Metabasalt Identification Guide

What Metabasalt Looks Like

Metabasalt is basalt that has been metamorphosed, typically at low to medium grade (greenschist to lower amphibolite facies). The original dark volcanic rock is partly recrystallized into green metamorphic minerals — chlorite, actinolite, epidote, and albite — giving the rock its common nickname greenstone. It is usually dark green to greenish-grey, fine- to medium-grained, and may retain ghostly relict volcanic textures like amygdules (filled gas bubbles) or pillow forms.

• Color: dark green, greenish-grey to grey-black
• Luster: dull to slightly satiny where chlorite/actinolite are present
• Transparency: opaque
• Habit/texture: massive to weakly foliated; may show relict pillows, amygdules now filled with epidote/quartz/calcite, or a faint schistosity

Step-by-Step Field-ID Checklist

1. Note the green color from chlorite, actinolite, and epidote — the giveaway of a metamorphosed mafic rock.
2. Look for relict volcanic features: pillow shapes, amygdules, or chilled margins indicate a basalt protolith.
3. Assess fabric: metabasalt is massive to weakly foliated; strongly foliated, banded equivalents grade into greenschist or amphibolite.
4. Spot epidote: yellow-green epidote in veins, patches, or amygdules is very common.
5. Check grain size: fine-grained overall, with new metamorphic minerals visible mainly with a hand lens.

Key Diagnostic Tests

• Hardness: mixed; actinolite/epidote ~6, chlorite soft (~2-2.5, flakes easily). Soft green flakes that smear indicate chlorite.
• Streak: pale green to grey.
• Density: moderately high (~2.9-3.1), typical of mafic rocks.
• Magnetism: weak to moderate where magnetite survives.
• Acid: generally inert, but calcite-filled amygdules or veins will fizz in dilute HCl.

Common Look-Alikes and How to Tell Them Apart

• Fresh basalt: dark grey-black and unaltered, lacking the green metamorphic minerals and any foliation. Greenness and relict-plus-metamorphic minerals signal metabasalt.
• Amphibolite: higher-grade equivalent dominated by black hornblende and plagioclase, coarser and more clearly foliated/lineated. Metabasalt is finer and greener (chlorite/actinolite rather than coarse hornblende).
• Serpentinite: also green but waxy/greasy, much softer (~3-4), and derived from ultramafic rock (peridotite), often with slickensided surfaces. Metabasalt is harder and less greasy.
• Greenschist: essentially strongly foliated metabasalt; the distinction is fabric — schistose vs. massive.

Where It Is Typically Found

Metabasalt is abundant in ancient greenstone belts (Archean cratons of Canada, Australia, South Africa), in ophiolite sequences (metamorphosed oceanic crust), and along convergent margins and accretionary terranes. Look for it in old shield areas and in fault-bounded slices of former seafloor.