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

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Igneous rocks are classified into two main types based on their formation location: intrusive (plutonic) and extrusive (volcanic). Here are the major types along with a brief description of their texture and composition:

Intrusive (Plutonic) Igneous Rocks

  1. Granite

    • Texture: Coarse-grained, with large crystals visible to the naked eye.
    • Composition: Primarily composed of quartz, feldspar (orthoclase and plagioclase), and mica (biotite and muscovite).

  2. Diorite

    • Texture: Coarse-grained, with a mixed color appearance due to the presence of light and dark minerals.
    • Composition: Contains plagioclase feldspar, along with biotite, amphibole, and occasionally quartz.

  3. Gabbro

    • Texture: Coarse-grained, similar to granite but darker in color.
    • Composition: Rich in mafic minerals; primarily composed of plagioclase feldspar, pyroxene, and olivine.

  4. Peridotite

    • Texture: Coarse-grained, often containing large olivine crystals.
    • Composition: Dominantly composed of olivine, with a significant amount of pyroxene, making it ultramafic.

Extrusive (Volcanic) Igneous Rocks

  1. Basalt

    • Texture: Fine-grained; crystals are too small to be seen without a microscope.
    • Composition: Composed primarily of plagioclase feldspar and pyroxene, and often contains olivine; it is mafic in character.

  2. Andesite

    • Texture: Fine-grained; may exhibit porphyritic texture with larger crystals embedded in a finer matrix.
    • Composition: Intermediate in composition, containing plagioclase, amphibole, and some biotite and pyroxene.

  3. Rhyolite

    • Texture: Fine-grained, with some porphyritic varieties showing larger crystals.
    • Composition: Rich in silica, primarily composed of quartz and alkali feldspar, with some biotite and amphibole.

  4. Pumice

    • Texture: Vesicular (contains many gas bubbles), light and porous.
    • Composition: Similar to rhyolite in terms of composition, often containing silicate minerals like quartz and feldspar.

  5. Obsidian

    • Texture: Glassy and smooth.
    • Composition: High in silica, similar to rhyolite, but with no crystalline structure.

Summary

  • Intrusive rocks typically have larger crystals due to slower cooling beneath the Earth's surface.
  • Extrusive rocks cool quickly on the surface, resulting in finer crystals or glassy textures.

These characteristics help geologists identify and classify igneous rocks based on their origins and mineral content.