The devastating earthquake that hit Lisbon, Portugal in 1755 fueled de-Christianization in Europe. Satirical French writer Voltaire, writing just after the disaster that killed tens of thousands of people, wondered, “Was there more vice in Lisbon, now destroyed/than in licentious Paris and London?”
It didn’t help that the disaster struck on All Saints’ Day, when the populace was out celebrating in the streets. They had been good Christians, or no worse than any others, and yet God had struck them down. Theologically, the Lisbon Earthquake made no sense.
Geologically, it makes no sense either. Earthquakes cluster around fault lines and subduction zones, where one tectonic plate jostles for space with another. But the sea floor beneath Lisbon is smooth and blameless.
New models suggest there’s something brewing under the surface, driving Lisbon’s illogical earthquakes.
Delamination forms many mountain ranges, such as the Sierra Nevadas in California. Photo: Wikimedia Commons
Fault lines and subduction
Two kinds of geological activity drive earthquakes. The first, which occurs on land, is called delamination. The top layer of the Earth, the crust, is hard and brittle. Beneath it lies the mantle. Continental mantle is squishy and dense, much denser than the magma asthenosphere beneath.
At the gaps between tectonic plates, the squishy mantle beneath one crust bumps into the other. They fight for dominance. When one loses and gets sucked beneath the other, it stretches and weakens. The lighter magma beneath starts pushing up, buoyed by its lower density, like a beach ball held under the waves.
The crust above buckles and splits as the magma lifts it into the air, peeling it from the mantle beneath. Mountains form, volcanoes burst, and earthquakes shudder. This process of delamination is the same mechanism driving the crackling of paint.
Oceanic mantle, which makes up the sea floor, isn’t as permeable as continental crust. It doesn’t allow the magma asthenosphere to push all the way up. Instead, it clings onto the crust above. The losing plate pulls the crust all the way under with it.
So that’s the model: Delamination peels up crust on the continent, and subduction sinks it in the ocean. Seismic surveys haven’t shown any evidence of subduction off the coast of Lisbon, and no delamination is occurring on the ground in Portugal.
Subduction occurs between two oceanic plates, or one oceanic plate and one continental plate. Photo: USGS
A possible solution to a 300-year-old mystery
If you were to show the results of the recent study published in Nature Geoscience to a survivor of the 1755 Lisbon earthquake who had received a 21st-century geological education from a time traveler, they would most certainly be agog.
“Oceanic delamination?” they would exclaim, clutching one hand to their chest. “But surely such a thing cannot happen! The oceanic mantle is too solid to allow for it! There must be another explanation.”
But oceanic delamination is exactly what the new paper proposes. The evidence:
1) There’s a strange blob of magma traveling deep below the ground.
2) A number of earthquakes seem to originate from the same region, with no obvious subduction.
3) All those earthquakes started from deep below the surface.
The blob of magma, dramatically called the “southwest Iberia anomaly,” indicates that the asthenosphere is straining against its boundaries. This, coupled with the earthquakes, suggests violent geological activity, even if no subduction is visible. And the deep origin of most of the earthquakes (at least 20km below the surface) means that the upper crust isn’t directly connected to the mantle beneath. If it were, activity in one region would trigger activity in another.
All this points to delamination, a theory backed up by the team’s simulations. It’s possible that the strong presence of serpentinite off the coast of Portugal may play a role in this geological oddity. Even though delamination isn’t supposed to occur on oceanic mantle, serpentinite is so slippery that it may be easier for the upper crust to peel off it.
So that’s the Lisbon earthquake, 270 years later: still theologically troubling. But geologically, an answer may be near.
A theorized model of the crust and mantle off the coast of Lisbon. Photo: Duarte et al 2025
