test 2

• Magma


  Molten rock of any chemical composition
• Lava


  Magma that flows out onto the surface of the earth
• Magma is a complex mixture of molten rock, crystals and gasses, and the relative proportions of these three components changes as the magma cools and crystallises
• The mineral species that crystallise from a magma are dependent on the composition of the liquid, but the gasses, commonly called the volatiles are mostly H2O and CO2
• Magma originates


  By melting of the lower crust and/or upper mantle at depths of between 50 and 200 km below the earth's surface
• Magma cooling


  Crystals grow to produce a tight interlocking network of grains
• The grains in igneous rock are dominantly silicate minerals which crystallise at relatively high temperatures 700 - 1 200 oC

  
• Magma


  Molten silicate material, composed of early-formed minerals, silicate liquid and dissolved gasses
• Major elements in magma


  • O
  • Si
  • Al
  • Ca
  • Na
  • Fe
  • Mg
• SiO2 (silica) is the principal constituent of a magma and forms between 30 and 75 % of virtually all magmas
• Physical properties of magma


  Directly related to the chemical composition and temperature
• Magma with a high Mg and Fe content (low Si) has a significantly lower viscosity and higher density than a magma that has a low Mg and Fe content (Si rich)
• Mg and Fe rich magma is often significantly hotter than Si-rich magma
• Magma series


  • High-silica (> 66 % SiO2)
  • Moderate-silica (52 to 66 % SiO2)
  • Low-silica (45 to 52 % SiO2)
  • Ultramafic (less than 45 % SiO2)
• High-silica magmas


  Crystallise at temperatures between 500 and 800 oC to produce rocks of the granite - rhyolite family
• Low-silica magmas


  Crystallise at temperatures between 900 and 1200 oC to produce rocks of the gabbro - basalt family
• Moderate-silica magmas


  Crystallise to produce rocks of the diorite - andesite family
• Ultramafic magmas


  Crystallise to produce a rock of the dunnite - komatiite family
• Silica (SiO2) and water (H2O)


  Largely control the physical properties of the magma
• Low-silica, mafic magmas


  Have a low viscosity
• High-silica, felsic magmas


  Have a high viscosity
• Mafic lavas tend to be very fluid and erupt relatively quietly, while felsic eruptions tend to be violent
• Crystallisation of magma


  Controlled by its temperature, composition and the presence of volatiles (especially H2O)
• The type of minerals which crystallise from the magma depends on the composition and the sequence in which the different types of minerals crystallise depends on the melting temperatures of the individual types of minerals
• Bowen's reaction series


  • Olivine
  • Pyroxene
  • Amphibole
  • Biotite
  • Ca-feldspar
  • Na-feldspar
  • K-feldspar
  • Muscovite
  • Quartz
• Olivine + quartz


  Produces pyroxene
• In felsic magmas, there is an excess of SiO2 and so quartz will crystallise and be present in the resulting rock
• In an ultramafic magma, there is a low percentage of SiO2 so olivine will be present in the rock
• Volcanic eruption


  1. Magma rises from the site of melting towards the surface of the earth

  2. If magma rises slowly or remains in a subterranean reservoir, high-temperature crystals may form and a porphyritic lava produced

  3. When magma reaches the surface, volatile components separate from the magma and the gasses escape

  4. Magma may rise through a pipe conduit called a vent or a sheet-like crack called a fissure
• Mafic magmas


  Rich in Mg and Fe, low in Si, have a relatively high temperature and low viscosity
• Felsic magmas


  Have a relatively low temperature and high viscosity
• Low viscosity mafic magma with low volatile content


  Eruption is relatively quiet, most magma forms lava flows with little explosive activity and ash production
• Low viscosity mafic magma with high volatile content


  Eruption is spectacular, with lava fountains and significant ejection of ash and pyroclastic material
• High viscosity felsic magma with low volatile content


  Eruption is unspectacular, lava does not travel far from vent, successive flows build up a dome-like structure
• High viscosity felsic magma with high volatile content


  Eruption is very violent, with explosive escape of volatiles and ejection of incandescent clouds of gas and lava droplets
• Felsic magmas are often too viscous and not hot enough to melt any pre-existing lava or pyroclastic material that may have blocked the volcanic vent, leading to a restrained eruption until a violent explosion occurs
• Volcanic eruptions generate intense convection currents in the atmosphere, often causing violent thunderstorms, and the resulting rain-water, displaced crater lakes and melted ice caps can flow down the volcano sides as landslides and mudflows called lahars
• Aa lava


  Cinder-like surface on the upper surface of a lava flow
• Pahoehoe


  Rope-like coils formed when the pliable surface of a lava flow is twisted by the flow of interior material
• Blocky lava


  Lava flow surface, often Aa, that has been fractured by subsurface flow