Question

Describe how a sedimentary rock can change into a metamorphic rock.

Answer 1

A sedimentary rock becomes a metamorphic rock by undergoing solid-state change in mineralogy, texture, and sometimes chemistry when it is subjected to increased temperature, pressure, and chemically active fluids — without melting. Key steps and processes:

- Burial and lithification: Sediments are first compacted and cemented into sedimentary rock (e.g., shale, sandstone, limestone).
- Increased temperature and pressure: Continued burial, tectonic compression (mountain building), or proximity to an igneous intrusion raises temperature and pressure on the rock. Temperature commonly rises because of depth or nearby magma; pressure increases from the weight of overlying rock or from directed stress during tectonic events.
- Recrystallization: Atoms migrate and new, stable minerals grow under the new conditions. Fine-grained minerals can recrystallize into coarser grains; new metamorphic minerals (e.g., mica, garnet, staurolite) may form.
- Development of texture: Directed pressure can align platy minerals and produce foliation (slaty cleavage, schistosity, gneissic banding). If pressure is equal in all directions, foliation may not develop.
- Role of fluids: Hot fluids can enhance recrystallization and bring or remove chemical components (metasomatism), changing mineral composition.
- No melting: If conditions reach melting, the rock would become igneous instead of metamorphic.

Examples:
- Shale → slate → phyllite → schist → gneiss as metamorphic grade increases.
- Limestone → marble (recrystallized calcite).
- Sandstone → quartzite (interlocking quartz crystals).

In short: burial/tectonics + heat + pressure (and fluids) transform the sedimentary rock’s minerals and texture into metamorphic rock over geological time.