Tsunamis are caused by undersea volcanoes or earthquakes that push massive amounts of energy through the water. Earthquakes are the most common cause, but landslides can create tsunamis as well.
Earthquake tsunamis begin when subduction occurs, a process where larger undersea plates slide under a lighter plate. In certain cases, the lighter plate shifts upward suddenly, stemming from the pressure of the other plate. This sends shoots of rocks upward, transferring powerful energy through the water and above sea level. The gravity forces the energy out in a horizontal fashion and along the surface, the equivalent of a ripple effect that is seen when an object is thrown into the water.
The energy spreads away from the disturbance area. The energy transfer formulates the tsunami, but the size depends on the water levels, since tsunamis travel faster in deeper waters. Tsunamis can travel in deep waters at hundreds of miles per hour. Tsunamis slow down when reaching land but increase in height. Tsunamis are usually no higher than 3 feet until the waves approach land.
In the case of landslide tsunamis, large amounts of sediment fall into the ocean at rapid speeds, forcing powerful energy levels into the water. The sudden energy travels at a faster speed than the water can absorb, causing tsunamis to develop.