Click on the arrow on this animation to see how volcanic eruptions can cause tsunami.
If a lot of water is displaced, there is the potential to generate tsunami. The effect is the same whether this displacement is generated by movement of the seafloor or pyroclastic mass falling into the sea.
The eruption of Krakatoa in 1883 is an example of this kind of event. The eruption resulted in the northern two-thirds of the island collapsing into the sea and generating tsunami that killed over 30,000 people in Indonesia.