How do tsunamis occur?

Tsunami are actually waves caused by sudden movement of the ocean due to earthquakes, landslides on the sea floor, land slumping into the ocean, major volcanic eruptions or large meteorite impacts.

Most tsunami are caused by large earthquakes at the seafloor, when large slabs of rock are forced to move past each other suddenly causing the overlying water to move. The resulting wave moves outwards and away from this event.

Underwater landslides can cause tsunami, and so can land which slumps into the ocean. Landslides happen when slopes become too steep to withstand gravity.

Less common are tsunami initiated by volcanic eruptions. These occur in several ways:

  • if an underwater volcano erupts, the hot lava may heat the surrounding water quickly and explosively.
  • massive flows of volcanic debris such as ash can travel down the side of a volcano and into the ocean, pushing water outwards.
  • the top of an underwater volcano may collapse downwards, so that the overlying water also drops.

Large meteorite impacts that occur at sea can trigger tsunami too.