According to the Theory of Plate Tectonics, the lithosphere, the outermost rigid shell of the Earth’s crust, is divided into 20 rigid plates, also called lithospheric plates, that float on the asthenosphere below.
The floating plates can: move away from each other; come together and collide; come together and slide against each other.
When two plates move away from each other, the crust thins until it disappears. These are areas of mid-ocean ridges and great continental rifts, where magma can rise up through the large faults created by the stretch.
When two plates approach and collide, the main phenomenon that occurs is subduction, i.e. one of the two arches and runs under the other. Obviously, the sliding process is not easy and often leads to the folding or orogenesis of the overlying plate. The sunken material heats up and tends to dehydrate, causing the fluids to rise, the melting temperature of the overlying lithosphere to decrease and magma to form in the arc-trench systems.
When two plates come together and slide next to each other, along a line of contact called a fault, there is neither production nor destruction of the earth’s crust.