Volcanoes and earthquakes release an enormous amount of energy from within Earth. This energy is passed on into ocean water creating waves that can he quite destructive.
Tsunamis, waves caused by volcanoes and earthquakes, can travel hundreds and thousands of miles (kilometers) before reaching a land mass.
Out in the ocean, they reach only a few feet (meters) high, but as they near land, they slow down and increase dramatically in size.
Tsunamis can be 100 feet (30 meters) tall when they reach shore. Their force has been known to destroy entire towns.