Continental and oceanic crust. Cutaway block artwork of the Earth's crust (brown) at the edge of a continent. It is thinnest beneath the ocean (far right), thicker beneath the continent (centre), and thickest beneath mountain ranges (the Andes, far left). Here, off Argentina, an extensive continental shelf ends in a slope down to the abyssal plain under the Atlantic. The crust-mantle boundary is the Mohorovicic discontinuity. The upper layer of the mantle (dark red) is rigid and moves with the crust, forming the lithosphere. Beneath the lithosphere lies the ductile part of the mantle, the asthenosphere (light red), over which the crust's tectonic plates move.