Petrology: Igneous Rocks

• Petrology is the study of rocks, concerning their composition, origin, distribution, structure, and classification
• It involves mineralogy, petrography, geochemistry, and the study of phase equilibria
• Petrology includes the study of igneous, sedimentary, and metamorphic rocks

Igneous Rocks

• Igneous rocks are formed through the cooling and solidification of magma or lava
• Magma is molten rock beneath the Earth's surface, while lava is molten rock erupted onto the surface
• Igneous rocks are classified based on their chemical and mineral composition, as well as their texture
• The texture of an igneous rock is determined by the size, shape, and arrangement of its mineral grains
• Intrusive (plutonic) igneous rocks cool slowly within the Earth, resulting in large crystal sizes (phaneritic texture)
• Extrusive (volcanic) igneous rocks cool rapidly on the Earth's surface, resulting in small or no crystals (aphanitic or glassy texture)
• Common igneous rocks include granite (intrusive), basalt (extrusive), and obsidian (extrusive)

Sedimentary Rocks

• Sedimentary rocks are formed from the accumulation and cementation of sediments
• Sediments can be fragments of other rocks (clastic), chemical precipitates, or organic matter
• Sedimentary rocks are classified based on the type of sediment they contain
• Clastic sedimentary rocks are composed of rock fragments and are classified by grain size:
  • Conglomerates contain gravel-sized particles
  • Sandstones are composed of sand-sized grains
  • Siltstones consist of silt-sized particles
  • Shales are made of clay-sized particles
• Chemical sedimentary rocks precipitate directly from solution and include:
  • Limestone (calcium carbonate)
  • Chert (silica)
  • Evaporites (salts)
• Organic sedimentary rocks are formed from the accumulation of organic matter:
  • Coal (plant remains)
  • Oil shale (organic-rich mud)
• Sedimentary structures, such as bedding, cross-bedding, and ripple marks, provide information about the depositional environment

Metamorphic Rocks

• Metamorphic rocks are formed when existing rocks are changed by heat, pressure, or chemically active fluids
• Metamorphism occurs in the solid state and involves changes in mineralogy, texture, and sometimes chemical composition
• Metamorphic rocks are classified based on their texture and mineral composition
• Foliated metamorphic rocks exhibit a layered or banded appearance due to the alignment of mineral grains under directed pressure:
  • Slate (fine-grained, from shale)
  • Schist (medium-grained, with visible platy minerals)
  • Gneiss (coarse-grained, with compositional banding)
• Non-foliated metamorphic rocks lack a layered appearance:
  • Marble (from limestone or dolostone)
  • Quartzite (from sandstone)
  • Hornfels (fine-grained, from various protoliths)
• Metamorphic grade refers to the intensity of metamorphism, with higher grades indicating higher temperatures and/or pressures

Petrogenesis

• Petrogenesis is the study of the origin and formation of rocks
• For igneous rocks, petrogenesis involves understanding the processes of magma generation, ascent, and crystallization
• Key factors influencing magma formation include:
  • Temperature
  • Pressure
  • Composition
  • Presence of volatiles (e.g., water)
• Magma can be generated by:
  • Decompression melting (at mid-ocean ridges and continental rifts)
  • Addition of volatiles (at subduction zones)
  • Heat transfer from mantle plumes
• Magma ascent is driven by buoyancy and pressure gradients
• Magma differentiation occurs through processes such as:
  • Fractional crystallization (removal of early-formed crystals)
  • Assimilation (incorporation of country rock)
  • Magma mixing (blending of different magmas)
• The petrogenesis of sedimentary rocks involves understanding the processes of weathering, erosion, transport, deposition, and diagenesis
• Weathering breaks down rocks at the Earth's surface through physical and chemical processes
• Erosion transports sediments away from their source by wind, water, or ice
• Deposition occurs when sediments accumulate in a sedimentary environment
• Diagenesis involves the physical and chemical changes that occur after deposition, including compaction, cementation, and recrystallization
• The petrogenesis of metamorphic rocks involves understanding the factors that control metamorphism, such as:
  • Temperature
  • Pressure
  • Fluid composition
  • Protolith composition
• Metamorphic environments include:
  • Contact metamorphism (around igneous intrusions)
  • Regional metamorphism (at convergent plate boundaries)
  • Hydrothermal metamorphism (at mid-ocean ridges and in geothermal systems)
  • Burial metamorphism (in sedimentary basins)
  • Shock metamorphism (during meteorite impacts)