Rock Classification

by Joe C.

What's a Rock?

Rocks are solid substances mainly composed of crystals. They are classified into one of three groups by their texture, physical appearance, and origin.

The Three Rock Classes

Igneous Rock (Left)

  • Formed by Melting & Solidification
  • Form inside or outside of volcanoes
  • Main types: Intrusive & Extrusive
  • Intrusive formed inside a volcano, cools slowly, has large crystals
  • Extrusive formed outside a volcano, cools quickly, has small crystals

Sedimentary Rock (Middle)

  • Formed by Compaction & Cementation
  • Form on the earth's surface
  • Main types: Clastic, Bioclastic, Chemical
  • Clastic formed when many sediments are compacted together
  • Bioclastic formed when living things or remains of living things (e.g. shells or plants) are compacted together
  • Chemical formed from chemical evaporates and/or precipitates

Metamorphic Rock (Right)

  • Formed by Heat & Pressure
  • Form deep within the earth
  • Main types: Foliated & Nonfoliated
  • Formed by either Regional (extreme pressure) or Contact (intense heat) metamorphism

Rock Examples


  • Formed outside a volcano--Crystals formed quickly
  • Texture is fine--Crystals are small
  • Non-Vesicular
  • Felsic--Light in color, low density
  • 15% Amphibole, 20% Biotite, 50% Plagioclase Feldspar, 15% Quartz, 5% Potassium Feldspar
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Bituminous Coal

  • Organic
  • Texture is very coarse
  • Composed of carbon
  • Contains compacted plant remains
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  • Texture is foliated
  • Grain size is fine to medium
  • Composition is mica, quartz, feldspar, amphibole, and garnet
  • Type of Metamorphism: Regional
  • Foliation surfaces shiny from microscopic mica crystals
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The Rock Cycle

The Rock Cycle is a flowchart that describes how one type of rock becomes another type of rock.
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