EM Ch 7

• Magma


  Molten rock generated by the partial melting of the mantle and crust
• Magma


  • Contains liquids, crystals, gases, and rock fragments in varying proportions depending on temperature, pressure, and chemistry
  • Only contains liquid and gases at temperatures of 1200°C and above

  
• Exsolution


  When gases separate from the magma due to decompression; gas bubble develop, expand, and rise towards the surface as a separate phase
• Pluton


  Magma chambers of various sizes, shapes, and depths that can store magma within the Earth
• Intrusive/Plutonic rocks


  • Form when magma cools within the surface
  • Commonly composed of coarse crystals (large ones) which grew large due to prolonged cooling

  
• Lava


  Magma that has erupted out the surface
• Extrusive/Volcanic rocks


  • Form from magma that rises and erupts from within due to dissolved gases acting as a fuel blaster
  • Produce rocks with small crystals; did not grow large enough due to fast cooling
• Igneous rock classification


  Based on composition and texture

  
• Igneous rock groups


  • Ultrabasic/Ultramafic
  • Basic/Mafic
  • Intermediate
  • Acidic/Felsic

  
• Ultrabasic/Ultramafic rocks


  • Komatiite
  • Peridotite

  
• Peridotite


  • Very dark-colored ultramafic rock depleted with silica and enriched with minerals like olivine, amphibole, and plagioclase
  • Rare and occur in the Earth's mantle

  
• Basic/Mafic rocks


  • Basalt
  • Gabbro

  
• Basalt


  • Very common volcanic rock (most common); encompassing the upper few kilometers of the oceanic crust

  
• Gabbro


  • Crystallizes more slowly at depth at the lower portions of the ocean basin
• Intermediate rocks


  • Andesite
  • Diorite

  
• Andesite


  • Most common volcanic rock in the Pacific Ring of Fire

  
• Diorite


  • Dioritic plutons underlies most andesitic volcanoes

  
• Acidic/Felsic rocks


  • Dacite
  • Granodiorite
  • Rhyolite
  • Granite

  
• Dacite


  • Also occurs along the Pacific Ring of Fire
• Granodiorite


  • Underlies most andesitic-dacitic volcanoes

  
• Rhyolite


  • Commonly erupts on thick continental crusts

  
• Granite


  • Crystallizes also occur at thick continental crusts
• Holocrystalline


  Wholly crystalline
• Hypocrystalline


  Partially crystalline / partially glass
• Holohyaline


  Wholly glass
• Euhedral minerals


  • Contain complete crystal faces that are not impinged by other crystals
  • Develop as early mineral phases in the crystallization of magma wherein crystals have abundant free space for growth, enhancing the likelihood to form perfectly formed crystal faces

  
• Subhedral minerals


  • Contain partially complete crystal forms in which at least one of the crystal faces is impinged by adjacent rock material
  • Nucleation on pre-existing surfaces such as early formed crystals on the margins of the magma chamber
  • Resorption in which pre-existing euhedral crystals are partially remelted
  • Growth can be aborted due to other secondary alteration processes that destroy pre-existing euhedral faces
• Anhedral minerals


  • Lack any observable crystal faces
  • The space available for the development of euhedral and subhedral crystals diminishes as crystallization progresses
  • Shape determined by the available space left (interstitial spaces)
• Pegmatitic texture


  • Characterized by large crystals averaging more than 30mm in diameter
  • Display large, early formed euhedral crystals surrounded by later formed subhedral grains
  • Commonly develops on granitic plutons with high volatile components
  • Source of many gemstones and valuable ore deposits of metals such as Sn, Ag, and Au

  
• Phaneritic texture


  • Crystal diameters between 1-30mm
  • Mineral crystals are visible to the naked eye
  • Early formed are euhedral, later formed are subhedral to anhedral
  • Can be subdivided into three: fine (1-3mm), medium (3-10mm), and coarse (10-30mm)
  • Fine-grained varieties are common o shallow intrusions like dikes and sills
  • Coarse-grained varieties are associated with larger and deeper intrusions

  
• Aphanitic texture


  • Small crystals less than 1mm; not visible to the naked eye
  • Associated with volcanic rocks that cool quickly on Earth's surface
  • Microcrystalline if minerals can be identified under the microscope
  • Cryptocrystalline if minerals cannot be identified individually under the microscope

  
• Porphyritic texture


  • Consists of two distinct size of crystals; large ones called phenocrysts while the finer-grained material is called the groundmass
  • Porphyritic-phaneritic texture = all crystals are visible but the phenocrysts are larger than groundmass crystals
  • Porphyritic-aphanitic texture = phenocrysts are embedded in an aphanitic groundmass composed of microcrystalline, cryptocrystalline, or glassy material
  • Shows a two-stage cooling process where larger phenocrysts form slowly at depth, while the finer grained groundmass crystals cool rapidly as magma approaches the surface

  
• Crystal nucleation


  • Involves the formation of new crystals through nuclei, or seed crystals which are large enough to persist and grow to even larger crystals
  • Crystal nucleation rate is the number of new seed crystals that develop per volume per unit time (commonly expressed as nuclei/cm3/s)
  • Nucleation peaks at regions below the liquidus line
  • Nucleation needs undercooling because crystal formation requires the development of bonds between ions which produces the heat of formation
• Crystal growth rate


  • Measure of the increase in crystal radius over time (commonly expressed as cm/s)
  • Governed by the rate of undercooling as well as the availability of elements and magma viscosity
  • Low nucleation rate = high growth rate (when magma experiences small levels of undercooling)
  • Prolonged undercooling results to phaneritic, porphyritic, and pegmatitic textures with euhedral-subhedral grains
• Ion availability


  • Refers to the availability of ions that can fill specific ionic site in a crystal lattice structure
  • Enhanced crystal growth = appropriate ions readily available and migrate immediately
  • Explains why minerals that require rare trace elements tend to be very small (apatite and zirconium)
• Viscosity


  • Resistance of a fluid to shear stress
  • Determined by silica content (higher silica content, more viscous), temperature (cooler, more viscous), and amount of dissolved gases (more volatiles, less viscous)
  • Higher viscosity = lower strain rate
• Chemical diffusion rate


  • Diffusion is the rate at which elements migrate through magma; depends on viscosity
  • Low magma viscosity increases the rate of diffusion and increases crystal growth rate
  • High magma viscosity decreases the rate of diffusion and decreases crystal growth rate
• Viscosity and chemical composition

  • Increased molecular linkages results in higher magma viscosity
  • Network formers are elements that tend to increase the molecular linkage thereby increasing viscosity (Si, Al, and O)
  • Network modifiers are elements that decrease molecular linkage thereby reducing viscosity
  • Silica-rich = more viscous; silica-poor but rich in Fe and Mg = less viscous
• Viscosity, molecular bonding, and heat

  Temperature inversely proportional to viscosity and molecular bonding (that's why hotter magma is more fluid and cooler magmas are more viscous)
• Role of gases


  • Magma contain 7 weight percent volatile gases (water, carbon dioxide, and sulfur dioxide) and minor gases (N, H, S, F, Ar, CO, and Cl)
  • Gases reduce molecular bonding; important network modifiers
  • More gases = less viscous because dissolved volatiles partially bonds with the corners of the silica tetrahedra