Igneous Rocks

A rock is an aggregate of one or more minerals.

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

• Igneous rocks have a distinctive texture of interlocking mineral crystals.
• Igneous rocks are mostly silicate minerals
• Crystallize at temperatures ranging from 700° C to 1200° C.
• Igneous rocks are records of the Earth's thermal history.
• Igneous rocks are classified by their texture and by their mineral (chemical) composition.

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.
   • 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.
   • Examples: granite and rhyolite.