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Formation of sedimentary and metamorphic rocks

Short answer

Sedimentary rocks form through weathering, erosion, deposition, compaction, and cementation of sediments. Metamorphic rocks form when existing rocks are changed by heat, pressure, and chemically active fluids without melting, producing new minerals and textures like foliation.

How are sedimentary and metamorphic rocks formed?

Formation of sedimentary rocks

Sedimentary rocks form at the Earth's surface.

  1. Weathering breaks parent rocks into fragments and dissolved ions.
  2. Erosion and transport by water, wind, ice or gravity.
  3. Deposition in layers in basins, rivers, lakes and seas. Chemical precipitation and accumulation of organic remains also add material.
  4. Compaction: weight of upper layers squeezes out water.
  5. Cementation: silica, calcite or iron oxide binds the grains.
  6. Lithification completes the change to rock. Examples: sandstone, shale, limestone.

Formation of metamorphic rocks

Metamorphic rocks form deep in the crust from existing rocks (igneous, sedimentary or metamorphic) in the solid state under changed conditions.

  1. Heat (from magma or burial) triggers recrystallisation.
  2. Pressure: confining pressure compacts the rock; directed stress aligns minerals into foliation.
  3. Chemically active fluids carry ions and speed up reactions.
  4. New minerals (garnet, kyanite, mica) and new textures develop without melting.

Types: contact (thermal) metamorphism near intrusions (limestone to marble), regional metamorphism during mountain building (shale to slate, schist, gneiss) and dynamic metamorphism along faults.

Examples: shale to slate to phyllite to schist to gneiss; limestone to marble; sandstone to quartzite.

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