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Metamorphic Rocks

Definition - Metamorphic rocks form from extreme heat and or pressure applied to any other rock. This can include igneous, sedimentary and even metamorphic rocks. 

Metamorphic Rocks Page Layout

• Websites

• Identifying Metamorphic Rocks

• Types of Metamorphic Rocks

• Petrogenesis (Creating Rocks)

• Metamorphic Grades

Websites

Kentucky Geological Survey

Identifying Metamorphic Rocks

The basic types of Metamorphic Rocks are listed below

Types of Metamorphic Rocks

There are 2 main types of sedimentary rocks

• Foliated - These are rocks that form mostly from rocks with many minerals in them. During the process of metamorphism the minerals are aligned parallel to one another (aka foliation)

  • Slate - Looks like shale but sheets break of smoother and easier. Usually black or dark gray in color.

   

  

   

  • Phyllite - Overall crystals not visible, but there might be a few large crystals interspersed. Color is usually shiny silvery color.

   

  

  Picture from csmres.jmu.edu

   

  • Schist - Usually composed with an abundance of micas. Sometimes contains other minerals like amphibole. Minerals grains are visible and form sheets in the rock.

   

  

  Picture from csmres.jmu.edu

   

  • Gneiss - The rock is banded. This is where similar colored (light or dark) minerals align in parallel bands across the rock giving it a striped appearance.

  

   

• Non-foliated - These are rocks that formed from other rocks with usually just one mineral in them. They lack the alignment of the minerals and are usually just considered massive.

  • Quartzite - Rock scratches glass. Also has no visible alignment of minerals.

   

  

  Picture from seis.natsci.csulb.edu

   

  • Marble - Rock reacts with acid. May have some lines of different colored minerals, usually not in parallel lines though.

  

Petrogenesis (Creating Rocks)

• There are 3 types of metamorphism and different types of rocks form from :

  • Contact - This occurs over a very small area when magma comes in contact with the surrounding country rock.

    • Metamorphism type - High Temp 

   

  • Regional - This occurs over a very large area when entire regions of rock are buried and subject to higher temperatures and pressure.

    • Metamorphism type - High Temp and High Pressure

   

  • Hydrothermal - This occurs when rock is subjected to heat (thermal) water (hydro) that continually flows through it. Most common in limestones.

    • Metamorphism type - High Temp

Metamorphic Grades

• Definition - Metamorphic Grade - This is the degree of metamorphism the original rock went through. Very Low grade metamorphism still resembles the original rock while High grade metamorphism results in a rock where the original rock is impossible to determine.

• Since 55% of all sedimentary rocks are shales and they are usually formed over large areas (think ocean basin or similar quiet water circumstances) that explains why most foliated rocks start from shales.

• Since the non-foliated rocks do not have any texture to define their degree of metamorphism their metamorphic grade is usually impossible to tell unless it is extremely low.

• Most hydrothermal metamorphic rocks do not have a high degree of metamorphism due to the temperature limits of the water.

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