Fusion crust: what it is and how to recognise it

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Fusion crust is the thin, glassy rind that forms on a meteoroid’s surface as it heats and partly melts while passing through Earth’s atmosphere. When the outer melt solidifies it creates a distinctive coating usually darker and smoother than the interior rock; on fresh falls it is often shiny and less than 1–2 mm thick.

How fusion crust forms

At hypersonic speeds the air ahead of the meteoroid compresses and heats rapidly, melting the outermost layer. Ablation strips much of that melt away; when the object slows enough the last molten film cools and solidifies as fusion crust. Its composition and thickness vary with meteorite type (stony vs. iron) and flight conditions.

Typical visual features

– Colour: usually black or very dark brown on fresh falls; weathers to rusty brown or tan over time.
– Texture: smooth, glassy or leathery surface; may show small contraction cracks (craquelure).
– Thickness: commonly <1–2 mm on stony meteorites, thinner on iron meteorites. 
– Flow features: fine flow lines or streaks showing melt movement; splash droplets and thin rims on trailing edges.
– Regmaglypts: thumbprint-like depressions produced by uneven ablation on some specimens.

Field tests and observations that support a fusion-crust ID

– Edge check: scratch a tiny edge (only on anonymous, non-valuable finds) — true fusion crust flakes or chips differently from desert varnish and may reveal a lighter interior.
– Fresh vs weathered: a shiny black crust with sharp contrast to interior indicates a relatively recent fall; uniform weathering or thick mineral encrustation suggests long terrestrial residence.
– Look for aerodynamic rounding and trailing-side lips or flow lines consistent with high-speed entry.

Common lookalikes and how to tell them apart

– Volcanic glass (obsidian): can be glassy and dark but often shows conchoidal fractures and lighter interior on broken faces; lacks regmaglypts and flow lines tied to high-speed ablation.
– Desert varnish: thin dark coating that forms chemically over centuries; it generally does not flake like fusion crust and lacks the glassy texture and flow features.
– Industrial slag: may be glassy or vesicular, often contains obvious bubbles (vesicles), bright iridescence, or concentrations of slag shapes; slag commonly shows molded shapes and internal flow structures not seen in meteorites.

When fusion crust is absent or altered

Many older finds have partially or completely lost their fusion crust through weathering, sand abrasion, or chemical alteration. Some fresh meteorites (rare) can have minimal crust. Absence of crust alone does not rule out a meteorite — use magnetism, density, interior metal flecks or chondrules as additional checks.

Practical tips for hobbyists

– Photograph the specimen in sunlight from multiple angles to record texture and flow lines before any handling.
– Avoid aggressive cleaning; fusion crust can be thin and easily damaged.
– Use a small magnet and a loupe: fusion crust plus metal flecks or a magnetic response increases confidence.
– If identification matters, submit the specimen to a university or recognised meteoriticist rather than etching or cutting it yourself.

Fusion crust is one of the strongest visual clues of extraterrestrial origin, but it is best used together with simple tests (magnet, density, interior features) and expert confirmation for a definitive ID.

Sources

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