Composition and Texture of Igneous Rocks

Composition of Earth's Crust and Mantle

  • Composition classes are vital in understanding plate tectonic settings.

    • Felsic Composition:

    • Dominates the continental crust.

    • Typically light-colored; contains high amounts of silica and lower amounts of iron and magnesium.

    • Mafic Composition:

    • Dominates the oceanic crust.

    • Darker in color; higher in iron and magnesium, lower in silica.

    • Intermediate Composition:

    • Characterizes transitional settings such as subduction zones.

    • In these zones, oceanic and continental rocks can melt together, creating a mix of compositions.

    • Ultramafic Composition:

    • Dominates the Earth's mantle.

    • Composed mostly of olivine and pyroxene; very low in silica.

Texture of Igneous Rocks

  • The texture of igneous rocks is primarily determined by the cooling and crystallization process of magma or lava.

    • Phaneritic Texture:

    • Results from slow cooling in an intrusive/plutonic setting (e.g., magma chambers).

    • Minerals are large enough to be seen without a microscope.

    • Characterized by coarsely crystalline forms.

    • Aphanitic Texture:

    • Occurs when lava cools rapidly in an extrusive/volcanic setting (near the surface of the Earth).

    • Minerals are too small to be seen without a microscope.

    • Characterized by fine-crystalline formation.

Special Textures of Igneous Rocks

  • Pegmatitic Texture:

    • Forms in an intrusive setting when water is incorporated into magma.

    • Water facilitates the movement of atoms within the magma, allowing for large mineral crystals to grow.

    • Pegmatites can have crystals several centimeters in size, larger than those in phaneritic rock.

  • Porphyritic Texture:

    • Represents igneous rocks with two distinct populations of crystal sizes, indicating two phases of crystallization.

    • First Phase:

    • Slow cooling allows larger mineral crystals to form.

    • Second Phase:

    • More rapid cooling creates smaller mineral crystals in the surrounding matrix or body of the rock.

    • Phases may be either both intrusive or one could be intrusive while the other is extrusive.