What causes certain materials inside the Earth to rise or sink?
The rocks that form the mantle layer are melted down by the heat generated by the Earth's core, moving the plates found at the surface of the Earth. The cause, then, of materials rising and sinking is their response to the pressure developing by hot molten rock from the hot Earth's core.
The movement of plates follows a pattern called convection cell, which is created when the heated parts rise and then they sink, as they become colder.
Plates consist of a thinner part, that is warmer and it is raised at the surface, and a thicker part that is cooler and it tends to sink.
New crust is formed as hot magma rises at the surface, while the colder old parts have the tendency to fall at subduction regions, since their temperature is below the magma temperatures.
The main sources that generate the heat in Earth's core are represented by the radioactive decay of naturally occurring chemical elements in the earth, such as uranium, thorium, and potassium, and the residual heat, which is remained at the Earth's core, since it's formation.