BUILDING HIGHER CLASSIFICATION OF COMMON ROCK A rock is a natural aggregate of minerals and certain non-mineral materials, such as fossils or glass. Just as minerals are the building blocks of rocks, rocks in turn are the natural building blocks of the Earth s lithosphere (the 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 (the 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 the continental or oceanic crust. Many rocks, formed beneath the surface and now exposed at the surface, were brought to the surface from great depths in the crust and in rare cases from the underlying mantle. There are two general ways that cause rocks to be exposed at the surface: formation at the surface (e.g., crystallization of lava, precipitation of calcite or dolomite from sea water) formation below the surface, followed by tectonic uplift and removal of the overlying material by erosion. beneath: al di sotto cool: raffreddare crust: crosta depth: profondità mantle: mantello molten: fuso overlying: sovrastante removal: rimozione spill out: fuoriuscire subclasses: sottoclassi tectonic uplift: sollevamento delle zolle tettoniche texture: struttura interna undergo: sottostare a, subire underlying: sottostante weathering: effetto degli agenti atmosferici There are three major classes of rocks: igneous, sedimentary, and metamorphic, with the following characteristics: igneous rocks form by crystallization from a material called magma. There are two subclasses of igneous rock: volcanic (sometimes called extrusive), and plutonic (sometimes called intrusive). Volcanic rocks form at the Earth s surface. They cool and crystallize from magma which has spilled out onto the surface from a volcano. At the surface, the magma is more familiarly known as lava. Plutonic rocks form from magma that cools and crystallizes beneath the Earth s surface. sedimentary rocks form from material that has accumulated on the Earth s surface. This material consists of the products of weathering and erosion, and other materials available at the surface of the Earth, such as organic material. metamorphic rocks form when a sedimentary or igneous rock is exposed to high pressure, high temperature, or both, deep below the surface of the Earth. The process produces fundamental changes in the mineralogy and texture of the rock. Because all metamorphic rocks form below the surface, to become exposed at the surface, they must undergo tectonic uplift and removal of the overlying material by erosion.