Metamorphic Rocks Study Notes

Metamorphic Rocks

  • Formation of Metamorphic Rocks

    • Metamorphic rocks are formed from preexisting parent rocks, known as protoliths, through the processes of heat, pressure, or chemically active fluids.

    • The metamorphic process alters the texture and/or mineral composition of the protoliths.

    • These rocks are formed under conditions that exceed those of sediment lithification but fall below the intense heat that would melt rock into magma.

    • The transformation depends on:

    • The identity and characteristics of the protolith (parent rock).

    • The intensity of the metamorphic conditions (heat and pressure).

  • Classification of Metamorphic Rocks

    • To classify a metamorphic rock correctly, one must understand:

    • The parent rocks (protoliths) and their potential metamorphic outcomes.

    • The intensity of metamorphism, which relates to the grade of the metamorphic process.

    • The specific characteristics of various textures, which can be foliated or non-foliated.

Foliated Textures

  • Definition of Foliation

    • Foliation refers to the alignment of platy and/or needle-like mineral crystals that occurs under directed pressure.

  • Formation in Regional Metamorphism

    • Occurs in regional metamorphic settings, such as the core of large mountain ranges that are formed when two continental plates collide.

    • The immense pressures during these collisions can cause minerals in existing rocks to realign.

    • Any new minerals that form during this process must also grow under the directed pressures present.

  • Degree of Foliation

    • The intensity of foliation is determined by the degree of pressure and temperature the rocks are exposed to, referred to as grade.

    • For foliation to develop, the parent rock must have a suitable chemistry that permits the growth of platy or needle-like minerals.

  • Common Protolith for Foliated Rocks

    • Shale is the most common protolith for foliated metamorphic rocks due to its chemical composition allowing for easy metamorphosis under the appropriate conditions.

  • Conditions Leading to Foliation

    • Under low-grade metamorphic conditions (low pressure and temperature), rocks will exhibit minimal foliation.