Volcanoes cause tsunamis through several distinct mechanisms, from massive landslides crashing into the ocean to underwater explosions and even atmospheric shockwaves that push on the sea surface. About 5 percent of all tsunamis are volcanic in origin, but they account for roughly 17 percent of all volcano-related deaths. That outsized fatality rate reflects how sudden and difficult to predict these waves can be.
Flank Collapse and Volcanic Landslides
The most common way a volcano generates a tsunami is by collapsing into the sea. Volcanic islands and coastal volcanoes build steep, unstable slopes over time. When a flank gives way, an enormous volume of rock, ash, and debris slides into the ocean at high speed, displacing a wall of water outward. Landslides are the second most important cause of tsunamis overall, behind earthquakes.
The size of the wave depends on how much material enters the water and how fast it moves. For a landslide above the waterline, the frontal area of the debris and its impact velocity largely determine the initial wave height. For slides that start underwater, the wave depends more on how the landslide evolves over time as it travels along the seafloor.
The 2018 collapse of Anak Krakatau in Indonesia is a recent and deadly example. A relatively small chunk of the volcano’s southwestern flank, less than about 0.3 cubic kilometers of rock, slid into the Sunda Strait while the volcano was actively erupting. That modest volume still sent waves of 15 to 30 meters crashing into the immediately surrounding islands, and coastal cities along Java and Sumatra received waves up to 3.4 meters high within 35 to 60 minutes. The eruption killed over 400 people. Hypothetical models of much larger collapses, like a 375 cubic kilometer slide from the Canary Islands, have been studied but represent extreme, unlikely scenarios.
Pyroclastic Flows Entering the Sea
Pyroclastic flows are superheated avalanches of gas, ash, and rock fragments that race down a volcano’s slopes at hundreds of kilometers per hour. When one of these flows reaches the ocean, it displaces water much like a landslide, but with added complexity. The flow has two layers: a dense, ground-hugging current packed with rocky debris, and a lighter, billowing cloud of hot gas and fine ash riding above it. The dense basal layer is what does most of the work in pushing water and generating waves.
Several other mechanisms contribute at the same time. Steam explosions occur when superheated material contacts seawater. The pressure of the expanding ash plume pushes down on the water surface. The shearing motion of hot gas racing across the ocean drags it forward. In practice, though, modeling shows that the pyroclastic debris flow component dominates wave generation in most situations. The other effects are secondary.
Underwater Eruptions and Steam Explosions
When a volcano erupts beneath the ocean surface, the explosive interaction between magma and seawater can blast water upward and outward, creating waves. Laboratory experiments simulating these eruptions have identified three regimes based on water depth: shallow, intermediate, and deep. The depth of the water above the eruption and the intensity of the blast are the two biggest factors controlling wave size.
Counterintuitively, the biggest waves don’t come from the shallowest eruptions. In very shallow water, most of the explosive energy escapes into the air as a fountain of spray. In very deep water, the ocean absorbs and contains the blast before it can disturb the surface. The sweet spot for maximum wave generation sits at intermediate depths, where enough energy reaches the surface to move large volumes of water without being wasted upward. Increasing the pressure of the eruption is far more effective at generating larger waves than simply extending the duration of the blast.
Atmospheric Shockwaves
The 2022 eruption of Hunga Tonga-Hunga Ha’apai in the South Pacific revealed a tsunami mechanism that caught many scientists off guard. The eruption was so powerful that it launched atmospheric pressure waves, called Lamb waves, that circled the entire planet multiple times. These pressure waves traveled faster than the ocean waves below them, and as they raced across the sea surface, they pushed the water down just enough to generate tsunami-like waves thousands of kilometers from the eruption site. Tide gauges registered unusual waves in the Atlantic, Pacific, and other ocean basins far too quickly for conventional ocean-traveling tsunamis to have arrived.
Research into this event found that the eruption process itself governed both the atmospheric pressure waves and the tsunami dynamics simultaneously. The volume of the crater created by the blast controlled how much volcanic material was ejected into the atmosphere, which in turn determined the strength of the pressure waves. Meanwhile, the corresponding loss of mass in the ocean where the volcano once stood drew water rushing inward, generating a more conventional tsunami at the source. The result was a coupled atmosphere-ocean event, two wave systems generated by a single eruption through entirely different physics.
Caldera Collapse
When a large eruption empties the magma chamber beneath a volcano, the roof can cave in, forming a wide, bowl-shaped depression called a caldera. If this collapse happens beneath the sea or in a flooded volcanic basin, the sudden vertical drop of hundreds of meters of rock could theoretically displace enormous volumes of water. The famous Late Bronze Age eruption of Santorini in the Aegean Sea has long been cited as an example.
In practice, this mechanism is more limited than it sounds. Research on Santorini has shown that the caldera was not yet flooded and connected to the open sea at the time of collapse, meaning the collapse itself could not have been the primary tsunami source. At most volcanic calderas, collapse occurs during the eruption rather than after, which means other mechanisms like pyroclastic flows and explosions are likely doing the wave-generating work. Caldera collapse remains a theoretically powerful mechanism, but evidence that it has directly triggered major tsunamis in the real world is thin.
Why Volcanic Tsunamis Are Hard to Predict
Earthquake-generated tsunamis benefit from decades of well-established warning systems. Seismometers detect the quake, algorithms estimate wave size and arrival time, and coastal populations receive alerts. Volcanic tsunamis don’t fit neatly into that system. A flank collapse may not produce a strong enough seismic signal to trigger standard earthquake-based warnings. The 2018 Anak Krakatau tsunami arrived at nearby coastlines in under an hour, and no official tsunami warning was issued.
Monitoring volcanic tsunami risk requires a different set of tools. Geologists watch for ground deformation on unstable flanks. Before Mount St. Helens collapsed in 1980, monitoring stations had detected a bulge on the north flank expanding by several meters per day. Satellites track thermal anomalies, gas emissions, and changes in a volcano’s shape using infrared and radar imaging. Underwater pressure sensors, hydrophones, and ocean buoys can detect waves once they form. After the Hunga Tonga event, researchers proposed incorporating air-pressure sensors into early warning networks, since the leading atmospheric pressure wave arrived well before the ocean wave and could serve as an advance signal.
High-frequency ocean radar is another emerging tool. Where continental shelves are wide enough, radar can spot tsunami currents at the shelf edge before waves reach shore. Ionospheric disturbances caused by volcanic atmospheric waves have even been detected using small GPS satellites, hinting at additional ways to catch these events early. The core challenge remains that volcanic tsunamis arise from varied, fast-moving processes, and a single detection method will never cover all of them.