IGNEOUS ROCKS form by crystallization from molten or partially material, called MAGMA. Magma comes mainly from two places where it is formed, (1) in the asthenosphere and (2) in the base of the crust above subducting lithosphere at a convergent plate boundary. There are two subclasses of igneous rock, VOLCANIC (sometime called EXTRUSIVE), and PLUTONIC (sometimes called INTRUSIVE).
SEDIMENTARY ROCKS form from material that has accumulated on the Earth's surface. The general term for the process of accumulation is DEPOSITION. The material consists of the products of weathering and erosion, and other materials available at the surface of the Earth, such as organic material. The process by which this otherwise unconsolidated material becomes solidified into rock is variously referred to LITHIFICATION (literally turned into rock), DIAGENESIS or CEMENTATION. Like volcanic rocks, some sedimentary rocks are "lithified" right at the surface, for instance by direct precipitation from sea water. Other sedimentary rocks, like plutonic igneous rocks, are "lithified" below the surface, when they are buried under the weight of overlying sediment. And like the plutonic rocks, sedimentary rocks which were lithified below the surface only become exposed at the surface by tectonic uplift and erosion of the overlying material.
VOLCANIC ROCKS form at the Earth's surface. They cool and crystallized from magma which has spilled out onto the surface at 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. In a sense, this is the portion of the magma that never makes it to the surface. For the plutonic rock to become exposed at the surface, it must be tectonically uplifted and the overlying material must be removed by erosion.
METAMORPHIC ROCKS form when a sedimentary or
rock is exposed to high pressure, high temperature, or both, deep below
the surface of the Earth. The process, METAMORPHISM,
fundamental changes in the mineralogy and texture of the rock.
original rock, prior to metamorphism, is referred to as the PROTOLITH.
The protolith can be either an igneous rock or a sedimentary rock,
as just indicated. The protolith could also be a previously
rock. Ultimately however, if you go far enough back into the
of a metamorphic rock you would find that the first protolith was
a sedimentary or igneous rock. Because all metamorphic rocks form
below the surface, for them to become exposed at the surface, they must
undergo tectonic uplift and removal of the overlying material by
In this exercise you will examine some common igneous, sedimentary and metamorphic rocks, and learn to classify them.
Just a note here with regard to grains size. The terms APHANITIC and PHANERITIC mean fine-grained and coarse-grained respectively. Generally, aphanitic means that the grains are too small to see or identify, while phaneritic means that the grains are big enough to see and identify, but the terms are used differently in each the classes of rocks. In igneous rocks the division between aphanitic and phaneritic is taken to be at a grain size of 1/16 mm. If the grain size is larger than 1/16 mm, the texture is said to be phaneritic. If the grain size is less than 1/16 mm, the texture is said to be aphanitic. In sedimentary rocks, the formal division between aphanitic and phaneritic is taken to be 1/256 mm. For metamorphic rocks the distinction between aphanitic and phaneritic is less quantifiable, but the general meanings are the same.