Tsunamis are massive waves that form when an ocean is disturbed by an earthquake, landslide, volcanic eruption or other disruptive event. Underwater earthquakes, which occur at the boundaries of tectonic plates, are one of the most common causes of tsunamis. When one plate moves up or down, it displaces water, and it is this displaced water that becomes the tsunami wave.
Tsunamis also form as a result of undersea landslides. The formation process is similar. Displaced water seeks a stable position, and in doing so, creates a tsunami. In rare cases, tsunamis occur when meteors strike the ocean and displace a large amount of water.
Not all earthquakes or landslides result in tsunami waves. Only those that occur with enough violence to displace a large amount of water very quickly have this devastating effect. Tsunamis are not single waves. Instead, they are a series of large waves in quick succession. These waves may not appear large on the open ocean, but as they approach shallow water, they become higher in comparison to the normal water level. Often, they reach distances of a mile or more offshore, causing destruction in their paths. Because of the large amount of water brought on shore, it takes many hours or days for the water levels to recede after a tsunami.