Igneous rocks crystallize from molten silicate material called magma that forms at high temperatures and pressures deep in the Earth. Magma rises through the crust and either reaches the surface by volcanoes (extrusive) or cools below surface (intrusive).

How Magmas Evolve – according to Bowen’s Reaction Series: discontinuous (different molecular structures of olivine [single tetrahedra], pyroxene [single chains], amphibole [double chains], biotite [sheets of tetrahedra]), and continuous (same structure but different proportions of sodium and calcium in plagioclase feldspar)

Repeated removal of crystals (crystal settling) from a magma chamber results in the melt crystallizing through a series of minerals according to their decreasing melting temperatures – remaining crystals react with the melt that is enriched in lighter elements (magmatic differentiation); dark crystals form early, settle, and take the heavy elements with them, then light crystals form later from the remaining light elements, eventually to quartz – thus granite can ultimately evolve from an original magma having the chemical composition of basalt

How Rocks Form: Igneous Rocks, Sedimentary Rocks, Metamorphic Rocks

Other processes affecting magma composition include assimilation where pieces of the host rock fall into the magma, melt, and change the magma composition; and magma mixing where one body of magma invades another, producing a composition intermediate between the two

Rock Composition: ranges from mainly dark (mafic, less silica) minerals to light (felsic, more silica) minerals

Texture: (size, shape, arrangement of crystals): fine aphanitic is formed by rapid cooling at or near the ground surface vs. coarse phaneritic formed by slow cooling below the surface; porphyritic with larger crystals surrounded by smaller ones; glassy if lava was quenched so fast that crystals had no time to form; pyroclastic (fragmental) when magma was ejected violently into the air then fell as particles onto the ground

Rocks and the Rock Cycle

Naming Igneous Rocks – use igneous rock textures and mineral compositions (i.e. dark vs. light minerals) to classify igneous rocks

  • Felsic (granitic) rocks (70% silica) ‑ granite and rhyolite are common in mountainous areas; granite is widely used for building stone, monuments, and headstones
  • Intermediate (andesitic) (60%) ‑ andesite and diorite found near subduction zones
  • Mafic (basaltic) rocks  (50%) ‑ basalt is the most common rock on Earth and is found mainly in the upper part of oceanic crust and in volcanic islands; gabbro is formed mainly in lower oceanic crust
  • Ultramafic  (45%) peridotite is probably the main rock in the upper mantle

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