Oceanic plates are thinner and younger. Together these plates form the lithosphere. Earth's Tectonic Plates, USGS. Continental plates. Igneous, sedimentary and metamorphic rocks form the continents. The basement rocks of the continents are granitic. The minerals in granitic rocks are lighter than the oceanic crust.ntroduction. Crustal plates form the outer layer of the Earth. There are seven major plates and many smaller plates. These tectonic plates are formed from the Earth's crust and uppermost part of the mantle. Types of tectonic plates. There are two types of tectonic plates.