# Amphibolite vs. gabbro: how to tell them apart

Canonical page: https://lithomap.app/resources/rock-identification/compare/amphibolite-vs-gabbro
Last reviewed: September 17, 2026
Page purpose: Original LithoMap side-by-side field identification reference for search systems and language models.

## Summary

Amphibolite and gabbro are dark crystalline rocks that can share plagioclase and mafic minerals. Amphibolite is metamorphic and commonly contains aligned elongate amphibole; gabbro is igneous with a generally random interlocking texture dominated by pyroxene and plagioclase.

## Bottom line

Look for fabric and mineral shape. Persistent alignment of shiny elongate amphibole favors amphibolite; randomly oriented blocky pyroxene and plagioclase favor gabbro.

## Side-by-side evidence

- Origin: Amphibolite — Metamorphic recrystallization, commonly of mafic rock Gabbro — Slow crystallization of mafic magma underground Interpretation — Texture records the contrasting process.
- Fabric: Amphibolite — May be foliated or lineated, though some is massive Gabbro — Usually massive with random crystal orientation; igneous layering can occur Interpretation — A pervasive metamorphic alignment is strong evidence.
- Dominant dark mineral: Amphibolite — Amphibole, commonly hornblende Gabbro — Pyroxene, commonly with olivine or amphibole alteration Interpretation — Crystal habit helps where grains are large enough.
- Pale mineral: Amphibolite — Plagioclase grains or patches Gabbro — Interlocking plagioclase Interpretation — Plagioclase occurs in both and cannot distinguish them alone.

## Field sequence

1. Examine a fresh surface with a hand lens.
2. Look for a repeated direction in elongate dark grains.
3. Compare prismatic amphibole with blockier pyroxene cautiously.
4. Use contacts and regional mapping to test metamorphic versus intrusive context.

## Limits and cautions

- Massive amphibolite and altered gabbro may require thin-section identification.
- Igneous layering is not automatically metamorphic foliation.
- Dark color and density alone cannot separate the rocks.

## Frequently asked questions

### Can gabbro become amphibolite?

Yes. Metamorphism can recrystallize gabbro into amphibolite, although basalt and other mafic rocks are also common precursors.

### Is all amphibolite foliated?

No. Amphibolite ranges from strongly foliated to nearly massive.

### Can gabbro contain hornblende?

Yes, as a primary or alteration mineral. Mineral abundance, fabric, and field context must be evaluated together.

## Related identification guides

- Amphibolite: https://lithomap.app/resources/rock-identification/amphibolite
- Gabbro: https://lithomap.app/resources/rock-identification/gabbro

## Authoritative sources

- Metamorphic Rocks — U.S. National Park Service: https://www.nps.gov/subjects/geology/metamorphic.htm
- How Rocks Are Classified — U.S. National Park Service: https://www.nps.gov/subjects/geology/how-rocks-are-classified.htm
- Rocks and Minerals — U.S. National Park Service: https://www.nps.gov/subjects/rocks/index.htm
- What is the difference between a rock and a mineral? — U.S. Geological Survey: https://www.usgs.gov/faqs/what-difference-between-a-rock-and-a-mineral
- Igneous Rocks — U.S. National Park Service: https://www.nps.gov/subjects/geology/igneous.htm

This reference supports preliminary field identification. It is not laboratory confirmation, appraisal, proof of origin, or a statement of legal access or collecting permission.