Correct option is B
The correct answer is (b).
Metamorphic rocks are uniquely formed through profound alterations of pre-existing rocks (protoliths) under extreme heat and pressure without entering a molten state.
This environment facilitates solid-state diffusion, intricate mineral phase transitions, and chemical recombination, completely recrystallizing the rock fabric while remaining entirely in the solid phase.
Information Booster
- The process of metamorphism is strictly a solid-state phenomenon; if the rock partially or fully melts, it transitions into the igneous realm.
- Solid-solid diffusion involves the extremely slow migration of atoms through crystal lattices, a primary mechanism for driving metamorphic chemical reactions over geologic time.
- Phase transitions occur when minerals become unstable at new temperature-pressure regimes, rearranging their atomic structures to form stable polymorphs (e.g., sillimanite to kyanite).
- Regional metamorphism is heavily associated with tectonic plate convergence, resulting in deeply buried rocks subjected to intense directed pressure and geothermal heat.
- Foliation, the distinct banding or layering observed in rocks like gneiss and schist, is a direct physical result of these directed pressures during solid-state alteration.
- Contact metamorphism occurs primarily due to the intense thermal gradients near intruding magma chambers, heavily relying on heat rather than pressure to drive reactions.
- Index minerals, such as chlorite, garnet, and staurolite, accurately indicate the specific grade and conditions of metamorphism the rock has endured.
- Option A: Igneous rocks are formed from the direct cooling and complete solidification of molten magma or lava, distinct from the solid-state changes defining metamorphism.
- Option C: Metasomatic rocks undergo metasomatism, a sub-process where the rock's bulk chemical composition is significantly altered by external hydrothermal fluids, rather than just internal solid-state diffusion.
- Option D: Sedimentary rocks are generated through the surface processes of weathering, erosion, deposition, and subsequent lithification of loose sediments at relatively low temperatures.
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