kilometers) under the oceans to 30--50 km for the continents.