3,2

• Igneous rocks


  Rocks forming specifically from the freezing or solidification of a melt (magma inside the earth and lava on the surface)

  
• Magma


  Molten rock occurring underground
• Lava


  Molten rock emerging through a vent (= volcano)

  
• From melts to igneous rocks

  1. Freezing/crystallising/solidifying to form igneous rocks

  2. Lava fountain where lava forcefully rises metres to 100s of metres into the air

  3. Lava lake where lava pools over the vent

  4. Lava flow where lava moves in a stream down a slope

  5. Pyroclastic debris (fragmented materials from explosive volcanoes)

  
• Extrusive igneous rocks


  • Freezing of lava aboveground after extrusion to the surface
  • Cementing or welding together of pyroclastic debris

  
• Intrusive igneous rocks


  • Magma intruded into pre-existing rock (wall rock) and solidifying underground as an igneous intrusion
  • Irregular magma chamber
  • Chimney-like columnar dykes
  • Sheet-like sills

  
• The intrusive realm lie underground, and the extrusive realm lies above ground. Lava flows and various types of explosive eruptions all produce extrusive rocks

  
• Source of heat inside planet

  1. Nebula theory – continuous collision and merging of planetesimals, kinetic energy converts to heat energy

  2. Thermal energy from the decay of radioactive elements

  3. Earth grows, with gravity pulling matter inward and squeezing it together more tightly with the weight of overlying material
• Igneous rocks are thought to be the first rocks of the Earth's crust to form

  
• Solid crust


  • Does not float on sea of magma (melt)
  • Deep rock is still solid, although it often behaves in a plastic manner due to the enormous pressures

  
• Forming melts (cause of melts)

  1. Decreases in pressure

  2. Addition of volatiles (CO2, H2O)

  3. Heat transfer

  
• Decompression melting


  Pressure decreases, but the rocks cool only a little and the rock begins to melt

  
• Flux melting


  Volatiles mix with hot, dry rock and help break chemical bonds, causing the rock to melt

  
• Heat-transfer melting


  Rising melt brings heat that is conducted into the wall rock around the intrusion, raising its temperature and causing melting
• Oxides


  Molecules composed of an element bonded to oxygen
• Dry melts


  Contain no volatiles
• Wet melts


  Contain up to 15% volatiles (e.g. H2O, CO2, N2, H2, SO2)
• Four major composition types of molten rock

  • Mafic melts (high proportion of MgO and FeO relative to silica)
  • Felsic melts or silicic melts (low proportion of MgO and FeO relative to silica)

  
• Factors affecting molten rock composition

  1. Source-rock composition

  2. Partial melting

  3. Assimilation ("contamination")

  4. Magma mixing

  
• Magmas formed from partial melting are more felsic than the source rock from which they were derived

  
• Assimilation ("contamination")


  Magma sitting in magma chamber prior to complete solidification: Heat provided by magma partially melts the wall rock, the new magma then mix with the original magma. May incorporate chemicals dissolved from the wall rock of the chamber, or from detached blocks of wall rock, "contaminating" the original magma.

  
• Xenoliths


  Unmelted country rocks within an igneous rock mass
• Magma mixing


  Different magmas formed in different locations from different source rocks may enter the same magma chamber. Originally distinct magmas may mix or dissolve in each other, producing a new, different magma.
• Magma mixing


  • Equal proportions felsic magma mixes with equal proportions of mafic magma, it produces an intermediate magma. If the magma remains heterogenous (not mixing) it can create xenoliths, dykes, etc.

  
• Magma rises in the Earth


  • It is less dense than the surrounding rock, resulting in a buoyancy force acting on it and forcing it upward
  • The weight of rock produces a pressure at a depth that squeezes magma upward

  
• Viscosity


  A liquid's resistance to flow. A viscous liquid flows slower than a non-viscous liquid (affects the speed of flow).
• Viscosity of molten rock

  • Temperature - hotter melts are less viscous (thermal energy breaks bonds and induces movement of atoms or molecules)
  • Volatile content - wetter melts are less viscous than dry melts (volatiles break silicates apart)
  • Silica content - mafic melts are less viscous than felsic melts (long chains of silicates tangle more than many isolated tetrahedra)

  
• Hot mafic lava


  Relatively low viscosity, flows in thin sheets over wide regions ("effusive")

  
• Cool felsic lava


  Relatively high viscosity, clumps up at volcanic vent to form a bulbous mound ("explosive")

  
• Freezing of magma


  • Occurs because a melt cools to below its freezing temperature
  • As magma rises, cooling happens because the temperature of the crust (moving towards the surface) decreases
  • If magma gets trapped underground (under the surface) as an intrusion, it slowly loses heat to surrounding wall rock, until it solidifies
  • If magma reaches the surface and extrudes as lava, it rapidly cools on contact with cooler air or water
  • Freezing sometimes occurs due to Degassing or removal of volatiles (addition of volatiles induces melting; removal induces freezing)

