The motion or movement that can lead to the formation of strings of volcanoes is the relative movement of tectonic plates. Most volcanoes form at the boundaries of Earths tectonic plates, which are huge slabs of Earths crust and upper mantle that fit together like pieces of a puzzle. These plates are not fixed, but are constantly moving at a very slow rate, only a few centimeters per year. Sometimes, the plates collide with one another or move apart, and volcanoes are most common in these geologically active boundaries. As the denser plate edge moves downward, the pressure and temperature surrounding it increases, which causes changes to the plate that melt the mantle above, and the melted rock rises through the plate, sometimes reaching its surface as part of a volcano. Over millions of years, the rising magma can create a series of volcanoes known as a volcanic arc. Therefore, the relative motion of tectonic plates can lead to the formation of strings of volcanoes.