Metamorphism in Geology

Metamorphism

• Occurs when rocks are altered by heat, pressure, or fluids.
• Changes the texture, mineral composition, and chemical composition of rocks without melting them.
• Derived from Earth's heat, buried under layers of rocks or adjacent to igneous intrusions.
• The pressure required for metamorphism is from either deep burial or tectonic plate compression.

Directed Stress

• Results in shearing and compressive stress.
• Causes rocks to deform and change shape.

Metamorphic Minerals

• Minerals in rocks undergo solid-state alterations.
• Change into new minerals that are more stable under the new temperature and pressure conditions.

Effects of Metamorphism

• Mineralogical Changes: Minerals change to reflect the new pressure and temperature equilibrium conditions.
  • Recrystallization: Existing crystals change in size and shape to form layers or interlocking crystals.
  • Recombination: Elements in pre-existing minerals recombine to form stable new minerals.
  • Addition/Subtraction of ions: Common in the presence of fluids during metamorphism.
• Metamorphic Textures:
  • Foliated metamorphic rocks are characterized by layers and bands of minerals due to pressure during metamorphism.
  • Foliation develops perpendicular to the direction of the pressure.
  • Nonfoliated metamorphic rocks have blocky crystal shapes with no layering.
  • Increasing grain size parallels changes in composition and development of foliation. This is not a factor for nonfoliated rocks.

Grades of Metamorphism

• Different combinations of temperature and pressure produce different grades: low, intermediate, and high.
• Each grade is associated with a specific set of minerals and textures.
• Metamorphism of shale produces the colorful minerals observed in slate.
• Prograde metamorphism: A series of increasing changes in mineral composition and textures as the temperature and pressure increases. Example: slate → phyllite → schist → gneiss → migmatite

Types of Metamorphism

• Regional Metamorphism: Occurs when high temperatures and pressures affect large regions of Earth's crust.
  • Produces large belts of metamorphism that range from low to high grade.
  • Results in mineral and rock type changes, folding, and deformation of rock layers.
• Contact Metamorphism: Occurs when molten material, such as in igneous intrusions, comes in contact with solid rock.
  • High temperatures and moderate-to-low pressure result in specific mineral assemblages.
• Hydrothermal Metamorphism: Occurs when very hot water reacts with rock, altering its chemical and mineral composition.
  • Commonly occurs near igneous intrusions and active volcanoes.
  • Chemical changes are common.

Economic Importance of Metamorphic Rocks and Minerals

• Many economic mineral resources are produced by metamorphic processes.
• Metallic Mineral Resources: Many metallic deposits are precipitated from hydrothermal solutions.
• Nonmetallic Mineral Resources: Metamorphism produces resources such as talc, asbestos, and graphite.

The Rock Cycle

• Igneous Rocks: Crystallize from magma.
• Sedimentary Rocks: Form from cemented or precipitated sediments.
• Metamorphic Rocks: Form from changes in temperature and pressure.