Petrology

• Rocks


  Naturally occurring and coherent aggregates of one or more minerals
• Major classes of rocks

  • Igneous rocks
  • Sedimentary rocks
  • Metamorphic rocks
• Igneous rocks


  • Solidified from molten material called magma
  • Formed at high temperatures
  • Originate from processes deep within Earth
• Sedimentary rocks


  • Deposited and lithified (compacted and cemented together) at the Earth's surface
  • Most are deposited from the land surface to the bottoms of lakes, rivers, and oceans
  • Generally stratified (have layering)
• Metamorphic rocks


  • Formed by changes in preexisting rocks under the influence of high temperature, pressure, and chemically active solutions
  • Changes can be chemical (compositional) and physical (textural)
  • Often produced by processes deep within Earth that recrystallize minerals
• Rock cycle


  1. Erosion (weathering and transportation)

  2. Deposition

  3. Diagenesis (compaction and cementation)

  4. Melting and recrystallization

  5. Uplift and erosion
• Texture


  The size, shape, and arrangement of the grains (for sedimentary rocks) or crystals (for igneous and metamorphic rocks)
• Common textural terms for rocks based on grain/crystal size


  • Very coarse
  • Coarse
  • Medium
  • Fine
  • Aphanitic
  • Phaneritic
  • Pegmatitic
• Porosity


  The volumetric portion of bulk rock that is not occupied by grains, crystals, or natural cementing material
• Sorting


  The tendency of sedimentary rocks to have grains that are similarly sized
• Poorly sorted sediment displays a wide range of grain sizes and has decreased porosity
• Well-sorted sediment has a fairly uniform grain size distribution and can have substantial porosity
• Total porosity

  Encompasses all the void space, including interconnected pores and sealed-off pores
• Apparent (effective, or net) porosity

  The proportion of void space that excludes the sealed-off pores
• Physical properties of rocks

  • Density
  • Specific gravity
  • Mechanical properties (stress, strain, deformation)
• Knowledge of the distribution of underground rock densities can assist in interpreting subsurface geologic structure and rock type
• Density measurements involve determining pore volume, bulk volume, or grain volume, along with the weight
• Rocks can be mechanically altered through processes like deformation from stress, confining pressure, internal pore pressure, temperature, rate of loading, and time
• Stress

  Force per unit area applied to a material
• Strain

  Change in dimension, volume, or shape of a material due to an applied stress
• Types of stress

  • Axial (tension or compression)
  • Shear (tangential)
  • Hydrostatic compression
• Elastic deformation

  Strain is recoverable when the stress is removed and linearly proportional to the applied stress
• Inelastic deformation

  Strain is permanent
• Elastic constants

  • Young's modulus
  • Shear modulus
  • Bulk modulus
  • Poisson's ratio
• Young's modulus

  Ratio of applied stress to fractional extension/shortening of sample length
• Shear modulus

  Ratio of applied stress to distortion (rotation) of a plane
• Bulk modulus

  Ratio of confining pressure to fractional reduction of volume
• Poisson's ratio

  Ratio of lateral strain to longitudinal strain
• The common units of stress are: 1 bar = 10^5 newtons per square metre (pascals), 10 kilobars = 1 gigapascal
• Rock mechanics

  Study of deformation resulting from the strain of rocks in response to stresses
• Geotectonics

  Study of deformation of large geologic structures in Earth's crust
• Stress-strain relationships

  • Elastic
  • Viscous
  • Plastic
  • Elasticoviscous
  • Firmoviscous
  • Plasticoviscous
• Thermal conductivity

  Measure of a rock's ability to conduct heat, depends on composition, fluid content, pressure, temperature, and homogeneity
• Thermal expansion

  Change in dimension of a rock with temperature, expressed as a coefficient
• Radioactive heat generation from decay of radioactive elements is an important factor in Earth's temperature gradient and thermal evolution
• Igneous rocks

  Crystalline or glassy rocks formed by the solidification of magma
• Types of igneous rocks

  • Plutonic intrusive
  • Volcanic/extrusive
  • Subvolcanic/hypabyssal intrusive
• Igneous rock composition

  • Felsic (high silica)
  • Intermediate
  • Mafic (low silica)
  • Ultramafic (very low silica)
• Felsic rocks

  Igneous rocks with >66% silica
• Mafic rocks

  Igneous rocks with 45-55% silica, enriched in iron and magnesium