1. Starting Material: Metamorphic rocks begin as pre-existing rocks, called protoliths. These protoliths can be:
* Igneous rocks: Formed from cooled magma or lava.
* Sedimentary rocks: Formed from cemented sediments.
* Other metamorphic rocks: Rocks that have already undergone metamorphism.
2. Heat and Pressure: The key ingredients for metamorphism are intense heat and pressure. These forces cause significant changes to the rock's mineral composition, texture, and structure.
* Heat: Can come from various sources like:
* Magma: When molten rock is close to existing rocks.
* Buried rocks: Deep burial under layers of sediment or rock creates immense pressure, which generates heat.
* Tectonic activity: The movement of tectonic plates can cause friction and generate heat.
* Pressure: Can also be caused by:
* Buried rocks: The weight of overlying rock layers creates enormous pressure.
* Tectonic activity: The collision of tectonic plates squeezes rocks together, generating pressure.
3. Changes: The heat and pressure cause several changes within the rock, leading to the formation of a metamorphic rock:
* Recrystallization: Minerals in the protolith change their size and arrangement due to the heat and pressure.
* Neocrystallization: New minerals are formed due to the changing chemical environment.
* Foliation: The alignment of platy minerals under pressure can create a layered or banded appearance (like in slate, schist, and gneiss).
* Texture changes: The rock's original texture can be altered, becoming finer-grained, more compact, or displaying other unique features.
4. Types of Metamorphism: The type of metamorphic rock formed depends on the intensity of heat and pressure and the composition of the protolith:
* Contact metamorphism: Occurs when heat from magma alters the surrounding rock.
* Regional metamorphism: Happens over vast areas due to tectonic plate collisions and the pressure of buried rocks.
In summary: Metamorphic rocks are created when existing rocks undergo transformation due to intense heat and pressure. These forces cause changes in mineral composition, texture, and structure, resulting in a rock with unique characteristics that distinguish it from its protolith.
