Pseudotachylyte
Rock formed by frictional melting during earthquakes, landslides, and impacts.
Wikipedia / Wikimedia Commons
Pseudotachylyte is a dark, glassy-looking rock that forms in thin veins when friction melts rock during earthquakes, huge landslides, or asteroid impacts, and the melt then cools almost instantly. Its makeup usually matches the surrounding rock, but it can be slightly richer in iron and magnesium because hydrous minerals like mica and amphibole tend to melt first. The rock was first described by Shand at the Vredefort impact structure, and he named it for its similarity to tachylyte, a natural basaltic glass.
Though pseudotachylyte often appears glassy, it alters easily, so pure glass is rarely preserved. Instead, it is usually devitrified into a very fine-grained material showing quench textures—chilled margins, spherulites (radial or concentric clusters of tiny crystals), radial crystal overgrowths on rock fragments, and skeletal or spinifex crystal forms.
**Formation**
**Seismic faulting** Pseudotachylytes are sometimes called "fossil earthquakes" because they provide clear evidence of seismic slip. During an earthquake, friction concentrates on a thin fault surface. Because rock is a good insulator, the heat builds up and melts the rock, creating a "fault vein." This vein often has "injection veins" branching off into cracks. For a while, some scientists argued that pseudotachylyte formed by extreme crushing, not melting, but evidence of direct crystallization from a melt has largely settled the debate—most researchers now agree it has a melt origin.
Lab experiments show that pseudotachylyte starts with flash melting of tiny contact points (asperities) on the fault. These melt patches grow and merge, forming a high-viscosity melt that initially raises friction and hinders sliding. As the patches continue to coalesce into a continuous, lower-viscosity melt layer, friction drops and the fault becomes lubricated, allowing easier sliding. Once the melt layer reaches a critical thickness, frictional heat can no longer be generated; the melt then quenches and crystallizes, increasing viscosity again and acting as a brake. When sliding stops, the quenched melt welds the fault shut, restoring its strength to that of the surrounding unfaulted rock.
**Abundance of seismic pseudotachylyte in nature** Pseudotachylyte seems rare in the geologic record compared to modern earthquake activity, raising questions about whether it is truly rare, hard to recognize in t
- type
- Rock
- first_documented_by
- Shand
- first_documented_location
- Vredefort impact structure
- formation_mechanisms
- Seismic faulting, landslides, impact events
- key_characteristic
- Extremely fine-grained to glassy, dark, cohesive rock
- also_known_as
- Fossil earthquakes
Lore & Background
Pseudotachylyte has been referred to as 'fossil earthquakes' as it represents definitive evidence of seismic slip. During seismic faulting, it forms through an extreme concentration of frictional sliding onto a thin surface of a fault, generating heat that melts the rock. This creates a 'fault vein' often accompanied by 'injection veins.' Laboratory experiments show that formation begins with flash melting of asperities that grow and join into high-viscosity melt patches, which then form a continuous melt layer that lubricates the fault. Once sliding stops, quenching welds the fault shut, restoring its strength.
Pseudotachylyte also occurs at the base of large-scale landslides, formed by the same frictional heating process. Notable examples include the Arequipa volcanic landslide deposit in Peru and the Langtang landslide deposit in Nepal. In impact structures, pseudotachylyte appears as irregular, anastomosing dike-like bodies containing rounded inclusions of target rock. Two types exist: S-Type (shock veins) formed by frictional and shock melting during shockwave expansion, and E-Type (endogenic) formed by frictional melting due to crater margin collapse.
There is an apparent lack of pseudotachylyte in the geologic record relative to observed seismicity, possibly due to rarity of production, lack of recognition, or poor preservation. It is often associated with other fine-grained rocks like mylonite and cataclasite, and is extremely prone to alteration, which may render it unrecognizable.
Reader's Guide
Pseudotachylyte holds significant importance as a direct record of ancient seismic events, landslides, and impacts, providing insights into the mechanics of faulting and melting processes. Its study has resolved debates about melt versus crush origin, with ample evidence supporting direct crystallization from a melt. The rock's susceptibility to alteration and its association with other fine-grained rocks highlight challenges in recognizing it in the field, suggesting that its apparent rarity may be due to underreporting rather than actual scarcity. In impact structures, distinguishing pseudotachylyte from impact melt is crucial, as pseudotachylyte reflects local wall-rock composition while impact melts show regional-scale homogenization. This distinction aids in understanding impact dynamics and crater evolution. Pseudotachylyte's role as a 'fossil earthquake' makes it a key tool for studying fault behavior and seismic cycles, while its presence in landslides and impact craters broadens its relevance to planetary geology and hazard assessment.
Did You Know?
- Pseudotachylyte was first documented by Shand in the Vredefort impact structure and named for its resemblance to tachylyte, a basaltic glass.
- During seismic faulting, pseudotachylyte forms through frictional melting that creates a melt layer, which initially raises friction but later lubricates the fault before quenching welds it shut.
- Pseudotachylyte has been found at the base of modern landslides, such as the one generated by the 1999 Taiwan earthquake.
- In impact structures, S-Type pseudotachylytes (shock veins) contain high-pressure mineral polymorphs like coesite and stishovite.
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