TECTONIC PLATES The solid lithosphere is believed to be made up of several crustal plates or tectonic plates (which are sixteen to twenty great slabs of rock) floating on the part of the mantle called the asthenosphere . They move in response to convection currents in the upper mantle. Magma rises from the core and lower mantle towards the surface and spreads out at mid-oceanic ridges. The plates are hot at mid-oceanic ridges but over a period of a million years or so, as these plates moved apart and cooled, the colder plate descended at the trenches dragging the surface plate with it. The asthenosphere showed at a subduction boundary A crustal plate is an area of continental and oceanic crust along with the upper mantle. These plates have been in constant motion and over millions of years they have formed continents and ocean basins and their irregularities. The shape of the continents is believed to have evolved and changed over millions of years due to crustal plat...