This process is called Differentiation . In addition, there are surface waves that roll along the surface. Seismologists use earthquakes to map the interior like a doctor uses a CAT scan to map the inside of a person. This is how we have such a clear picture of the interior of the Earth. There is a complex transition between Mantle and Crust Region where basaltic lavas form.
Crust floats on a shallow upper mantle.
The plates of the Lithosphere move about, driven by convection currents in the mantle. Convergent Boundary Places where two plates are colliding head-on. Sites of deep, powerful earthquakes. Magma from below wells up and fills the gap, building new crust. Splitting Iceland into two parts.