Rock classification

Two families, set by how they form

• Rocks divide into two large families by how they form, igneous rock and sedimentary rock.
• Igneous rock is magma that cooled and hardened. Where it cools decides whether it becomes volcanic or plutonic rock.
• Sedimentary rock is settled grains pressed together. Grain size and composition divide it into six kinds.

• Igneous rock magma cools and hardens
  • Volcanic rock cools quickly
    • Rhyolite
    • Andesite
    • Basalt
  • Plutonic rock cools slowly
    • Granite
    • Diorite
    • Gabbro
• Sedimentary rock settled grains press together
  • Conglomerate grains over 2mm
  • Sandstone grains 0.06 to 2mm
  • Mudstone grains under 0.06mm
  • Tuff volcanic ash
  • Limestone shells and remains
  • Chert shells and remains

Where the magma cools

• Deep underground, rock melts at high temperature into magma.
• Magma that rises to or near the surface cools quickly and becomes volcanic rock.
• Magma that stays deep underground cools slowly and becomes plutonic rock.

Cooling speed decides the texture

• Inside magma, mineral crystals grow little by little.
• Cool it quickly and fine grains and glass, the groundmass, surround the large crystals that grew first, the phenocrysts. This is porphyritic texture.
• Cool it slowly and crystals of about one size lock together with no gaps. This is equigranular texture.

The mineral ratio decides the color

• The more colorless minerals (quartz, feldspar), the paler the rock.
• The more colored minerals (biotite, hornblende, pyroxene, olivine), the darker the rock.
• Color falls into three bands, pale, middle and dark, and volcanic and plutonic rock each take their own name.

• Pale Volcanic rock (porphyritic) Rhyolite Plutonic rock (equigranular) Granite
• Middle Volcanic rock (porphyritic) Andesite Plutonic rock (equigranular) Diorite
• Dark Volcanic rock (porphyritic) Basalt Plutonic rock (equigranular) Gabbro

Settled, then pressed together

• Weathering and erosion. Rock at the surface breaks down into gravel, sand and mud.
• Transport. Running water wears the corners round as it carries the grains.
• Deposition. The large, heavy grains settle first, so gravel, sand and mud land in that order out from the shore.
• Pressed together over a long age, they become sedimentary rock.

• Conglomerate grains over 2mm Gravel set together with sand and finer material.
• Sandstone grains 0.06 to 2mm Sand pressed together. 0.06mm is 1/16mm.
• Mudstone grains under 0.06mm Mud too fine to see, pressed together.
• Tuff volcanic ash Volcanic ejecta set together. The grains stay angular, which marks past eruptions.
• Limestone shells and remains Mostly calcium carbonate. Dilute hydrochloric acid makes it give off gas.
• Chert shells and remains Mostly silica. Very hard, and acid does nothing to it.