On the morning of November 1, 1755, Lisbon was alive with celebration. It was All Saints’ Day, and the city’s churches were filled with worshippers. Then, without warning, the ground began to shake. Walls crumbled, chandeliers crashed, and in minutes one of Europe’s grandest capitals became a ruin.
The earthquake that followed was one of the most catastrophic in European history. Its tremors tore through Portugal and were felt as far away as North Africa and Northern Europe. Fires swept through Lisbon, devouring what the quake had spared. Then, as if nature had not unleashed enough fury, a massive tsunami surged inland, drowning thousands more. By the time the chaos subsided, tens of thousands of people were dead, and Lisbon lay in ruins.
For centuries, scientists puzzled over the source of such devastation. Portugal sits far from the dramatic fault lines that slice through other regions of the world, such as the San Andreas Fault in California or the Ring of Fire circling the Pacific. So why did earthquakes of such staggering force strike here?
The answer, it turns out, lay hidden deep beneath the ocean floor.
Echoes of Quakes Through the Centuries
The 1755 Great Lisbon Earthquake was not the first, nor the last, to rock Portugal. Historical records reveal that in 1356, another powerful earthquake shook the region, although far less is known about its scale. More recently, in 1969, a magnitude 7.9 earthquake rattled the Iberian Peninsula. This time, modern seismometers captured the event, pinpointing its source: the Horseshoe Abyssal Plain, a relatively flat stretch of seafloor southwest of Portugal.
But here was the mystery—this plain was not near any major known tectonic boundaries, the places where plates collide, slide, or pull apart, causing the world’s most violent earthquakes. Why would such destructive events originate in a seemingly quiet region of the Atlantic Ocean?
For decades, this puzzle remained unsolved. Until, recently, scientists discovered that beneath the placid surface of the Horseshoe Abyssal Plain, a slow-motion geological drama was unfolding.
A Strange Anomaly Beneath the Seafloor
A breakthrough came when geophysicists combined data from land-based seismometers with instruments anchored on the ocean floor. Using seismic imaging—essentially creating an ultrasound of Earth’s interior—they detected something strange.
Beneath the Horseshoe Abyssal Plain lay a block of lithosphere behaving in a way no one had seen before. Normally, oceanic lithosphere, the rigid “skin” of Earth’s outer shell, remains stable for millions of years before eventually being swallowed at subduction zones, where one plate dives beneath another. But here, the lithosphere appeared to be splitting apart.
The imaging showed that the lower portion of this plate had detached and begun to sink downward into the mantle, a process called delamination. This was astonishing because delamination had only ever been observed in continental plates—not in oceanic ones. For the first time, scientists were watching the ocean floor itself begin to peel apart.
Serpentinization: Water as the Hidden Trigger
But what could cause such a dramatic rupture? The answer lies in a deceptively gentle process: water infiltration.
As cracks opened in the lithosphere, seawater seeped deep into the rock. There, under high pressure and heat, the water transformed minerals into serpentinite—a greenish, slippery rock named for its scaly appearance. This process, known as serpentinization, weakens the crust, allowing it to decouple from the underlying mantle. Over time, the once-rigid plate begins to peel and sink, much like wallpaper loosening from a damp wall.
This delamination creates new stress and instability, setting the stage for earthquakes. In Portugal’s case, two fracture zones acted like boundaries, isolating a block of lithosphere that eventually began sinking northwards. This led to the formation of a major thrust fault—the perfect recipe for the kind of catastrophic quakes Lisbon has endured.
A Prelude to Subduction
Even more remarkably, researchers believe this delamination might be the beginning of something much larger: a new subduction zone. Subduction—the process where one tectonic plate dives beneath another—drives the planet’s most violent earthquakes and fuels volcanic arcs.
If delamination is indeed the precursor to subduction, then off the coast of Portugal, we may be witnessing the early stages of a tectonic transformation. Over millions of years, today’s quiet Atlantic margin could evolve into a subduction zone as dramatic as those ringing the Pacific.
In this sense, the earthquakes that devastated Lisbon may not be isolated tragedies but signs of Earth’s restless creativity—its way of reshaping continents and oceans.
Lessons for the Future
The scientific breakthrough is more than a curiosity of geology—it carries profound implications for the millions of people living in Portugal, Spain, and beyond. By uncovering the hidden mechanics beneath the Horseshoe Abyssal Plain, researchers have taken a crucial step toward understanding why the Iberian Peninsula remains seismically active.
This knowledge can inform better earthquake preparedness and risk mitigation. Knowing that the region sits atop a nascent tectonic boundary, governments can strengthen infrastructure, improve early warning systems, and educate communities about the risks of future quakes and tsunamis.
It also underscores the need for continued research. If oceanic lithosphere can delaminate here, it may be happening elsewhere, too. Similar hidden processes could explain other puzzling earthquake hotspots across the globe.
When Earth Reminds Us of Its Power
The Lisbon earthquake of 1755 was more than a natural disaster. It was a turning point in history, shaking not only buildings but also philosophies. Thinkers like Voltaire and Rousseau grappled with the event, questioning faith, reason, and humanity’s place in a fragile world.
Now, nearly 270 years later, science has given us a glimpse of the forces that caused that terrible morning. Beneath the calm Atlantic waters lies a restless plate, splitting, sinking, and reminding us that Earth’s crust is never as solid as it seems.
Understanding this mystery does not erase the tragedy of Lisbon, but it transforms it into something larger—a chapter in Earth’s ongoing story of creation and destruction. For Portugal, for Europe, and for the world, it is a reminder that the ground beneath our feet is alive, and that the same forces that bring devastation also sculpt the mountains, shape the oceans, and build the very landscapes we call home.
More information: João C. Duarte et al, Seismic evidence for oceanic plate delamination offshore Southwest Iberia, Nature Geoscience (2025). DOI: 10.1038/s41561-025-01781-6
