What causes tsunami?

Earthquakes

Most tsunami are caused by large earthquakes on the sea floor when slabs of rock move past each other suddenly, causing the overlying water to move. The resulting waves move away from the source of the earthquake event.

Landslides

Landslides can happen on the seafloor, just like on land. Areas of the seafloor that are steep and loaded with sediment, such as the edge of the continental slope, are more prone to undersea landslides.

When an undersea landslide occurs (perhaps after a nearby earthquake) a large mass of sand, mud and gravel can move down the slope. This movement will draw the water down and may cause a tsunami that will travel across the ocean.

Volcanic eruptions

Tsunami initiated by volcanic eruptions are less common. They occur in several ways:

• destructive collapse of coastal, island and underwater volcanoes which result in massive landslides
• pyroclastic flows, which are dense mixtures of hot blocks, pumice, ash and gas, plunging down volcanic slopes into the ocean and pushing water outwards
• a caldera volcano collapsing after an eruption causing overlying water to drop suddenly.