Boninite Identification Guide
Identify boninite, a rare high-magnesium volcanic glass-rich lava from subduction zones, and distinguish it from basalt and andesite.
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What Boninite Looks Like
Boninite is a rare, high-magnesium andesitic volcanic rock typically dark gray to greenish-black or brown, often glassy to fine-grained with a pitch-like to dull luster. A defining visual feature is abundant small phenocrysts of clinopyroxene (commonly bronze-colored) and orthopyroxene set in a glassy or microcrystalline groundmass, usually with little or no plagioclase feldspar. Fresh surfaces can look almost like dark glass; altered surfaces are dull and greenish. It may show spherulitic or quench textures from rapid cooling.
Step-by-Step Field ID Checklist
- Confirm a dark, fine-grained to glassy volcanic rock.
- Look with a hand lens for small pyroxene phenocrysts but a near-absence of feldspar laths.
- Note any glassy, pitch-like groundmass or quench/spherulitic texture.
- Consider the setting: boninite is tied to young or fossil subduction zones and forearcs.
- Test hardness and check the lack of cleavage and absence of acid reaction.
Key Diagnostic Tests
- Mohs hardness: the rock is hard overall (pyroxene ~5.5–6.5; glass ~5.5); it resists a knife.
- Streak: gray to greenish-gray.
- Fracture: conchoidal to splintery in glassy varieties.
- Density: moderate to fairly high (SG ~2.7–2.9) owing to magnesium-rich pyroxenes.
- Acid: no reaction (distinguishes from carbonate-cemented rocks).
- Magnetism: weak; not strongly magnetic.
- Geochemistry (high MgO with high SiO2 and low Ti) is the formal definition but requires lab work.
Common Look-Alikes and How to Tell Them Apart
- Basalt: contains abundant plagioclase feldspar and olivine; boninite is feldspar-poor and pyroxene-rich with more silica.
- Andesite: typically has visible plagioclase phenocrysts and is less magnesian; boninite lacks the feldspar and is glassier.
- Obsidian: a felsic glass that is usually black and very high-silica with conchoidal fracture but no pyroxene phenocryst population.
- Komatiite: an ultramafic lava with spinifex olivine blades, not pyroxene-in-glass.
Where Boninite Is Found
Boninite is named for the Bonin (Ogasawara) Islands of Japan. It occurs in intra-oceanic forearcs and ophiolites, including the Izu-Bonin-Mariana arc, the Troodos ophiolite (Cyprus), New Caledonia, and parts of the western Pacific. It is genuinely uncommon, so most dark glassy lavas you find are basalt or andesite, not boninite.
Frequently asked questions
How can you tell if a rock is boninite?
Look for a dark, glassy to fine-grained volcanic rock with small pyroxene phenocrysts but almost no plagioclase feldspar, formed in a subduction-zone or ophiolite setting; confirmation needs geochemistry showing high magnesium and silica.
What does boninite look like?
It looks like a dark gray to greenish-black, pitch-like or glassy lava speckled with tiny bronze pyroxene crystals and lacking obvious feldspar.
Boninite vs basalt: what's the difference?
Basalt is rich in plagioclase feldspar and olivine, while boninite is feldspar-poor, pyroxene-rich, more silica-rich, and unusually high in magnesium.
Where does the name boninite come from?
It is named after the Bonin (Ogasawara) Islands of Japan, where the rock was first described.