The most common cause of tsunamis is from undersea earthquakes on the ocean floor. Submarine landslides and volcanic eruptions also cause tsunamis. On very rare occasions, a meteorite impact in the ocean results in a tsunami. Any event that displaces a significant amount of water in a short period of time can cause a tsunami.
Not all undersea earthquakes cause tsunamis. The quake must cause part of the crust to lift up like a gigantic paddle, displacing a large volume of water. If the quake happens deep in the crust or the crust is not displaced significantly, then a tsunami is not likely to occur. Strong, shallow quakes produce more tsunamis. Tsunamis are difficult to detect on the open ocean since the wave does not rise very far above the ocean's surface. As the tsunami approaches shallower water, the water piles up and causes a much taller wave.
Volcanic eruptions also cause tsunamis, such as the waves produced when the Indonesian volcano Krakatoa erupted in 1883. Superheated gas and rock from the volcano flowed into the sea, which displaced a huge volume of water and caused tsunamis. These waves were felt as far away as South Africa. Many villages were destroyed by these waves, and more than 36,000 people died.