The 10 km thick rock layer between the Earth's surface and the upper part of the mantle is called a plate. The Earth's surface is divided into ten plates that move in different directions at a speed of several centimeters a year. Magma rises from the mantle and creates a new plate at oceanic ridges. Oceanic plates sink under continental plates at trench spots, thus returning down to the mantle. Many earthquakes and high volcanic activity occur around plate boundaries.