metamorphic rocks

METAMORPHIC ROCKS

  • Metamorphism: The process of solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and chemically active fluids.

    • Mineralogical and crystallographic changes occur.
    • Weathering and diagenesis changes at Earth's surface are NOT classified as metamorphism.

  • Formation of Metamorphic Rocks: Can originate from:

    • Igneous rocks
    • Sedimentary rocks
    • Other metamorphic rocks

  • Process: Transition between rock types occurs via temperature and/or pressure while keeping the rock solid.

TYPES OF METAMORPHISM

  • Contact (Thermal) Metamorphism:

    • Characteristics: High temperature, low pressure;
    • Occurs around intrusive igneous rocks due to thermal effects of magma.
    • Results in hornfels (fine-grained).
    • A zoned area of metamorphic alteration called aureole forms around the intrusion.

  • Regional Metamorphism:

    • Characteristics: High temperature, high pressure;
    • Associated with orogenic events (mountain building)
    • Occurs due to plate collisions that create compressional forces.
    • Produces large quantities of metamorphic rock.

  • Dynamic (Dislocation) Metamorphism:

    • Characteristics: Low temperature, high pressure;
    • Associated with zones of high strain such as faults; can cause crushing and grinding of rocks.
    • Produces rocks with cataclastic texture.

  • Hydrothermal Metamorphism:

    • Caused by hot, ion-rich fluids circulating through rocks.
    • Common along mid-ocean ridges and can form breccia with angular fragments.

  • Prograde Metamorphism:

    • Progressive increase in mineralogical changes with rising temperature and pressure.

  • Retrograde Metamorphism:

    • Occurs when a rock undergoes lower-grade metamorphism after a higher-grade metamorphism.

  • Burial Metamorphism:

    • Occurs when sediments are deeply buried under additional sediment weight, increasing pressure/temperature.

  • Impact (Shock) Metamorphism:

    • Results from high-speed meteorite impacts, forms impactite.

AGENTS OF METAMORPHISM

  • Heat:

    • Most critical agent for metamorphic changes.
    • Sources:
      • Contact Metamorphism: Heat from magma.
      • Geothermal Gradient: Temperature increases due to depth within the Earth.

  • Pressure and Differential Stress:

    • Increases with depth, influencing texture and structure.
    • Confining Pressure: Equal stress from all directions (isostatic).
    • Differential Stress: Unequal stresses that affect mineral orientation.
    • Effects: Mechanical rotation and elongation of minerals.

  • Chemically Active Fluids:

    • Mostly water with volatile components, enhancing ion migration and recrystallization.
    • Fluid Sources: Pore spaces in sedimentary rocks, fractures in igneous rocks, hydrated minerals such as clays and micas.

METAMORPHIC ROCK TEXTURES

  • Foliation: Planar arrangement in metamorphic rocks, often leaf-like (from Latin folium). Formed from the alignment of platy or elongated minerals.

    • Layers can vary in thickness.

  • Lineation: Linear structures formed due to deformation; can occur alongside foliation.

  • Schistosity: A type of foliation with subparallel orientation of platy minerals, larger grains visible without aid.

  • Gneissic Texture: Banding due to segregation of minerals, typically in high-grade metamorphic rocks (gneiss).

  • Non-foliated Textures: Develop under low deformation, with equidimensional crystals, includes porphyroblastic textures (large crystals within a fine-grained matrix).

METAMORPHIC FACIES

  • Zeolite Facies: Low-temperature and low-pressure conditions with characteristic minerals like zeolite.
  • Prehnite-Pumpellyite Facies: Associated with burial, minerals like Prehnite, Pumpellyite, Quartz.
  • Blueschist Facies: Low-temperature, high-pressure environments, blue minerals like glaucophane.
  • Greenschist-Ampibolite-Granulite Facies: Medium to high temperature and pressure, green minerals indicative of greenschist.
  • Eclogite Facies: High-temperature and high-pressure, basic rocks transform into eclogite with minerals such as omphacite and garnet.

CLASSIFICATION OF METAMORPHIC ROCKS

  • Common Foliated Rocks:

    • Slate: Very fine-grained, excellent rock cleavage, derived from shale, mudstone.
    • Phyllite: Fine-grained with glossy sheen, between slate and schist.
    • Schist: Medium to coarse-grained, containing various minerals predominantly micas.
    • Gneiss: High-grade, banded appearance, forms from granite.
    • Migmatite: High-grade with igneous characteristics.

  • Common Non-foliated Rocks:

    • Marble: Derived from limestone, crystalline texture.
    • Quartzite: From quartz-rich sandstone, very hard and resistant.
    • Hornfels: Produced by contact metamorphism, tough and non-foliated.

GENERAL PROPERTIES OF METAMORPHIC ROCKS

  • Typically exhibit a crystalline texture.
  • May show foliation at low transformation intensity.
  • New minerals formed during metamorphism are common.
  • Fossils may rarely appear at very low metamorphic grades.
  • Can form massive occurrences in the Earth's crust.

ENGINEERING PROPERTIES OF ROCKS

  • Engineers prioritize the physical and mechanical properties of rocks over names.
  • Important properties include:
    • Specific Gravity
    • Porosity
    • Water Absorption
    • Unit Weight
    • Strength
    • Bearing Capacity
    • Deformation
  • Metamorphic rocks are generally strong but not often used in construction except for marble and quartzite.