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
- Intrusive Rocks - slowly cool beneath the Earth's surface.
- Large crystals or grains visible with the naked eye
- Examples: granite and gabbro.
- 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.
- 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
- 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.
- Intermediate rocks (andesitic)
- Mineral makeup between mafic and felsic.
- Examples: diorite and andesite.
- Felsic rocks (granitic)
- Light colored rocks and minerals rich in feldspar and silica.
- Less dense: ≈ 2.3 g/cm3
- Examples: granite and rhyolite.