The Melted Glass That Records a 2-Billion-Year-Old Quake

Fossil earthquakes are real: pseudotachylyte veins are frictional melt produced during seismic slip, quenched in the rock record and preserved for billions of years. In the Cora Lake shear zone of the Canadian Shield, those veins are tied to fault movement roughly 1.9 billion years ago — and they formed around 30 kilometres down, in lower crust that should have flowed rather than snapped. This video covers how earthquakes generate frictional melt, how Richard Sibson established pseudotachylyte as a record of seismic slip on the Outer Hebrides Thrust in 1975, why vein thickness only reveals an earthquake's magnitude below about M3, and how melt lubrication destroys that relationship for anything larger. It also covers why these veins are so rare: hydrothermal alteration wipes out the diagnostic microstructure within days to months, which is why ancient, cold, dry cratons preserve earthquakes that active fault zones cannot. The San Andreas Fault has a century of instrument data behind it. The Canadian Shield has a receipt written in melted stone. 1. Sibson, R.H. (1975) — Generation of Pseudotachylyte by Ancient Seismic Faulting, Geophysical Journal International 43(3) https://academic.oup.com/gji/article/... 2. Orlandini & Mahan (2020) — Rheological evolution of a pseudotachylyte-bearing deep crustal shear zone in the western Canadian shield https://www.sciencedirect.com/science... 3. Regan et al. (2014) — The Cora Lake shear zone, Athabasca granulite terrane, Canadian Journal of Earth Sciences 51 https://cdnsciencepub.com/doi/10.1139... 4. Fondriest et al. (2020) — Pseudotachylyte Alteration and the Rapid Fade of Earthquake Scars From the Geological Record, GRL https://agupubs.onlinelibrary.wiley.c... 5. Scaling seismic fault thickness from the laboratory to the field (Durham repository) https://durham-repository.worktribe.c... 6. Dating fossil lower-crustal earthquakes by in-situ apatite U-Pb geochronology (Lofoten), EPSL 2024 https://www.sciencedirect.com/science... 7. Earthquake dynamics from pseudotachylyte microstructure (Gole Larghe Fault Zone), EPSL 2025 https://www.sciencedirect.com/science... 8. Pseudotachylyte — mineral data, friction melt temperatures and melting duration (Mindat) https://www.mindat.org/min-48674.html