Short answer
Igneous rock forms when magma or lava cools and solidifies. Metamorphic rock forms when a preexisting rock changes under heat, pressure, or reactive fluids while remaining solid. The formation process, not color or hardness alone, decides the category. 1 2
Both can involve heat, but melting and solidification define igneous formation; solid-state transformation defines metamorphism.
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At a glance
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| Question | Igneous rock | Metamorphic rock |
|---|---|---|
| Formation | Cooling of melt | Change of existing solid rock |
| Complete melting needed? | Yes, before solidification | No |
| Common example | Granite or basalt | Gneiss or marble |
The table summarizes the cited definitions and the article’s stated scope. 1 2
What each thing is
Igneous rock. Rock that forms when molten material cools and solidifies. 1
Metamorphic rock. Rock substantially changed from a preexisting rock by heat, pressure, or fluids without complete melting. 2
Key differences
USGS describes intrusive igneous rocks crystallizing underground and extrusive rocks forming near the surface. Metamorphism can recrystallize minerals and alter texture without turning the entire rock into liquid. 1 2
How to tell them apart
Use mineral composition, texture, field context and surrounding rock. Interlocking crystals can occur in both groups, so one visual feature is not conclusive. A geologic map or thin-section analysis may be needed. 1 2
Where they overlap
Both can be crystalline and both may form in environments with high temperature. An igneous rock can later become metamorphic, so the rock cycle links the categories. 1 2
Edge cases
Partial melting blurs a simple classroom boundary; rocks with mixed igneous and metamorphic histories exist. Metamorphic rocks can also be transformed again. This article covers dominant formation process. 1 2
Why the distinction exists
Rock names preserve how material formed, which helps geologists reconstruct the environment and sequence of Earth processes. 1 2
Common misconceptions
Heat alone does not make rock igneous. Marble is metamorphic even though it formed at elevated temperature. 1 2
Examples
Basalt cooled from lava and is igneous. Limestone altered by heat and pressure into marble is metamorphic. 1 2
Sources
Sources checked October 3, 2026.
- USGS — What Are Igneous Rocks?. Intrusive and extrusive.
- USGS — What Are Metamorphic Rocks?. Process of metamorphism.