Tsunami Generation: Volcanoes

Tsunamis generated by volcanoes, both above and below water, are infrequent. Additionally, a volcano must be near the coast or not far below the sea surface to generate a significant tsunami. Like landslide-generated tsunamis, tsunamis generated by volcanic activity usually lose energy quickly and rarely affect distant coasts.

The Augustine Volcano in Alaska (2006). In 1883, a debris flow from the collapse of the north face of the Augustine Volcano's peak caused a local tsunami. Damage to homes and fishing boats was reported. Source: U.S. Geological Survey, Game McGimsey
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Several types of volcanic activity can displace enough water to generate destructive tsunamis, including:

Pyroclastic flows (flowing mixtures of rock fragments, gas, and ash)
Submarine explosions relatively near the ocean surface
Caldera formation (volcanic collapse)
Landslides (e.g., flank collapse, debris flows)
Lateral blasts (sideways eruptions)

Examples of volcano-generated tsunamis include:

August 27, 1883 Indonesia - The volcano Krakatau (Krakatoa) exploded and collapsed, generating one of the largest and most destructive tsunamis ever recorded. Waves reaching 135 feet (41 meters) high destroyed coastal towns and villages along the coasts of Java and Sumatra and killed more than 34,000 people.
May 21, 1792 Kyushu Island, Japan - At the end of the four-month eruption of the Unzen volcano, a flank collapse generated a tsunami with waves reaching 180 feet (55 meters) high that caused destruction around the Ariake Sea and more than 14,000 deaths.

To learn more about volcanoes, visit the U.S. Geological Survey's Volcano Hazards Program.