# Basalt vs. gabbro: how to tell them apart

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

## Summary

Basalt and gabbro are mafic igneous rocks with broadly similar compositions, but basalt cools at or near the surface and is fine grained while gabbro cools underground and develops visible interlocking crystals.

## Bottom line

Use grain size first. A dark rock with a nearly uniform microscopic groundmass favors basalt; visible interlocking plagioclase and pyroxene throughout favor gabbro.

## Side-by-side evidence

- Grain size: Basalt — Fine grained; most crystals are too small to identify unaided Gabbro — Coarse grained; principal minerals are visible Interpretation — Cooling rate recorded by grain size is the clearest distinction.
- Cooling setting: Basalt — Lava or shallow volcanic material Gabbro — Slowly cooled intrusive magma Interpretation — Outcrop relationships can confirm what the texture implies.
- Vesicles: Basalt — Common in some flows and cinders Gabbro — Generally absent Interpretation — Gas-bubble cavities support basalt but dense basalt may have none.
- Minerals: Basalt — Microscopic plagioclase and pyroxene, commonly with olivine Gabbro — Visible plagioclase and pyroxene, commonly with olivine Interpretation — The similar mineral system explains why these rocks are compositional counterparts.

## Field sequence

1. Inspect a fresh dry surface with a hand lens.
2. Ask whether crystals are visible throughout or only as sparse phenocrysts.
3. Look for vesicles, glassy margins, or flow structures.
4. Use outcrop contacts or a geologic map to test volcanic versus intrusive context.

## Limits and cautions

- Diabase or dolerite has an intermediate grain size and may bridge the field appearance.
- Altered gabbro can resemble metamorphic greenstone or amphibolite.
- A porphyritic basalt may contain large crystals but still has a fine-grained groundmass.

## Frequently asked questions

### Do basalt and gabbro have the same composition?

They broadly occupy the same mafic compositional family, though individual samples and formal classifications vary.

### Can gabbro have holes like basalt?

Typical gabbro lacks vesicles because it crystallizes under pressure underground. Cavities may instead be weathering pits or later fractures.

### Which rock is darker?

Both are usually dark. Color is less reliable than whether the texture is fine grained or visibly crystalline.

## Related identification guides

- Basalt: https://lithomap.app/resources/rock-identification/basalt
- Gabbro: https://lithomap.app/resources/rock-identification/gabbro

## Authoritative sources

- Igneous Rocks — U.S. National Park Service: https://www.nps.gov/subjects/geology/igneous.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

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