The crust in conjunction with the upper mantle comprises the lithosphere. Both continental and oceanic crust rest on peridotite of the mantle. Basalts are extrusive igneous rocks derived from the partial melting of the upper mantle. Basaltic magma is formed from lherzolites in the mantle. Others suggest an origin in the deep mantle (mantle plume). This concentration is comparable with that of magma in Earth's upper mantle. A shell secreted by the mantle covers the upper surface. An alternate theory to the mantle plume model was proposed in 2018. Earthquakes are caused by movements within the Earth's crust and uppermost mantle. Plate tectonics is driven by convection cells in the mantle. It has a geochemically distinct crust, mantle, and core. Titania may be differentiated into a rocky core surrounded by an icy mantle. At high degrees of partial melting of the mantle, komatiite and picrite are produced. These plates are destroyed by subduction into the mantle at subduction zones.