Petrology of Rocks

Rocks

• Form the Earth, comprised of minerals
• A hard mass of mineral matter with at least two rock forming minerals (e.g. granite & gneiss)
• Physical and chemical composition varies depending on the minerals
• Formed from molten magma beneath the Earth's surface

Petrology

• The study of rocks (from Greek words "petra" for rock and "logos" for science)
• Petrography deals with rock description, including its composition and properties
• Petrogenesis studies the origin and formation of rocks

Rock Occurrence

• Sedimentary rocks are most abundant, with shales making up 52% of the total
• Igneous rocks are the second most abundant, with granite accounting for 15%

Rock Formation Processes

• Igneous Rocks:
  • Formed through cooling and consolidation of magma (either within or on the Earth's surface)
  • Also known as primary rocks
• Sedimentary Rocks:
  • Formed by disintegration and decomposition of pre-existing rocks, followed by transportation and cementation of fragmented material
  • Also referred to as secondary or clastic rocks, often stratified
• Metamorphic Rocks:
  • Result from the alteration of pre-existing igneous or sedimentary rocks under high temperature and pressure

Types of Rocks

• Classified based on their mode of formation:
  • Igneous Rocks: Solidification of molten magma, often non-laminar with a massive structure
  • Sedimentary Rocks: Formed by the consolidation of sediments derived from the breakdown of older rocks, transported by wind or water
  • Metamorphic Rocks: Formed by the transformation of pre-existing rocks under intense heat, pressure, or both

Sedimentary Rock Formation Stages

• Four main stages:
  • Weathering: Breakdown of pre-existing rocks
  • Transportation: Movement of sediments by wind or water
  • Deposition: Sediment accumulation in low-lying areas
  • Diagenesis: Transformation of unconsolidated sediments into hard rock

Metamorphic Rocks

• Result from chemical or physical changes to the original rock
• Formed from the transformation of igneous and sedimentary rocks under high temperature and pressure
• Two main types of metamorphism:
  • Thermal Metamorphism: Dominated by heat
  • Dynamothermal Metamorphism: Combined effect of pressure and heat
• Examples:
  • Sandstone forms quartzite
  • Shale forms slate or schist
  • Limestone forms marble
  • Granite forms granite gneiss
  • Dolerite forms hornblende