Tsunamis happen when earthquakes, landslides or volcanic eruptions disturb the ocean floor. Most tsunamis occur when there are earthquakes at tectonic plate boundaries. When the ocean floor at the boundary rises or falls, it moves the water above and creates the huge waves that become tsunamis.
Tsunamis grow as they reach the shore and enter shallower water. The top of the waves moves faster than the bottom, causing the wave to rise quickly. The lowest point of the wave, known as the trough, reaches the shore first and sucks coastal water toward the sea. The rest of the tsunami usually hits approximately 5 minutes later.