A volcano can cause a tsunami, a destructive wave series most commonly associated with underwater earthquakes. Unlike tectonic tsunamis, volcanic tsunamis are generated by the rapid displacement of water through non-seismic volcanic processes. These events are characterized by extreme speed and localized destruction, often arriving on nearby coastlines with little to no warning. While less frequent than those caused by tectonic activity, volcanic tsunamis can be just as devastating.
Major Water Displacement Events
The most catastrophic mechanism for a volcanic tsunami involves the wholesale failure of the volcano’s physical structure. This process is known as a flank collapse or a large-scale landslide, where an immense volume of the volcano’s unstable slope suddenly plunges into the sea. Structural failures can be triggered by internal pressure changes from rising magma, gravity acting on weakened rock, or large earthquakes.
When a substantial portion of a volcano’s edifice slides into the ocean, it instantaneously pushes out a colossal amount of water. This rapid injection of debris creates a powerful initial wave that propagates outward, similar to a rock dropped into a pond. The resulting tsunamis tend to have shorter wavelengths and periods than tectonic tsunamis, meaning they lose energy faster but pose a significant danger to nearby areas.
Another significant displacement mechanism is the rapid subsidence following a large eruption, known as caldera collapse. During this event, the emptied magma chamber can no longer support the weight of the mountain, causing the summit to crash down and form a caldera. If this collapse occurs in a submarine or coastal setting, the rapid downward movement instantly displaces the overlying water column. This drop creates a void that the surrounding water rushes in to fill, generating powerful tsunami waves.
Direct Hydrovolcanic Interactions
A secondary way a volcano can generate a tsunami is through the direct interaction of eruptive material with the surrounding ocean. High-velocity pyroclastic flows, which are fluidized mixtures of hot gas, ash, and rock fragments, travel rapidly down a volcano’s flank and surge across the water surface. As this dense, fast-moving current enters the sea, its immense momentum pushes the water outward.
This rapid transfer of energy creates a displacement wave. The resulting tsunami is generally more localized than a wave from a flank collapse, but it can be destructive in the immediate vicinity of the volcano and nearby coastlines. Violent submarine explosions, or the shockwave of a massive eruption, can also create pressure disturbances strong enough to generate local, short-period waves.
Case Studies of Volcanic Tsunamis
The most famous example of a volcanic tsunami occurred during the 1883 eruption of Krakatoa in Indonesia’s Sunda Strait. The eruption caused the partial collapse of the volcanic island into a newly formed caldera, displacing a monumental volume of seawater. This event generated waves that reached heights of up to 46 meters, obliterating 165 coastal villages and causing an estimated 36,000 fatalities on the coasts of Java and Sumatra.
A more recent event was the 2018 Anak Krakatau tsunami, also in the Sunda Strait. This disaster was caused by the sudden failure of the volcano’s southwest flank, where approximately 0.2 cubic kilometers of the cone slid into the ocean. The resulting tsunami struck the coasts of Java and Sumatra within 30 minutes, with wave run-ups reaching up to 13 meters in localized areas. This event demonstrated that even a relatively small flank collapse can generate a devastating “silent” tsunami without the preceding earthquake that standard warning systems detect.
The 1792 Unzen disaster in Japan also saw a catastrophic flank collapse generate a massive tsunami. This event resulted in over 14,000 deaths, validating the hazard posed by large-scale debris avalanches.
Detecting and Predicting Volcanic Tsunami Hazards
Forecasting tsunamis generated by volcanoes presents unique challenges because their triggers are often less predictable than tectonic earthquakes. Standard Tsunami Warning Systems (TWS) are primarily designed to respond to large seismic events, which typically provide minutes to hours of warning based on seismic wave travel time. Volcanic tsunamis, particularly those from flank collapses, can occur with little to no seismic precursor, meaning the collapse itself is the first indication of a threat.
To address this, scientists rely on integrated, real-time volcano monitoring techniques. Tracking subtle shifts can indicate the instability of a volcano’s edifice. Monitoring techniques include:
- Utilizing seismic networks to detect subtle ground vibrations and rockfalls that might precede a flank collapse.
- Using GPS systems to measure ground deformation on the volcano’s slopes.
- Employing satellite radar systems to measure ground deformation on the volcano’s slopes.
- Deploying specialized sea-level gauges near at-risk volcanic islands to detect rapid water level changes.
Linking this volcano-specific data directly to Tsunami Warning Centers is a priority to improve response time for these fast-arriving hazards.