Igneous Rocks

Geology 200

Geology for Environmental Scientists

Magma Compositions

• Ultramafic - composition of mantle
• Mafic - composition of basalt, e.g. oceanic crust. 900-1200^oC, 50% SiO₂
• Intermediate - mix of oceanic and continental crust
• Felsic or Silicic or Sialic - composition of continental crust. <850^oC, 75% SiO₂

Two Major Divisions

• Intrusive igneous rocks - magma that solidifies below the surface of the earth
• Extrusive igneous rocks - magma that cools on the surface of the earth
• The rock textures are different for each division.

Texture of Igneous Rocks

• Glassy: caused by rapid cooling; no crystals
• Pyroclastic: fragments of class and crystals; volcanic ash and bombs
• Aphanitic: microscopic crystals
• Phaneritic: macroscopic crystals
• Porphyritic: grains of two distinct sizes; can occur in phaneritic or aphanitic matrix

Origin and Differentiation of Magma

• Source rock composition - what rocks were melted?
• Partial melting - produces magmas of different composition than source rocks; more felsic or silicic.
• Fractional crystallization - increases silica content of remaining melt.
• Magma mixing
• Assimilation - xenoliths fall into magma chamber

Temperature of Crystallization, or Bowen’s Reaction Series

The order of crystallization for the discontinuous series from olivine to pyroxene to amphibole to biotite to quartz and K-feldspar follows a sequence of increasingly complex arrangements of silicon-oxygen tetraheda. These arrangements progress from single tetrahedra, to single chains, to double chains, to sheets, and finally to framework arrangement of tetrahedra.

Igneous Rocks and Plate Tectonics

• Divergent plate boundaries - convection cells move mantle peridotite closer to the surface. These rocks are at high temperatures so the pressure reduction causes partial melting and production of basaltic magmas.

Igneous Rocks and Plate Tectonics

• Convergent plate boundaries - the subduction of oceanic crust produces partial melting of the hydrated basalt. This produces intermediate (andesitic) magmas which rise to form mountains (e.g. Andes). These magmas can in turn melt continental crust producing granitic magmas.

Igneous Rocks and Plate Tectonics

• Over geologic time these processes first created oceanic crust from ultramafic mantle rocks. In turn, the mafic oceanic crust created felsic continental crust from partial melting of hydrated basalt. Continental crust is less than 1% of earth’s mass, so there isn’t a lot of felsic material in the earth. Most has risen to the crust.