Weather and Mega-Tsunamis: When Storms Topple Mountains

The popular image of a tsunami is straightforward: an earthquake beneath the ocean floor displaces a column of water, which races across the sea and devastates a distant coastline. This is accurate for most tsunamis — but it is not the complete picture. Some of the largest and most devastating tsunamis in Earth's history were not caused by earthquakes at all but by massive landslides — the sudden collapse of mountains, volcanic islands, or unstable coastal cliffs into the ocean, displacing billions of tonnes of water in a single catastrophic event and generating waves that dwarf anything an earthquake can produce. These mega-tsunamis, as they are called, have created waves hundreds of metres high — taller than skyscrapers, taller than the cliffs they struck, taller than anything the modern world has ever experienced. And weather plays a role in making them happen: the same storms, rainfall, and erosion that reshape the landscape above the waterline also destabilise the slopes that, when they finally collapse, generate waves of almost incomprehensible scale.

What Makes a Mega-Tsunami

A mega-tsunami differs from an ordinary tsunami in both cause and scale. Ordinary tsunamis are generated by the vertical displacement of the ocean floor during earthquakes — the upward or downward movement of a fault raises or lowers the water column above it, creating a wave that propagates across the ocean. The displacement is typically 1–10 metres vertically over an area of hundreds to thousands of square kilometres, producing waves that are devastating but usually less than 30 metres high at the coast. A mega-tsunami is generated by a landslide — the rapid entry of a massive volume of rock, ice, or volcanic material into water, displacing a volume of water proportional to the volume of the landslide. Because landslides can be vastly larger in volume than earthquake displacements, the resulting waves can be vastly larger — hundreds of metres rather than tens.

The 1958 Lituya Bay event in Alaska remains the benchmark for documented mega-tsunamis. A magnitude 7.8 earthquake triggered a massive rockslide (approximately 30 million cubic metres of rock) from the steep walls of a narrow fjord. The rockslide entered the bay at tremendous speed, displacing the water and generating a wave that ran up the opposite slope to a height of 524 metres — higher than the Empire State Building. The wave stripped vegetation, soil, and rock from the slopes of the bay, leaving a clearly visible trimline that documented the wave's extreme height. Two of three fishing boats in the bay at the time were destroyed; remarkably, one boat and its two occupants survived by riding over the wave crest.

Even larger events are documented in the geological record. Submarine landslides on the flanks of volcanic islands have produced some of the largest mass movements on Earth — individual slides involving tens of cubic kilometres of material. The Canary Islands show evidence of at least 14 major flank collapses in the past few million years, with individual slides displacing up to 35 cubic kilometres of rock into the Atlantic Ocean. The waves generated by these collapses are debated, but even conservative models suggest wave heights of tens of metres at distances of hundreds of kilometres — large enough to devastate Atlantic coastlines from Africa to the Americas.

Weather as Destabiliser: How Storms Trigger Landslides

Weather's role in mega-tsunami generation is not to create the wave directly but to destabilise the slopes whose collapse generates the wave. Rainfall — the most significant weather factor — infiltrates the fractures and pores of rock and soil, increasing pore water pressure and reducing the frictional forces that hold slopes in place. The relationship between rainfall and landslides is well-established: intense or prolonged rainfall is the most common trigger for landslides worldwide, and coastal and mountain slopes that have been destabilised by rainfall have produced some of the largest landslides in recorded history.

On volcanic islands — where the mega-tsunami hazard is most significant — rainfall infiltration combines with other destabilising factors to create slopes that are preconditioned for catastrophic failure. Volcanic rock is inherently weak: it is often fractured, porous, and altered by hydrothermal activity that converts strong minerals to weak clays. Volcanic islands are steep because they are built by eruptions that pile material at angles near the limit of stability. And volcanic islands in tropical and subtropical climates receive intense rainfall that saturates the already-weak material. The combination — weak rock, steep slopes, heavy rain — makes volcanic island flanks among the most landslide-prone terrain on Earth.

Storm waves provide an additional destabilising mechanism on coastal slopes. Wave action at the base of a cliff undercuts the slope, removing supporting material and creating an overhang that eventually collapses. On volcanic islands, where cliffs plunge directly into deep water, storm waves can undercut unstable slopes at sea level while rainfall saturates them from above — a dual attack that accelerates the slope's movement toward failure. Major storms that combine heavy rainfall with large waves represent the maximum weather-driven destabilisation, and it is during or shortly after such storms that coastal landslides are most likely to occur.

Climate Change and Slope Stability: Thawing the Danger

Climate change introduces new dimensions to the weather-mega-tsunami connection through two mechanisms that are already observable. First, in high-latitude and high-altitude regions, warming temperatures are thawing permafrost and retreating glaciers, exposing steep rock faces that were previously stabilised by ice. The ice acted as a buttress — holding fractured rock slopes in place through the mechanical support of frozen material in fractures and the physical support of glaciers at the slope's base. As the ice disappears, these slopes become free to move, and several recent large landslides have been attributed to glacier retreat and permafrost thaw.

The 2015 Taan Fiord landslide in Alaska — which generated a tsunami approximately 193 metres high within the fjord — occurred when a rock face destabilised by glacier retreat collapsed into water that was exposed by the same glacier's retreat. The glacier had both stabilised the slope (through ice buttressing) and protected the water below (by filling the fjord with ice that prevented the rock from reaching the water). When warming removed both the stabilising ice and the protective ice, the landslide occurred and the tsunami was generated — a direct consequence of climate change creating a hazard that did not previously exist.

