Question

List the major types of igneous rocks, and briefly describe their texture and composition

Answer 1

Igneous rocks are primarily classified based on their texture and composition. Here are the major types:

### 1. **Granite**
- **Composition:** Felsic, primarily composed of quartz, feldspar (usually orthoclase and plagioclase), and mica.
- **Texture:** Coarse-grained (phaneritic), meaning the individual mineral crystals are large enough to be seen with the naked eye. Typically has a light color.

### 2. **Rhyolite**
- **Composition:** Felsic, similar to granite but with a higher proportion of volcanic glass and usually contains quartz and feldspar.
- **Texture:** Fine-grained (aphanitic), where crystals are too small to see without a microscope. Often has a lighter color and may show flow banding.

### 3. **Diorite**
- **Composition:** Intermediate, composed of plagioclase feldspar, hornblende, and sometimes quartz or biotite.
- **Texture:** Coarse-grained (phaneritic) with a salt-and-pepper appearance due to the contrasting colors of the minerals.

### 4. **Andesite**
- **Composition:** Intermediate, primarily made up of plagioclase feldspar with some biotite, hornblende, or pyroxene.
- **Texture:** Fine-grained (aphanitic) or porphyritic, containing larger crystals (phenocrysts) in a finer-grained matrix.

### 5. **Basalt**
- **Composition:** Mafic, predominantly composed of pyroxene, olivine, and plagioclase feldspar.
- **Texture:** Fine-grained (aphanitic) with a dark color. Some basalts may exhibit a vesicular texture, containing small gas bubbles.

### 6. **Gabbro**
- **Composition:** Mafic, composed mainly of plagioclase feldspar, pyroxene, and sometimes olivine.
- **Texture:** Coarse-grained (phaneritic) with a dark color, similar mineral composition to basalt, but with larger crystals.

### 7. **Pumice**
- **Composition:** Felsic to intermediate, usually formed from volcanic glass with a high gas content.
- **Texture:** Light-colored, very porous, and glassy. It can float on water due to its low density.

### 8. **Obsidian**
- **Composition:** Felsic, composed mainly of silica.
- **Texture:** Volcanic glass, usually glassy and very smooth, lacking a crystalline structure.

### 9. **Scoria**
- **Composition:** Mafic, composed mainly of basaltic material.
- **Texture:** Vesicular, with numerous gas bubbles, giving it a light weight and reddish-brown to black color.

These major types highlight the diversity in igneous rock formation processes, mineral compositions, and textural characteristics.