LE2 Geol 11

• Magma


  Completely or partially molten rock beneath Earth's surface
• Magma


  • 600-1200C
  • Composed of varying amounts of liquid (melt; ions of Si, O, Al, K, Na, Fe, Mg), solid (early crystallizing or residuum after partial melting), and gas (dissolved volatiles like H2O, CO2, SO2)
• Silica content


  Amount of SiO2
• Viscosity


  • Resistance to Flow
  • Higher viscosity = more resistant = "thicker"
  • Lower viscosity = less resistant = "thinner"
• Types of Magma


  • Felsic (Granitic, High SiO2, Low Fe-Mg, Lower temperature, Higher viscosity, Explosive eruption)
  • Intermediate (Andesitic)
  • Mafic (Basaltic, Oceanic crust)
  • Ultramafic (Picritic, High Fe-Mg, Higher temperature, Lower viscosity, Gentle eruption)
• Origin and Formation of Magma


  1. Increase temperature to melting point (Hotspot volcanism)

  2. Decrease pressure (Adiabatic decompression, Rift volcanism, Oceanic spreading ridge)

  3. Add volatiles (Lowers melting temperature, Subduction zone)
• How Magma Evolves


  1. Assimilation of host rock

  2. Magma mixing / mingling

  3. Fractional crystallization and crystal settling
• Assimilation of host rock


  Molten body moves up to "country rock" and dislodges "foreign" rock which melt and is incorporated to magma body
• Magma Mixing / Mingling


  • Magma intrudes another that has different composition
  • Younger magma mixes with older magma
• Fractional Crystallization / Settling


  • Change composition of melt as crystallization progresses
  • Crystals may become isolated from melt
• Igneous Rocks


  • Solidification of magma (intrusive)
  • Lava that flowed out to Earth's surface (extrusive/pyroclastic)
• Crystallization


  • Ions pack closer to each other as rate of vibration is lowered
  • Minerals crystallizing first will occupy greatest space
• Texture of Igneous Rocks


  • Intuitive: Based on how much it progressed out of the volcano and size of the grains
  • Larger size, slower cooling
  • Outside volcano, miscellaneous
• Other textures


  • Vesicular (Vesicles from escaping gases in volcanic and pyroclastic rocks, Scoria = vesiculated basalt, Pumice = frothy glass)
  • Pegmatitic (Interlocking crystals > 3 cm)
• Mode of Emplacement


  • Extrusive (volcanic, Basalt, Andesite, Rhyolite)
  • Intrusive (formed at depth, Gabbro, Diorite, Granite)
• Bowen's Reaction Series
• Intrusive Igneous Activity (Plutons)


  • Tabular (Dike, Sill)
  • Massive (Batholiths, Stock, Laccolith)
• Dike


  Discordant, cut across bedding surfaces
• Sill


  Nearly horizontal, concordant, along bedding surfaces
• Batholith


  Granitic, >100 km^2 in outcrop area
• Stock


  Smaller than batholith, <100 km^2
• Laccolith


  Concordant pluton with dome, mushroom-like roof and planar base, injected between sedimentary strata
• Mineral Resources and Igneous Processes


  • Hydrothermal Ore Deposits
  • Geothermal
• Hydrothermal Ore Deposits


  • Hot, metal-rich fluids
  • Remnant of late-stage magmatic process
• Geothermal


  • Heating of water underground as it passes through subsurface region of hot rocks
  • Hot water brought to surface (steam) to drive turbines
• Elements for Geothermal


  • Heat source (Magma, shallow (<6km), recent past (<100,000 yrs))
  • Reservoir rock (Permeable and porous)
  • Cap Rock (Impermeable)
  • Water (Sufficient to sustain hydrothermal convection)
• Porphyry Copper Deposits


  • Far SE-Lepanto
  • Boyongan-Surigao
  • Tampakan-S-Cotabato
• Volcanism is the phenomenon by which magma is erupted to surface through volcanoes as lava
• Volcanism is closely tied to plate boundaries
• Volcano


  • Geomorphic feature where magma is exhumed to surface
  • Not always conical
• Parts of a Volcano


  • Conduit (Pipe)
  • Lava
  • Vent
  • Crater
  • Parasitic Cone
• Pyroclastic material


  • Bombs
  • Ash
• Reasons for Volcanic Eruptions

  • Influx of new magma (basaltic)
  • Addition of magma causes chamber to swell and fracture
  • Magma with different composition causes magma mingling
  • Degassing of magma
• Degassing of magma


  • Depressurization, ponding, and consequent differentiation concentrates volatiles forming bubbles
  • Bubbles coalesce and expand
  • Pressure of gas > pressure of overburden
  • Fracture occurs
• Strength of Eruption


  • Dependent on physical and chemical characteristics of erupting magma
  • Viscosity: Temperature and composition
  • Volatile content (Basaltic (1-2%) < Rhyolitic (4-6%))
• Volcano Explosivity Index (VEI)


  • Relative measure of explosiveness of volcanic eruptions
  • Based on volume of products, height of eruption cloud, and frequency
• Types of Volcanic Eruptions

  • Explosive (Pyroclastic rocks or fragmental rocks, Ash fall, Pyroclastic flows, Debris avalanches, Pyroclastic surges, Lahars, Usually rhyolitic flows (Aa))
  • Non-explosive/Effusive (Lava flows, spatter cones, lava fountains, flood basalt, Usually basaltic flows (Pahoehoe))
• Magmatic Eruption Types


  • Hawaiian (Lava flows from vent, Basaltic lavas, Low viscosity, low content of gas, high temperature, Low amount of ash)
  • Icelandic (Similar to Hawaiian, but lava flow from fissure)
  • Strombolian (Short-lived eruption of lavas, Lava fountain, Deposit are mostly scoria)
  • Vulcanian (Highly viscous lava, Volcanic bombs and blocks, Andesitic - dacitic rather than basaltic)
  • Pelean (COLLAPSE of rhyolite, dacite, or andesite lava domes that create large eruptive columns, Glowing gases)
  • Plinian Type (Voluminous explosive ejections of pumice and pyroclastic flows, Usually accompanied by caldera genic collapse, Volatile-rich dacitic to rhyolitic lavas, Typically in stratovolcanoes)
  • Phreatomagmatic (Surtseyan, Basaltic magma interacting with water in shallow sea or lake)
  • Subglacial (Lava interacting with ice)
  • Phreatic (Geysers, Usually periodic, No magma, Steam, Fumaroles, Meteoric fluids)
• Products of Volcanism


  • Lava flows (Cooling joints reflecting high temperatures during emplacement)
  • Pyroclasts (Fragmented crystals or rocks from crystallizing magma or from volcano edifice)
  • Volcanic gases (Juvenile, Heated meteoric, Ground or surface, SO2, CO2, CO, H2O)
• Types of Volcanoes


  • Based on Age of Activity (Active, Potentially Active, Inactive)
  • Based on Architecture (Shield Volcano, Composite Volcano, Dome, Side Volcano, Cone, Pyroclastic Cones, Maar, Scoria Cone, Tuff Ring, Tuff Cone, Caldera, Plug Domes)