Second, climate change is increasing the intensity of extreme rainfall events globally, which increases the frequency of rainfall-triggered landslides. In mountainous coastal regions — Norway's fjords, Alaska's coast, the volcanic islands of the Atlantic and Pacific — the combination of warming-induced slope destabilisation and intensifying rainfall creates conditions for more frequent and potentially larger landslides into water. The Norwegian fjord system, where several large rock slopes are monitored for potential collapse into inhabited fjords, represents one of the most closely watched mega-tsunami hazards in the world — and climate change is accelerating the movement of several of these slopes.

Volcanic Island Collapses: The Ultimate Mega-Tsunami

The largest potential mega-tsunami source on Earth is the collapse of a volcanic island flank — a catastrophic event in which a significant portion of a volcanic island slides into the ocean as a single massive landslide. The geological record shows that such events have occurred repeatedly throughout the history of volcanic ocean islands: the Hawaiian Islands show evidence of at least 68 major flank collapses, the Canary Islands show at least 14, and similar evidence exists in the Azores, Cape Verde, Réunion, and volcanic islands throughout the Pacific.

The Cumbre Vieja volcano on La Palma in the Canary Islands has received the most attention as a potential mega-tsunami source, following a 2001 study by Steven Ward and Simon Day that modelled the consequences of a catastrophic flank collapse during a future eruption. The study projected that a collapse of the volcano's western flank — an unstable block of approximately 500 cubic kilometres — could generate a tsunami that would cross the Atlantic and strike the Americas with waves tens of metres high. The study was controversial, and subsequent analyses have generally produced lower wave estimates, suggesting that a collapse would be more gradual and generate smaller (though still significant) waves. The 2021 eruption of Cumbre Vieja did not produce a flank collapse, but it renewed attention to the volcanic and structural instability of the island.

The probability of a mega-tsunami-generating flank collapse at any given volcanic island is very low in any human timescale — the return period for such events is typically tens of thousands to hundreds of thousands of years. But the consequences are so extreme — potential wave heights of hundreds of metres near the source and significant waves at trans-oceanic distances — that the hazard merits ongoing monitoring and research. The challenge is characteristic of extreme-consequence, low-probability hazards: the event is unlikely in any given century but would be civilisation-altering if it occurred, and the difficulty of maintaining preparedness for an event that most living people will never experience is substantial.

Warning Systems and Preparedness: Can We See It Coming?

Unlike earthquake-generated tsunamis, which can be detected by ocean-floor pressure sensors and provide hours of warning to distant coastlines, mega-tsunamis from landslides provide minimal warning because the wave source is local and the waves arrive almost instantly at nearby coastlines. The 1958 Lituya Bay wave reached the opposite shore in less than two minutes. A volcanic island flank collapse would send waves to neighbouring coastlines within minutes — far less time than any evacuation could accomplish. The primary defence against mega-tsunamis is therefore not warning but monitoring: identifying the unstable slopes that could generate them, measuring their movement, and establishing hazard zones based on modelling of potential wave heights.

Norway operates the world's most advanced landslide-tsunami monitoring system, with continuous surveillance of several large rock slopes in western fjords that could produce devastating local tsunamis if they collapse. The Åknes rock slope in Storfjorden — a moving mass of approximately 54 million cubic metres — is monitored by an extensive network of sensors that would trigger evacuation warnings for downstream communities if the slope accelerates toward failure. Similar monitoring exists at other Norwegian fjord slopes and is being developed in Alaska and other regions where landslide-tsunami hazard has been identified. These systems represent the state of the art in mega-tsunami preparedness — a recognition that the hazard cannot be prevented but can be anticipated, and that the minutes or hours of warning that monitoring provides may be sufficient to save lives.

The Mediterranean Context: Volcanic Islands and Enclosed Seas

The Mediterranean Sea, despite its relatively small size, is not immune to mega-tsunami hazard. The volcanic islands of the Aeolian archipelago (Stromboli, Vulcano, Lipari), the Hellenic arc (Santorini), and the submarine volcanoes of the Tyrrhenian and Aegean seas all present some degree of flank collapse potential. The 2002 eruption of Stromboli produced a landslide-generated tsunami that struck the nearby coast of the island with waves up to 10 metres high — a small-scale demonstration of the mechanism that, at larger scale, could produce a devastating event in the enclosed Mediterranean basin.

The Santorini caldera — formed by the catastrophic Minoan eruption approximately 3,600 years ago — generated tsunamis during that eruption that archaeological evidence suggests reached heights of 9–35 metres on the northern coast of Crete, approximately 110 kilometres to the south. While the Minoan eruption tsunami was generated primarily by the caldera collapse and pyroclastic flows entering the sea (rather than by a flank collapse landslide), it demonstrates the potential for volcanic events in the Aegean to generate significant tsunamis that affect distant coastlines.

The enclosed nature of the Mediterranean amplifies the tsunami hazard: waves generated within the basin cannot disperse across an open ocean but reflect off coastlines and interfere constructively, potentially producing higher wave heights at greater distances than the same event in an open ocean would generate. A major flank collapse of an Aeolian or Aegean volcanic island could produce waves that would reach densely populated coastlines — Naples, Messina, Athens, the Cretan coast — within minutes, providing little time for warning or evacuation. The combination of active volcanism, potential slope instability, enclosed basin geometry, and dense coastal population makes the Mediterranean a region where the mega-tsunami hazard, while low in probability, warrants serious attention from civil protection authorities and volcanologists.