The crust is the top layer of Earth's makeup, and it's also the thinnest layer. The crust takes up 1% of Earth's mass. Earth's crust is made up of two parts: oceanic crust and continental crust. The moho is a boundary that lies between the crust and the mantle. It is about 20 miles beneath the continents and 6 miles beneath the ocean floor. It was named after a Croatian seismologist, Andrija Mohorovičić, in 1909.