A rock is a naturally occurring aggregate of minerals, and certain non-mineral materials such as fossils and glass. Just as minerals are the building blocks of rocks, rocks in turn are the natural building blocks of the Earth's LITHOSPHERE (crust and mantle down to a depth of about 100 km), ASTHENOSPHERE (although this layer, in the depth range from about 100 to 250 km, is partially molten), MESOSPHERE (mantle in the depth range from about 250 to 2900 km), and even part of the CORE (while the outer core is molten, the inner core is solid). Most rocks now exposed at the surface of the Earth formed in or on continental or oceanic crust. Many such rocks, formed beneath the surface and now exposed at the surface, were delivered to the surface from great depths in the crust and in rare cases from the underlying mantle.