Pseudotachylite Identification Guide
Recognize pseudotachylite, a dark glassy fault-melt rock, by its injection veins, survivor clasts, and brittle host setting.
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What Pseudotachylite Looks Like
Pseudotachylite is a dark, glassy to flinty rock formed by frictional melting along faults or during meteorite impacts. It is typically black, dark brown, or gray, very fine-grained (aphanitic) to glassy, and occurs as thin veins, networks, and injection seams cutting through a brecciated or crushed host rock. Embedded within it are often rounded "survivor" clasts of the host that did not melt.
It does not form large uniform bodies; instead it appears as dark cross-cutting veins that may branch and inject into fractures.
Step-by-Step Field ID Checklist
- Setting first: Identify a fault zone, shear zone, or impact structure with crushed/brecciated rock.
- Vein form: Look for dark, fine, glassy veins cross-cutting the host along fractures.
- Clasts: Check for rounded, embayed fragments of host rock floating in the dark matrix.
- Margins: Note sharp, sometimes chilled or flow-banded contacts and branching injection veins.
- Texture: The matrix is aphanitic to glassy, sometimes with tiny spherulites or microlites.
Key Diagnostic Tests
- Hardness: variable (~5–6) depending on matrix composition and devitrification.
- Luster: glassy to dull/flinty.
- Fracture: conchoidal in glassy varieties.
- Field relationship is the key "test": it must be a melt-derived vein hosted within a fault or impact breccia, not a sedimentary or volcanic layer.
Common Look-Alikes and How to Tell Them Apart
- Basalt dike: a dike is a continuous igneous intrusion with consistent thickness; pseudotachylite forms irregular, branching, fault-hosted veins containing host-rock clasts.
- Mylonite/ultramylonite: these are foliated, ductile fault rocks with stretched grains; pseudotachylite is glassy and brittle-melt, lacking penetrative foliation (though the two can coexist).
- Tachylite/obsidian: these are volcanic glasses; pseudotachylite is generated in situ within a host rock and carries survivor clasts.
- Cataclasite: crushed but not melted — lacks the glassy/flow-banded melt matrix.
Diagnosis rests on the combination of a dark glassy matrix, injection veins, survivor clasts, and a fault or impact host.
Where It Is Found
Pseudotachylite occurs in major fault and shear zones and in impact structures. Famous occurrences include the Vredefort impact structure (South Africa), the Sudbury impact structure (Canada), the Outer Hebrides of Scotland (the classic locality), and numerous Alpine and crustal fault zones worldwide.
Frequently asked questions
What is pseudotachylite and how do you identify it?
It is a dark, glassy rock formed by frictional melting along faults or impacts. Identify it by dark glassy injection veins cutting a brecciated host, containing rounded survivor clasts.
How is pseudotachylite different from a basalt dike?
A basalt dike is a continuous igneous intrusion of uniform width, while pseudotachylite forms irregular, branching, fault-hosted melt veins that carry fragments of the surrounding host rock.
Pseudotachylite vs mylonite — what's the difference?
Mylonite is a foliated rock formed by ductile shearing with stretched grains, whereas pseudotachylite is a glassy, brittle frictional melt without penetrative foliation, though both can occur together.
Where is pseudotachylite found?
It occurs in major fault and shear zones and in impact craters, with famous examples at the Vredefort and Sudbury impact structures and the Outer Hebrides of Scotland.