Tsunamis occur when there is a very sudden movement. Such as earthquakes, landslides in the sea floor, major volcano eruptions or a large meteorite impact. The majority of tsunamis are caused by large earthquakes in the sea floor. If a volcano eruption causes a tsunami it can happen in many different ways such as if a volcano erupts under water then lava will proberly heat the surrounding water very quickly and explosively. Another way a volcano eruption can cause a tsunami is when there is a eruption underwater ash could travel down the side therefor forcing the water outwards. The least likely way to start a tsunami by a volcano eruption is when the under water volcano colaps, so the overlying water also drops.