Formation and Classification of Rocks

General Classification of Rock Types

• Igneous Rocks: Rocks formed when magma or lava cools and solidifies.
• Sedimentary Rocks: Rocks formed from sediments that are deposited, compacted, and cemented.
• Metamorphic Rocks: Rocks formed when existing rocks are changed by heat and pressure.

Formation of Named Rocks

• Igneous Rocks

  • Granite:
  • Formation Process: Forms underground when magma cools and crystallises slowly.
  • Crystalline Structure: Slow cooling produces large crystals.
  • Basalt:
  • Formation Process: Forms at the Earth's surface when lava cools and crystallises quickly.
  • Crystalline Structure: Fast cooling produces small crystals.

• Sedimentary Rocks

  • Limestone:
  • Organic Accumulation: Forms from the accumulation and compaction of shells and skeletons of marine organisms, which are mainly composed of calcium carbonate.
  • Chemical Precipitation: Can also form when calcium carbonate precipitates directly from water.
  • Sandstone:
  • Formation Process: Forms when layers of sand are deposited, compacted, and cemented together.
  • Shale:
  • Formation Process: Forms from very small particles of mud and clay that are deposited, compacted, and cemented together.

• Metamorphic Rocks

  • Marble:
  • Parent Rock: Limestone.
  • Formation Process: Forms when limestone is subjected to high heat and pressure.
  • Structural Transformation: High heat and pressure cause its constituent minerals to recrystallise.
  • Slate:
  • Parent Rock: Shale.
  • Formation Process: Forms when shale is subjected to heat and pressure.
  • Structural Transformation: Heat and pressure change its minerals and internal structure.