Metamorphic Rock
Metamorphic rock is formed when existing igneous, sedimentary, or other metamorphic rocks are transformed by intense heat, pressure, or chemically active fluids deep within Earth's crust — a process called metamorphism — without the rock melting completely. The transformation alters the mineral composition, texture, and crystal structure of the original rock, often producing distinctive features such as foliation (banding or layering) in rocks like schist and gneiss. Metamorphic rocks are indicators of past geological events such as mountain building, and include economically valuable materials like marble, slate, and quartzite.
Common Metamorphic Rocks, Their Precursors, and Conditions
| Metamorphic Rock | Parent Rock (Protolith) | Temperature (°C) | Pressure (kbar) | Foliation |
|---|---|---|---|---|
| Slate | Shale / Mudstone | 200–400 | 2–5 | Yes (slaty cleavage) |
| Phyllite | Slate / Shale | 300–450 | 3–7 | Yes (silky sheen) |
| Schist | Phyllite / Mudstone | 450–600 | 5–12 | Yes (schistosity) |
| Gneiss | Granite / Schist | 600–800 | 8–15 | Yes (gneissic banding) |
| Marble | Limestone / Dolostone | 400–700 | 3–10 | No (non-foliated) |
| Quartzite | Sandstone | 300–700 | 2–10 | No (non-foliated) |
Interactive Tools
Khan Academy — Metamorphic Rocks
Video lessons and articles on metamorphic rock formation, types, and properties
Open ToolUSGS Metamorphic Rocks
USGS educational resource on metamorphic processes and rock classification
Open ToolMindat.org Metamorphic Rocks
Database of metamorphic rock types with mineralogy and photographic reference
Open Tool
Wikimedia Commons, CC BY-SA
Related Terms
Rock Cycle
The rock cycle is a continuous geological process by which rocks are transformed from one type to another through processes such as melting, cooling, erosion, deposition, compaction, cementation, heat, and pressure over geological timescales. The three main rock types — igneous, sedimentary, and metamorphic — are interconnected in this cycle, each capable of being converted into another type under the right conditions. Understanding the rock cycle helps geologists interpret Earth's history, locate mineral resources, and predict geological hazards.
Igneous Rock
Igneous rock is formed by the cooling and solidification of magma (underground molten rock) or lava (magma that reaches the surface), making it one of the three fundamental rock types in Earth's crust. Intrusive igneous rocks such as granite solidify slowly deep underground, forming coarse-grained crystals, while extrusive igneous rocks such as basalt cool rapidly at the surface, forming fine-grained or glassy textures. Igneous rocks make up about 90% of Earth's crust by volume and are essential raw materials for construction, ceramics, and as indicators of volcanic history.
Sedimentary Rock
Sedimentary rock is formed from the accumulation and compaction of sediments — particles of minerals, organic matter, or chemical precipitates — deposited by water, wind, ice, or gravity over time. This process, called lithification, involves compaction (pressure from overlying sediment) and cementation (mineral-rich groundwater binding particles together). Sedimentary rocks cover about 75% of Earth's land surface, host nearly all fossil records, and contain major reserves of coal, oil, natural gas, and economically important minerals.
From Greek "metamorphoun" (to transform, to change shape) — "meta" (change) and "morphe" (form). The term was introduced into geology by Scottish geologist Charles Lyell in his "Principles of Geology" (1833) to describe rocks that had been transformed from their original state by heat and pressure deep in the crust.