Lecture 5 - Metamorphism and Metamorphic Rocks

Metamorphism

  • The process of transforming minerals and structures of rocks involving heat and distortion, together or separately

  • Reactions happen mainly in the solid state: new crystals form in fissures or as replacements of other minerals

  • May be closed-system environment (chemical reworking of original constituents) or open-system environment (rock is “cooked” with additions from magmatic source)

  • Two types:

    • Regional Metamorphism - rocks are buried deep (10-20 km typically) and spans thousands of square kilometers

    • Contact Metamorphism - a body of magma in the upper part of crust can be the source of heat and metamorphose rock around it. The zone of contact metamorphism is a few meters up to tens of meter

  • Main factors:

    • Parent rock mineral fabric

    • Temperature

    • Pressure

    • Fluid availability (usually water)

    • Time

  • Parent rock is considered to be the igneous or sedimentary rock origin of the metamorphic rock

    • Ex: mudstone (sedimentary) metamorphoses to slate (metamorphic) which metamorphoses to schist (metamorphic)

      • Mudstone is the parent rock of slate and schist

      • The feature that caused metamorphosis is the minerals’ stability (of parent rock), and at different conditions, they become unstable and recrystallize into different minerals

  • Temperature controls what kind of metamorphism may take place

    • Further, from igneous rocks, minerals crystallize at certain temp, therefore, mineral stability is a function of temperature

    • Minerals have certain temperature ranges where they are stable. If the pressure is changed and/or water is available, then the temperature range where they are stable will also change

  • Pressure affects the texture of the resulting metamorphic rock. It may also cause metamorphic rocks to have foliation

    • Equal confining pressure - same pressure from all directions

    • Directed pressure - pressure from sides is higher than pressure at top and bottom

    • Shear stress - pressure pushes one side in one direction and the other side in the opposite direction

  • Some minerals will crystallize into one of its polymorphs based on the temperature and pressure it is in. Polymorphs have the same composition but different crystal structure.

    Polymorphs of Al2SiO5

  • Water facilitates ion transfer and may make metamorphic process go faster. It may have high ion (dissolved elements) concentration and may transport ions from one place to another

  • Time is needed for metamorphic process to happen — estimates of new mineral growths are around 1mm every million years. However, since tectonic process that cause metamorphic processes are also slow, allowing metamorphic rocks to form over a long period

Metamorphic Rocks

  • formed from metamorphism

  • These rocks have altered fabric form of the parent rock by different mineral sizes, arrangements, and textures

  • Two types:

    • Foliated - directed pressure or shear stress is needed in the environment to form this kind of rock

    • Non-foliated - no directed pressure environment or envi with little pressure (such as near surface)

    • * Some rocks do not show foliation even if they are formed in an environment with directed pressure (such as quartzite and marble)

  • Foliation

    • “layering” or “banding” in metamorphic rocks causing “sheet-like” metamorphic rocks causing “sheet-like” structure

    • not necessarily always seen as physical layers or bands by the naked eye but show up as weakness planes of the rocks

    • If a rock is squeezed (directed pressure), it may deform and texture will change such that the minerals seem elongated along an axis perpendicular to the bigger stress/squeeze

Foliated Metamorphic Rocks

  • can be classified on type of foliated texture it has and grade of metamorphism. These textures are listed below (and in increasing grade of metamorphism)

    • Slaty

    • Phyllitic

    • Schistose

    • Gneissic

  • Slate

    • From low-grade metamorphism of shale

    • Microscopic clay and mica crystals have grown and aligned perpendicular to the largest stress

    • Breaks into very thin flat sheets along cleavage using a chisel. This property is called slaty cleavage.

  • Phyllite

    • Similar to slate but heated to a higher temperature

    • Has larger mica crystals but still barely visible to the eye as sheen on the surface

    • Can form in wave/silky/crenulated layers

  • Schist

    • High enough temperatures forming mica big enough to be seen by the naked eye, possibly with quartz, feldspar, etc. visible

    • Strongly foliated and often bonded with course crystals

  • Gneiss

    • Minerals have been separated into distinct bands

    • Dark bands are usually biotite and hornblende and light bands are quartz and feldspar

    • Foliation is not as intense as schist

Non-Foliated Rocks

  • Form in areas where pressure is just confining pressure (same pressure from all sides) or lower-pressure conditions

  • Mostly, these rocks are not buried deep enough and the heat is coming from a nearby body of magma

  • Quartzite - metamorphosed sandstone

  • Marble - metamorphosed limestone

Engineering Considerations

  • Metamorphic Rocks and Weathering

    • Fresh metamorphic rocks are usually sufficiently sound for almost any engineering purpose

    • Weathering may lower the quality of the rock material. It may expose the foliation which are the weakest parts of a metamorphic rock

  • Foliation

    • Foliated metamorphic rocks usually presents anisotropy

    • Anisotropy is the difference of a material’s property at different directions

    • The planes of foliation provide direction along which the tensile and shear strengths are considerably less than in all other directions

    • The direction of rock foliation should be included in the geologic investigation

    • The anisotropy should be considered in applications

    • Creep and slides may be persistent along the foliation direction for rock slopes and excavations. Interventions perpendicular to the weak planes may be necessary.

  • Construction material

    • Non foliated metamorphic rocks are usually used as building materials

    • Quartzite may often be used a railroad ballast

    • Marbles are used a building stone