Metamorphic Rocks
Rock Cycle: Metamorphic
Metamorphic rocks are formed when another type of rock undergoes profound physical or chemical changes. Metamorphic rocks can be formed from any type of igneous or sedimentary rock.
(In Greek, metamorphic means ‘to change form.’ Consider when we say something ‘morphs’ into something else… if you were a ‘90s kid like me, you would have watched the Mighty Morphin’ Power Rangers, or read the Animorphs book series about kids who turn into animals!)
These changes are caused through four main processes:
- Increased heat: Applying intense heat to rock can force atoms within the minerals to break their chemical bonds, move, and form new bonds. This creates new mineral formations and assemblages within the rock.
- Increased pressure: Increased pressure can change mineral structure due to atoms becoming packed more closely and densely.
- Increased heat and pressure together: The combination of heat and pressure makes the original mineral assemblage unstable, causing changes as outlined above.
- Compression and shear: Rocks can be compressed by overlying weight, then subject to shear when two parts of the mass move in different directions. This changes the shape of the rock and changes how its minerals are aligned.
These four processes can result from a few causes:
- Shifting of lithospheric plates, particularly at the subduction zone between convergent plates
- Orogenesis, i.e. mountain formation
- Magma rising within the crust, ‘cooking’ adjacent rocks – this is known as contact metamorphism, and generally happens near igneous intrusions
- Sediments collect in a large depression in Earth’s crust, creating enough pressure through its own weight to transform the bottom layers of sedimentary rock into metamorphic rock.
Metamorphic rocks are generally denser and more compact than their original source material. As a result, they are harder and more resistant to weathering and erosion. They can also be foliated, meaning banded or striped. Foliation happens due to the re-alignment of minerals that were in the original rock. While not all metamorphic rocks are foliated, a rock that is foliated is almost certainly a metamorphic rock!