ES1089G: Lec9

Earth Sciences 1089G

Lecture 9: Metamorphic Rocks

Part 1: Metamorphic Processes

  • Definition: Metamorphic rocks are formed by the transformation of pre-existing rocks under the influence of high temperatures, pressures, and chemically active fluids.

Types of Metamorphism

  1. Contact Metamorphism

    • Involves heat with uniform pressure.

    • Occurs when rock is heated by an igneous intrusion, causing a metamorphic halo or “aureole” in adjacent rock.

    • Overall composition of the rock remains the same.

    • Example: Hornfels (from shale).

  2. Regional Metamorphism

    • Pre-existing rock is subjected to heat and pressure on a regional scale.

    • Commonly associated with mountain-building events and involves compressive forces.

  3. Metasomatism

    • Involves chemically active fluids that alter the mineral composition of the rock.

Part 2: Metamorphic Rock Types

Foliation

  • Foliation results from differential stress applied to rocks, leading to alignment of platy minerals (like mica) and elongate minerals.

  • Key to note: Regional metamorphism generally leads to foliation, whereas contact metamorphism does not.

Common Foliated Metamorphic Rocks

  • Shale > Slate > Phyllite > Schist > Gneiss

    • The sequence shows increasing intensity of metamorphism, with changes in grain size and foliation type.

    • Shale transforms into slate with very low metamorphic grade, continuing to gneiss with high metamorphic grade.

Foliated Texture

  • Definition: Preferred orientation of mineral grains in metamorphic rocks due to directional pressure.

  • It is not to be confused with bedding, which is layering from a sedimentary parent rock.

Uses of Foliated Metamorphic Rocks

  • Foliated rocks, like slate and phyllite, are durable and can be split into sheets; common uses include:

    • Slate roofing

    • Walkway tiles

    • Gneiss in countertops, headstones, and houses.

Non-Foliated Metamorphic Rocks

  • Non-foliated rocks lack platy/elongate minerals.

  • Common examples include:

    • Hornfels: formed from shale in contact metamorphism.

    • Marble: formed from limestone; lacks foliation but is valuable in sculpture for its texture.

    • Quartzite: formed from quartz sandstone.

Non-Foliated Rocks and Their Uses

  • Non-foliated rocks, like marble, are used in sculptures and architectural features due to aesthetic variation and durability.

Metasomatism

  • Process of altering the chemical composition of a rock through fluids.

  • Example Reaction: Olivine reacts with water to form serpentine.

Metasomatized Metamorphic Rocks

  • Serpentine: Used in sculpture since ancient times.

  • Soapstone: Primarily made of talc, widely used by Aboriginal peoples in Canada.

  • Jade: Composed of jadeite or nephrite; notable for toughness and intricate carvability.

Conclusion

  • Metamorphic rocks display a variety of textures and compositions based on the conditions of their formation, showing significant geological processes and applications.