envisci

• The atmosphere is the gaseous envelope surrounding Earth
• Sizes and shapes of minerals are dependent on the rate of cooling of the magma
• Textures common among volcanic rocks

  • Vesicular texture (presence of vesicles like scoria)
  • Hyaline texture (glassy texture like obsidian)
• Example of zoned minerals

  • Crystallization process in solid solutions (olivine group) where the core of the minerals is Mg-rich while the outer zones are enriched with Fe
• 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
• Textures of Igneous Rocks

  • Phaneritic texture (crystals visible to the naked eye, mostly observed among plutonic rocks)
  • Aphanitic texture (crystals visible with magnifiers, characteristic of volcanic rocks)
  • Porphyritic texture (two sizes of crystals observed - phenocrysts and groundmass)
• Igneous rocks are differentiated based on texture, such as crystal sizes
• Shapes of minerals

  • Euhedral (defined perfect planar surfaces)
  • Subhedral (intermediate faces)
  • Anhedral (no observable faces)
• Porphyritic textures are formed by early slow crystallization at depths followed by rapid crystallization near the surface
• A longer cooling period produces coarser-grained minerals with better-formed crystal faces, while faster cooling (quenching) produces finer-grained minerals with atoms not able to form the regular internal geometrical arrangement
• Textures of rocks help infer the environment of formation based on sizes, shapes, and arrangements of minerals
• Types of Igneous Rocks based on mode of emplacement

  • Plutonic rocks (intrusive igneous rocks)
  • Volcanic rocks (extrusive igneous rocks)
• Sizes of minerals

  • Coarse-grained (>5 mm)
  • Medium-grained (2-5 mm)
  • Fine-grained (<2 mm)
• Rapid cooling can result in zoned minerals with distinct compositions
• With decreasing temperatures, different plutonic rocks vary from ultramafics, gabbros, diorites, to granites while different volcanic rocks vary from basalts, andesites, dacites to rhyolites
• Pyroclastic flow deposits exhibit characteristics of welded tuff with flattened lava glass fragments (fiamme) commonly identified as ignimbrites
• Grain sizes in pyroclastic textures

  • Ash (< 2 mm)
  • Lapilli (2-64 mm)
  • Block and bomb (> 64 mm)
• Ultrabasic rocks have < 45% SiO2, basic rocks have about 45-52% SiO2, intermediate rocks have 52-66% SiO2, and acidic rocks have > 66% SiO2
• Granite and rhyolite


  Have the same chemical composition but differ in texture
• 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)
• Breccias have predominantly angular rock components
• Bowen’s Reaction Series is used in the classification of different igneous rocks
• If M < 90%, neglect all mafic contents and recalculate Q+A+P or F+A+P = 100%. If M > 90%, use other diagrams for calculation
• Quartz enrichment happens at lower temperatures
• Plutonic rocks exhibit coarse-grained textures (phaneritic) while volcanic rocks exhibit finer-grained textures (aphanitic to porphyritic)
• Different types of igneous rocks were derived from a single basaltic magma undergoing differentiation and fractionation
• Tuff has rock components predominantly made up of ash and lapilli
• Classification of igneous rocks

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

  2. Mineralogical classification (modal) using the IUGS system
• Textures of violently extruded volcanic rocks are important to know
• Common Igneous Rocks

  • Plutonic: Granite, Diorite, Gabbro, Peridotite
  • Volcanic: Rhyolite, Andesite, Basalt
• Modal analysis involves identification using Q for quartz, A for alkali feldspars, P for plagioclases, F for feldspathoids, and M for mafic minerals
• Various minerals are formed from olivine, pyroxene, amphibole, biotite to muscovite, and plagioclases from Ca-rich plagioclases (anorthite) to Na-rich plagioclases (albite)
• Agglomerates have coarse and rounded rock components
• Granite has phaneritic textures while rhyolite has aphanitic to porphyritic textures
• 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
• Sedimentation


  • Requires fundamental information on sediments particularly on how they are formed and how they react to different surficial conditions
  • Incorporates subprocesses such as weathering, transport, and deposition which are fundamentals to the formation of sedimentary rocks
• Mineralogy influencing weathering

  Minerals that crystallize first in the Bowen’s Reaction Series are more susceptible to chemical weathering than those that crystallize last
• Chemical weathering


  • Breakdown of rocks accompanied by changes in chemical and mineralogical composition
• Sedimentation

  1. Weathering

  2. Erosion

  3. Transportation

  4. Deposition
• Solution weathering

  • Carbonation