Igneous Rocks

A rock is an aggregate of one or more minerals.

An igneous rock forms from the cooling and crystallization of liquid rock.

Texture of Igneous Rock

  1. Phaneritic
    • Intrusive Rocks - slowly cool beneath the Earth's surface.
    • Large crystals or grains visible with the naked eye
    • Examples: granite and gabbro.
  2. Aphanitic
    • Extrusive Rocks - form at the Earth's surface and cool quickly.
    • Small crystals or grains not visible with the naked eye
    • The rock is glassy or the crystals are small.
    • These are the rocks and ash of volcanoes.
    • Examples: basalt and rhyolite.
  3. Porphyritic
    • Large crystals surrounded with a fine grained or glassy matrix.
    • Formed when partially cooled intrusive rock is expelled to the surface and cools rapidly.
    • Many rhyolites are porphyritic.

Composition of Igneous Rock

  1. Mafic rocks (basaltic)
    • Dark rocks and minerals rich in magnesium and iron (ferric).
    • Some felsic minerals present.
    • Mafic minerals crystallize at higher temperatures than felsic minerals.
    • More dense: ≈ 2.6 g/cm3
    • Examples: basalt and gabbro.
  2. Intermediate rocks (andesitic)
    • Mineral makeup between mafic and felsic.
    • Examples: diorite and andesite.
  3. Felsic rocks (granitic)
    • Light colored rocks and minerals rich in feldspar and silica.
    • Less dense: ≈ 2.3 g/cm3
    • Examples: granite and rhyolite.
Igneous Rock Formations