rocks

• Plutonic rocks are formed from magma that cooled below the surface of the earth and are also called intrusive igneous rocks
• Igneous Rocks


  • Commonly differentiated based on texture, such as crystal sizes
  • Textures include Phaneritic, Aphanitic, and Porphyritic
• Textures common among volcanic rocks include vesicular texture characterized by vesicles and hyaline texture characterized by a glassy texture
• Volcanic rocks are formed from magma that cooled on the surface of the earth and are also called extrusive igneous rocks
• Porphyritic texture consists of two sizes of crystals: phenocrysts and groundmass, formed by slow crystallization at depths followed by rapid crystallization near the surface
• Formation of Igneous Rocks

  1. Magma crystallizes or solidifies to form igneous rocks

  2. Magma is a mobile molten material composed partially of melted rocks in the earth's interior or beneath the surface

  3. Lava is molten rock material extruded on the surface of the earth
• Mineral sizes and shapes in rocks can help infer the environment of formation, with different sizes and shapes dependent on the rate of cooling of the magma
• Types of Igneous Rocks

  • Plutonic rocks
  • Volcanic rocks
• Rapid cooling can result in zoned minerals with distinct compositions, such as the olivine group
• The composition of igneous rocks depends on the minerals formed during the crystallization of magma/lava
• Breccias have predominantly angular rock components
• Textures of violently extruded volcanic rocks are important to know
• Classification of igneous rocks by composition

  1. Based on silica content and relative amounts of K, Na, Fe, Mg, and Ca

  2. Classified as felsic, intermediate, mafic, and ultramafic igneous rocks
• Various minerals are formed from olivine, pyroxene, amphibole, biotite to muscovite, and plagioclases from Ca-rich plagioclases (anorthite) to Na-rich plagioclases (albite)
• Basalts are mafic rocks with 45-55% silica
• Granites are felsic rocks with > 65% silica
• Tuff has rock components predominantly made up of ash and lapilli
• Grain sizes in pyroclastic textures

  • Ash (< 2 mm)
  • Lapilli (2-64 mm)
  • Block and bomb (> 64 mm)
• Peridotites are ultramafic rocks with < 45% silica
• Quartz enrichment happens at lower temperatures
• Plutonic rocks vary from ultramafics, gabbros, diorites, to granites with decreasing temperatures
• Granite has phaneritic textures while rhyolite has aphanitic to porphyritic textures
• Ultrabasic rocks have < 45% SiO2, basic rocks have about 45-52% SiO2, intermediate rocks have 52-66% SiO2, and acidic rocks have > 66% SiO2
• If M > 90%, other diagrams need to be used for classification
• Granite and rhyolite have the same chemical composition but differ in texture
• If M < 90%, neglect all mafic contents and recalculate Q+A+P or F+A+P = 100%
• Different types of igneous rocks were derived from a single basaltic magma undergoing differentiation and fractionation
• Classification of igneous rocks

  1. Chemical classification (normative) based on % weight of SiO2 in the rocks

  2. Mineralogical classification (modal) using the IUGS system
• Bowen’s Reaction Series is used in the classification of different igneous rocks
• Pyroclastic flow deposits exhibit characteristics of welded tuff with flattened lava glass fragments (fiamme) commonly identified as ignimbrites
• Agglomerates have coarse and rounded rock components
• Andesites are intermediate rocks with about 55-65% silica
• Modal analysis or mineralogical classification involves Q for quartz, A for alkali feldspars + albite, P for plagioclases, F for feldspathoids, and M for mafic minerals
• Volcanic rocks vary from basalts, andesites, dacites to rhyolites with decreasing temperatures
• Formation of igneous rocks

  1. Dependent on the rate of cooling of the magma

  2. Longer cooling period produces coarser grained minerals with better formed crystal faces

  3. Faster cooling (quenching) produces finer grained minerals with atoms not able to form the regular internal geometrical arrangement (e.g. volcanic glass)
• Common Igneous Rocks

  • Plutonic: Granite, Diorite, Gabbro, Peridotite
  • Volcanic: Rhyolite, Andesite, Basalt
• Plutonic rocks exhibit coarse-grained textures (phaneritic) while volcanic rocks exhibit finer-grained textures (aphanitic to porphyritic)
• Color indices

  • Leucocratic (0-35), Mesocratic (35-65), Melanocratic (65-90)
• Highly altered igneous rocks

  • Spilites, Serpentintite, Kimberlite
• Major processes in the formation of sedimentary rocks

  Physical and chemical weathering of parent rocks, transportation, deposition in sedimentary basins, compaction, and cementation of sediments