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Igneous Rocks Niti Mankhemthong. Modified from http://www.authorstream.com/Presentation/kcollazo-170754-igneous-rocks-science-education-ppt-powerpoint/. Igneous Rocks. Igneous rocks form when magma cools and hardens.

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Igneous rocks niti mankhemthong

Igneous RocksNitiMankhemthong

Modified from http://www.authorstream.com/Presentation/kcollazo-170754-igneous-rocks-science-education-ppt-powerpoint/

Igneous rocks
Igneous Rocks

Igneous rocks form when magma cools and hardens.

Igneous from the Latin meaning “Fire formed”, is a rock type that forms from the solidification of a molten mineral solution.

Photo used with permission from Mike Jarvis, Naperville Central HS,

Naperville, IL

All igneous rocks have a random fabric of fused mineral grains.

Igneous rocks1
Igneous Rocks

  • Intrusive (Plutonic)rocks form inside the Earth (underground) when magma cools slowly to create large crystals.

    • (Intrusive = Inside)

  • Extrusive (Volcanic)rocks form on the Earth’s surface (above ground) when lava cools quickly to create small crystals.

    • (Extrusive = Exit/outside)

  • Intrusive large crystals
    Intrusive: Large crystals

    Which is which? How do you know?

    Extrusive: Small crystals

    Igneous rocks2
    Igneous Rocks

    • There are 3 types of igneous rocks

      1. Felsic= Light color, high in silica, viscous (thick and slow), low in iron, calcium or magnesium

      Felsic=Feldspar+ Silica

      2. Mafic= Dark color, low in silica, not viscous (thin and watery), high in iron, calcium and magnesium

      Mafic=Magnesium + Iron (Fe)

      3. Ultramafic= Very dark in Color

    Felsic light overall color
    Felsic: Light overall color

    Which is which? How do you know?

    Mafic: Dark overall color

    Igneous rocks3
    Igneous Rocks

    • Texture refers to the crystal size.

    • Textures are divided into five main types:

      1. Fine-grained (aphanitic)

      • crystals are invisible or hard to see with the naked eye

      • They often have cooling cracks.

      • They make poor building materials.

      • Such as basalt

    2. Coarse-grained (Phaneritic)

    • Crystals are easily visible (granite)

    • Crystals fit together

    • They are generally the strongest igneous rocks due to interlocking btw grains

    • Such as granite

    Coarse grained large crystals
    Coarse Grained: Large crystals

    Which is which? How do you know?

    Fine Grained: Small crystals

    3. Aporphyritictexture

    • is a rock with large crystals surrounded by fine-grained crystals.

    • Large and small crystal are not touching each other. It is easy to confuse a porphyritic rock with a phaneritic rock.

    • Porphyritic rocks are both intrusive and extrusive.

      This is a porphyriticandesite similar to the rocks we see on the campus.

    Porphyritic texture
    Porphyritic Texture:

    Rhyolite: Light colored (felsic), fine grained (volcanic) igneous rock

    Igneous rocks4
    Igneous Rocks

    4. Aglassytexture

    • like broken glass, and you can’t see any grains, no matter how hard you try (no crystal).

    • This magma cools so quickly that crystals have no time to form at all.

    • Glasses are not very stable, since they are not crystalline. They are not a good aggregate for concrete or good building stones, because they take on water.

    Glassy Texture:

    Volcanic Glass/Obsidian: is usually mafic. Even when felsic it appears dark nonetheless because the small % of mafic minerals in the rock form a homogenous solution with the felsic minerals tinting the overall color to dark. Notice the conchoidal fracture.

    5. Vesicular:

    • Vesiclesare holes, so a vesicular rock is a “holey” rock.

    • The holes are formed when gas and other volatiles escape the lava before it hardens.

    • Vesicular rocks are extrusive origin.

    • The holes make the rock weak, and it is therefore a poor building material.

    • This material is often used for landscaping/xeriscaping in the El Paso area.

    Pumice fine grained felsic vesicular volcanic igneous rock

    Vesicular Texture:

    Pumice: Fine grained, felsic, vesicular volcanic igneous rock.

    Scoria: Fine grained, mafic, vesicular volcanic igneous rock.

    Igneous rocks5
    Igneous Rocks

    Rocks can be grouped into families:

    • Granite family = felsic, light, contains quartz, feldspar, & muscovite.

      (granite, rhyolite, obsidian, pumice)

    • Gabbro family = mafic, dark, contains feldspar, olivine, biotite.

      (gabbro, basalt, scoria)

    • Diorite family = intermediate composition and color.

      (diorite, andesite, obsidian)

    Warming up
    Warming up!!

    Let’s try some examples, tell me if the following rock samples are:

    1. Felsic or Mafic

    2. Fine grained or Coarse grained

    3. Intrusive or Extrusive

    Be sure to tell me how you knew.


    Mafic = Dark color

    Fine grained= Small crystals

    Extrusive= Small crystals = Formed on surface


    Felsic = Light color

    Coarse grained= Large crystals

    Intrusive = Large crystals = Formed underground


    Mafic = Dark color

    Coarse grained= Large crystals

    Intrusive= Large crystals = formed underground


    Felsic = Light color

    Fine grained= Small crystals

    Extrusive= Small crystals = Formed above ground


    Mafic = Dark color

    Fine grained= Small crystals

    Extrusive= Small crystals = Formed above ground