Classification of Common Rocks:
Igneous, Sedimentary, and Metamorphic

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.  There are two general ways that rocks come to be exposed at the surface:
  1. Formation at the surface (e.g., crystallization of lava, precipitation of calcite or dolomite from sea water)
  2. Formation below the surface, followed by tectonic uplift and removal of the overlying material by erosion
There are three major classes of rocks, IGNEOUS, SEDIMENTARY, and METAMORPHIC, with the following attributes:

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).

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.

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.

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, METAMORPHISM, produces fundamental changes in the mineralogy and texture of the rock.  The 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 metamorphosed rock.  Ultimately however, if you go far enough back into the history of a metamorphic rock you would find that the first protolith was either 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 erosion.


In this exercise you will examine some common igneous, sedimentary and metamorphic rocks, and learn to classify them.

The classification of rocks is based on two criteria, TEXTURE and COMPOSITION.  The texture has to do with the sizes and shapes of mineral grains and other constituents in a rock, and how these sizes and shapes relate to each other.  Such factors are controlled by the process which formed the rock.  Because igneous, sedimentary, and metamorphic processes are distinct, so too the resulting textures are distinct.  Thus there are distinct igneous textures, distinct sedimentary texture, and distinct metamorphic textures.  For the purposes of this exercise and routine classification, the kinds of minerals and their proportions, or MINERALOGY, are taken as the natural expression of composition.  Fortunately for you, just as the three classes of rocks each have distinct textures, so too do they have distinct mineralogies.  Details of TEXTURE and COMPOSITION are discussed in the individual sections on igneous, sedimentary and metamorphic rocks.

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.

The Laboratory Exercise: CLASSIFY ROCKS
You laboratory instructor will supply a set of igneous rocks, a set of sedimentary rocks, and a set of metamorphic rocks.  Select a rock, and take the appropriate path below to classify the rock.  In each of the sections, you will find brief descriptions of TEXTURES and MINERAL COMPOSITIONS, and a classification chart based on these characteristics.


Designed by R.N. Abbott, Jr., Department of Geology, Appalachian State University, Boone, NC 28608, as modified from Abbott, R.N., Jr., Callahan, J.E., Cowan, E.A., McKinney, F.K., McKinney, M., Raymond, L.A., and Webb, F, 2000, Laboratory Problems in Physical Geology, 11th edition, Department of Geology, Appalachian State University, Boone, NC.