  
• Cooling rates of magma

  • Vary based on how fast the surroundings can transfer heat
  • Wall rock surrounding a magma intrusion insulates it and causes it to cool slowly
  • Lava erupting is rapidly cooled by air or water
  • Depth of intrusion - deeper intrusions cool slower than shallow (close to the surface) intrusions
  • Shape and size of magma body - greater surface area for given volume, greater the heat loss
  • Presence of circulating groundwater - water carries away heat, and as such magma interacting with groundwater will cool quicker

  
• Cooling of extrusions (lava)

  • Depends on composition of the lava - Thin lava flows cool quicker than thicker lava flows because of the ratio of surface area to volume
  • Shape and size of lava body - Tiny droplets of lava sprayed into the air in an explosive eruption freeze much quicker than coherent lava flows, also due to surface area: volume ratio
  • Presence of water - Lava immersed in water cools faster than lava immersed in air, because water carries heat away quicker from rock than air can
• Fractional Crystallisation


  1. Melts contain multiple different chemicals

  2. When a mafic magma cools to freezing temperature, mafic minerals such as olivine and pyroxene crystallise first (i.e. the iron and magnesium-rich minerals)

  3. These solid crystals are denser than the remaining melt, so they sink

  4. Some of the crystals react with the remaining magma, and some become isolated from the magma - the process of sequential crystal formation = fractional crystallization

  5. This extracted iron and magnesium from the magma, resulting a mafic crystals forming, and the remaining melt becoming more felsic

  6. If magma freezes completely before too much fractional crystallization can occur, a mafic igneous rock forms

  7. If magma remains after a lot of fractional crystallization has occurred, the melt becomes progressively more felsic. Freezing of remaining felsic magma, yields a felsic igneous rock

  
• With decreasing temperature, fractional crystallization begins and the composition of the remaining magma becomes more felsic.

  
• Fractional Crystallisation


  • The cooling rate of molten rock depends on the size and shape of the magma, or lava body, and on its depth
  • The process of fractional crystallization results in a progressive change in magma composition during freezing

  
• The classification of igneous rocks. This graphic model describes the difference between nine common igneous rocks based on texture of mineral grains, temperature of crystallization, relative amounts of typical rock forming elements, and relative proportions of silica and some common minerals.

  
• Bowen's Reaction Series


  1. In a cooling melt, olivine and calcium-rich plagioclase form first.

  2. As the melt cools, the plagioclase that forms contains more sodium (Na). This sodium-rich plagioclase may encase earlier-formed crystals or may grow as new crystals.

  3. Meanwhile, some olivine crystals react with the remaining melt to produce pyroxene - which may encase olivine crystals or replace them.

  4. Some of the olivine and calcium-rich plagioclase crystals may become isolated from the melt, taking iron, magnesium, and calcium atoms with them.

  5. By this process the remaining melt becomes enriched with silica

  6. As the melt continues to cool, pyroxene crystals react with the melt from amphibole, and then some amphibole reacts with the remaining melt to form biotite, while crystals continue to become isolated, and the remaining melt continues to become more felsic.

  7. At temperatures between 650C and 850C only 10% of the melt remains liquid, and this melt has high silica contents. At this stage, the final melt freezes, yielding quartz, potassium feldspar (K-feldspar or orthoclase) and muscovite.

  8. The discontinuous reaction series refers to the sequence olivine, pyroxene, amphibole, biotite, K-feldspar/muscovite/quartz; each step yields a different class of silicate mineral.

  9. The continuous reaction series refers to the progressive change from calcium-rich to sodium-rich plagioclase because the steps yield different versions of the same mineral.

  10. Notably not all minerals listed in the series appear in all igneous rock. For example, a mafic magma may completely crystallize before felsic minerals such as quartz or K-feldspar can grow.

  11. The succession of minerals in the discontinuous series is not random - it begins with minerals having isolated silicon-oxygen tetrahedra (olivine) and progresses to those having single chains tetrahedra (pyroxene), then double chains (amphibole) and finally sheets (mica) or three-dimensional framework (quarts).

  12. Minerals that crystallize later in the discontinuous series have more Si-O-Si bonds and smaller O/Si reactions than those that crystallized earlier.

  
• Minerals


  • Common rock-forming minerals are mostly silicate minerals
  • In which the silica tetrahedra build towards framework silicates as crystallisation temperature decreases

  
• Rock


  Coherent, naturally occurring, aggregate of minerals or glass
• Silicate minerals


  • Olivine
  • Pyroxene
  • Amphibole
  • Micas
  • Feldspars