1. Preexisting Rock: Metamorphic rocks originate from pre-existing rocks, which can be either igneous, sedimentary, or even other metamorphic rocks.
2. Solid State Transformation: Metamorphic rocks are formed when pre-existing rocks experience changes without completely melting. This transformation occurs in the solid state, driven by heat, pressure, and/or chemical reactions.
3. Heat and/or Pressure: The primary processes involved in metamorphism are intense heat and/or high pressure. These factors cause changes in the mineral composition, texture, and overall structure of the original rock.
4. Recrystallization: Minerals within the pre-existing rock undergo recrystallization during metamorphism. This means they dissolve and re-crystallize to form new mineral grains with different textures and structures.
5. Foliated or Non-Foliated: Metamorphic rocks can be foliated or non-foliated, depending on the type and intensity of metamorphism. Foliation refers to the alignment of mineral grains, such as mica, in parallel bands, creating a layered appearance. Non-foliated metamorphic rocks, on the other hand, lack this preferred mineral alignment.
6. Preservation of Original Features: While undergoing metamorphism, rocks may retain certain features from their original form, such as fossils or other remnants of their sedimentary or igneous heritage.
7. Classification by Composition and Texture: Metamorphic rocks are classified based on their mineral composition, texture, and foliation. Some common types include slate, schist, gneiss, marble, quartzite, and amphibolite.
In summary, a rock is considered metamorphic when it has experienced solid-state transformations due to intense heat and/or pressure, resulting in changes in its mineral composition, texture, and overall structure.