Metamorphic rocks are formed when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are transformed by intense heat and pressure. Here are their key characteristics:
1. Foliation:
* Definition: A layered or banded appearance due to the alignment of mineral grains under pressure. This creates a distinct "sheet-like" structure.
* Types:
* Schistosity: Shiny, platy minerals like mica align in parallel layers.
* Gneissic banding: Light and dark bands of minerals alternate.
* Slatey cleavage: Fine-grained, breaks into flat, thin sheets.
* Absence: Some metamorphic rocks, like marble and quartzite, lack foliation due to the absence of platy minerals or pressure being applied equally in all directions.
2. Recrystallization:
* Definition: Existing minerals in the original rock are transformed into new, more stable minerals due to heat and pressure.
* Example: Limestone (sedimentary) turns into marble (metamorphic).
3. Texture:
* Definition: Describes the size, shape, and arrangement of mineral grains.
* Examples:
* Foliated textures: Schistose, gneissose, slaty.
* Non-foliated textures: Granular, massive, and others depending on the mineral composition.
4. Hardness:
* Variable: Can be soft or hard depending on the mineral composition.
* Example: Slate is relatively soft, while quartzite is extremely hard.
5. Color:
* Variable: Highly dependent on the mineral composition.
* Examples: Marble can be white, gray, or even colored by impurities.
6. Chemical Composition:
* Variable: Changes in the chemical composition can occur during metamorphism.
* Example: The addition of fluids during metamorphism can introduce new elements.
7. Origin:
* Derived from other rocks: Metamorphic rocks are always formed from preexisting rocks.
* Types of parent rocks: Igneous, sedimentary, or even other metamorphic rocks.
8. Formation Process:
* Heat: Increased temperature provides the energy for recrystallization.
* Pressure: Forces mineral grains to align and change shape.
* Fluids: Can introduce new elements and enhance chemical reactions.
9. Types of Metamorphism:
* Regional metamorphism: Large-scale transformation due to tectonic plate collisions.
* Contact metamorphism: Localized change near igneous intrusions.
* Dynamic metamorphism: Occurs along fault zones due to intense pressure.
10. Examples of Metamorphic Rocks:
* Foliated: Slate, phyllite, schist, gneiss
* Non-foliated: Marble, quartzite, hornfels
Understanding these characteristics helps us identify and classify metamorphic rocks and learn about the geological processes that shaped them.
