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• Igneous rocks


  Rocks that form as molten rock cools and solidifies
• Magma


  The parent material of igneous rocks, formed by melting that occurs at various levels within Earth's crust and upper mantle
• Lava


  Molten rock that reaches Earth's surface
• Components of magma


  • Liquid component (melt)
  • Solid component (silicate minerals)
  • Gaseous component (volatiles)
• Melt


  The liquid portion of magma, composed mainly of mobile ions of silicon, oxygen, and lesser amounts of other elements
• Volatiles


  Gaseous components of magma that will vaporize at surface pressures, such as water vapor, carbon dioxide, and sulfur dioxide
• Crystallization of magma


  1. Silicon and oxygen atoms link together to form silicon-oxygen tetrahedra

  2. Tetrahedra join with each other and other ions to form embryonic crystal nuclei

  3. Nuclei grow as ions lose mobility and join the crystalline network

  4. Earliest formed minerals have space to grow and tend to have better-developed crystal faces

  5. Eventually all of the melt is transformed into a solid mass of interlocking silicate minerals
• Crystallization of magma is more complex than the simple solidification of a compound, as it spans a temperature range of 200°C or more
• Magmas differ in chemical composition, amount of volatiles, and rate of cooling, which influence the crystallization process and the appearance and mineral makeup of igneous rocks
• Intrusive igneous rocks


  Igneous rocks that form when magma crystallizes at depth, coarse-grained with visible mineral crystals
• Extrusive igneous rocks


  Igneous rocks that form when lava solidifies at the surface, fine-grained
• Intrusive igneous rocks are observed at the surface where uplifting and erosion have exposed them, such as in Mount Washington, Stone Mountain, the Black Hills, and Yosemite National Park
• Extrusive igneous rocks are abundant in western portions of the Americas, forming volcanic peaks in the Cascade Range and Andes Mountains, as well as many oceanic islands
• Major silicate mineral groups in igneous rocks

  • Dark (ferromagnesian) silicates (olivine, pyroxene, amphibole, biotite mica)
  • Light (nonferromagnesian) silicates (quartz, muscovite mica, feldspars)
• Granitic (felsic) composition


  Igneous rocks composed almost entirely of light-colored silicates like quartz and feldspar
• Basaltic (mafic) composition


  Igneous rocks containing substantial dark silicate minerals and calcium-rich plagioclase feldspar, but no quartz
• Intermediate (andesitic) composition


  Igneous rocks with a composition between granitic and basaltic, containing at least 25% dark silicate minerals
• Ultramafic composition


  Igneous rocks composed almost entirely of ferromagnesian minerals like olivine and pyroxene
• Silica (SiO2) content is an important indicator of igneous rock composition, ranging from about 40% in ultramafic rocks to over 70% in granitic rocks
• Silica content


  Inversely related to iron, magnesium, and calcium content, but positively related to sodium and potassium content
• Silica content of magma

  Influences its viscosity and eruption temperature - high silica granitic magma is more viscous and erupts at lower temperatures, while low silica basaltic magma is more fluid and erupts at higher temperatures
• Silica content


  Directly infers the chemical makeup of an igneous rock
• Silica content in magma

  Strongly influences its behavior
• Granitic magma


  Has high silica content, is quite viscous, and may erupt at temperatures as low as 700°C
• Basaltic magma


  Has low silica content, is generally more fluid, and erupts at higher temperatures between 1100 and 1250°C
• Igneous rock groups based on light and dark mineral proportions

  • Granitic (felsic) rocks
  • Basaltic (mafic) rocks
  • Intermediate (andesitic) rocks
  • Ultramafic rocks
• Felsic igneous rocks


  Composed almost entirely of light-colored minerals quartz and feldspar
• Mafic igneous rocks


  Contain abundant dark silicate minerals in addition to plagioclase feldspar
• Dark (ferromagnesian) silicate minerals

  Different from light (nonferromagnesian) silicate minerals
• People have been making glass in roughly the same way for at least 2000 years
• Texture


  The overall appearance of a rock based on the size, shape, and arrangement of its mineral grains
• Factors affecting crystal size in igneous rocks

  • Rate of cooling
  • Amount of silica present
  • Amount of dissolved gases in the magma
• Aphanitic (fine-grained) texture


  Igneous rocks that form at the surface or in the upper crust where cooling is relatively rapid
• Phaneritic (coarse-grained) texture


  Igneous rocks that form when large masses of magma slowly crystallize at great depth
• Porphyritic texture


  Large crystals (phenocrysts) embedded in a matrix of smaller crystals (groundmass)
• Glassy texture


  Unordered ions "frozen in place" before they are able to unite into an orderly crystalline structure
• Obsidian


  A common type of natural glass
• Pyroclastic (fragmental) texture


  Igneous rocks composed of individual rock fragments ejected during a violent volcanic eruption
• Pegmatitic texture


  Exceptionally coarse-grained igneous rocks with interlocking crystals all larger than a centimeter in diameter
• Porphyritic texture


  Indicates that an igneous rock cooled in two stages - slow cooling followed by rapid cooling