ABSTRACT
The main classification of rocks – into igneous, sedimentary and metamorphic types – is based on the way in which the rocks are formed. Igneous rocks are formed by the cooling of molten magma, which wells up from deep beneath the Earth's surface. The magma is a melted mixture of rock-forming minerals, and, as cooling proceeds, the individual minerals crystallize out and become solid at different temperatures. The rocks of the Earth's crust are continuously being eroded. Sedimentary rocks are, therefore, characteristically formed from small particles of weathered rocks and minerals that are bound together by a related matrix substance. It is possible to imagine a long-term rock cycle as follows: Following a mountain-building period, and the uplift of parts of the Earth's surface, possibly by buckling of the crust associated with the movement of continental plates, erosion begins, followed by the deposition under water of transported sediments.
