The statement that the Earth is differentiated refers to the radial stratification of the Earth by density. Plates are destroyed at subduction zones. All volcanic activity occurs at subduction zones. Plates are created at mid-ocean ridges. The Earth's lithosphere is broken into plates.
Most earthquakes occur at plate boundaries.
The bending of seismic waves in the interior of the Earth. Because we know the temperature is high enough to melt iron. Volcanoes indicate that the interior is liquid. Click here for the answers.