G1 Rock Forming Processes

• • Sodium rich plagioclase - albite
  • Calcium rich plagioclase - anorthite
• Hjulstrom Curve

  • Graph used to determine if a river will erode, transport or deposit sediment
  • Takes into account particle size and water velocity
• Walther’s Law

  • Transgressions
  • Sandstones move further inland
  • Nearshore environments become deeper and are overlain by finer sediment
  • Regressions
  • Sandstones move out to sea
  • Offshore environments become shallower and are overlain by coarser sediment e.g. sandstones
• • Walther’s Law is most obviously seen along coastlines
  • Former positions of coastal environments can be seen in a vertical sequences of facies.
• • Fractional Crystallisation
  • Gravity Settling
  • Early formed minerals will settle out forming a layer at the base of the magma chamber or sill
  • Filter pressing
  • During crystallisation of magma, there is a point where crystals and liquid exist together as a slushy mass. Due to the weight of the overlying crystals, the liquid gets squeeze out forming a separate layer
• • Replenishment
  • Magmatic events may represent different phases. Injection of fresh magma may change the composition of the melt.
  • Magma Mixing
  • Mixing two magmas that are compositionally different will produce a magma of intermediate composition
• Granitic Magma

  • Granitic magma is being created by partial melting of granite in continental crust just below 600 degrees C
  • Silicic magma may also form via assimilation and processes of differentiation
  • Although silicic magma can reach the surface this often stalls at depth because of the high viscosity of the melt and relatively low temperatures near the surface
  • These can form extensive batholiths
• Granitic magma can also form at continental collision zones. This does not result in volcanism at the surface
• Types of metamorphic fabric

  • Slaty cleavage
  • Schistocity
  • Gneissose banding
  • Porphyroblastic
  • Granoblastic
  • Mylonitic
• • Slaty cleavage
  • Rocks with slaty cleavage will split into thin sheets along the cleavage planes. Occurs in fine grained rocks formed by low grade regional metamorphism
  • Bedding and fossils may not be completely destroyed by low grade metamorphism leaving traces or relict structures. Fossils may be deformed due to compressive stress.
• • Schistosity
  • Found in phyllites and schists
  • Results in alignment of flat, platy minerals commonly muscovite mica
  • Alignment of muscovite gives the rock a micaceous sheen. Garnet porphyroblasts are often present in schist and they distrupt alignment of mica minerals
• • Gneissose banding
  • Found in gneisses
  • Light and dark minerals are separated into bands
  • Mica-rich layer is foliated and pale layer is granoblastic
• • Porphyroblastic fabric
  • Occurs in both regional and contact metamorphic rocks
  • Porphyroblasts are large crystals that grow during metamorphism and are surrounded by a finer groundmass
• • Granoblastic fabric
  • Unfoliated fabric, formed by thermal metamorphism. Pressure is not a factor in formation of granoblastic fabric
  • Randomly orientated
  • Equidimensional
  • Usually occurs in rocks with only one mineral (monomineralic)
• • Factors controlling width of metamorphic aureole
  • Size of igneous intrusion
  • Temperature of intrustion
  • Permeability of country rock
  • Dip of the contact - shallow angle of dip allows wide aureole, steep angle of dip allows narrow aureole
• • Index minerals are used to determine the grade of metamorphism. Chemical reactions producing index mineral are primarily controlled by temperature and pressure
• • Important index minerals tested in lab:
  • Kyanite - intermediate grade regional rocks
  • Andalusite - common in contact metamorphic rocks
  • Silimanite - common in high grade regional and contact metamorphic rocks
• Zoned crystals


  Crystallisation of the melt takes place too rapidly - early formed crystals do not have time to react back fully with the melt. Leaves a crystal remnant behind and around this remnant, later crystals will grow which in turn will become surrounded by later forming crystals as temperature falls - forming a zoned crystal