Igneous rocks are classified into two major types based on their origin: intrusive (plutonic) and extrusive (volcanic) igneous rocks. Here are the major types along with their textures and compositions:
1. Granite (Intrusive)
- Texture: Coarse-grained (phaneritic), meaning that the crystals are large enough to be seen with the naked eye.
- Composition: Primarily composed of quartz, feldspar (both plagioclase and alkali), and mica (biotite and muscovite). It has a high silica content.
2. Diorite (Intrusive)
- Texture: Coarse-grained (phaneritic), with visible crystals but less uniform than granite.
- Composition: Intermediate composition between granite and gabbro; it contains plagioclase feldspar, quartz, biotite, hornblende, and/or augite.
3. Gabbro (Intrusive)
- Texture: Coarse-grained (phaneritic), characterized by its dark color and large crystals.
- Composition: Rich in mafic minerals, mainly plagioclase feldspar and pyroxene, with lesser amounts of olivine and amphibole.
4. Peridotite (Intrusive)
- Texture: Coarse-grained (phaneritic), usually containing large crystals and is often described as ultramafic.
- Composition: Composed mostly of olivine and pyroxene, with very low silica content.
5. Basalt (Extrusive)
- Texture: Fine-grained (aphanitic), with small crystals that are not visible without magnification. It may also have a vesicular texture (with gas bubbles) if formed with volcanic gas.
- Composition: Mainly composed of plagioclase feldspar and pyroxene, often enriched in iron and magnesium.
6. Andesite (Extrusive)
- Texture: Intermediate-grained (can be aphanitic or porphyritic, with larger crystals embedded in a finer matrix).
- Composition: Contains a mix of plagioclase feldspar, pyroxene, and sometimes biotite or hornblende, representing a composition between basalt and rhyolite.
7. Rhyolite (Extrusive)
- Texture: Fine-grained (aphanitic), often exhibiting a porphyritic texture as well.
- Composition: High in silica, primarily composed of quartz and alkali feldspar, with smaller amounts of mica and amphibole.
8. Pumice (Extrusive)
- Texture: Vesicular (contains many small gas bubbles), usually lighter and less dense.
- Composition: Similar to rhyolite, but with a high concentration of vesicles due to rapid gas escape during volcanic eruptions.
9. Obsidian (Extrusive)
- Texture: Glassy, with no crystal structure visible, resulting from rapid cooling of lava.
- Composition: Similar to rhyolite, composed of silica but lacks crystalline structure.
These classifications help geologists understand the history of the rock formation, the conditions under which they formed, and their mineralogical composition.