Tsunamis usually occur when earthquakes take place underneath the ocean at tectonic plate boundaries, or when volcanic eruptions or landslides take place in the ocean. Underwater earthquakes cause the ocean floor to fall and rise abruptly, which results in large volumes of water being displaced. This displaced water then forms into massive rolling waves that eventually become a tsunami.
Tsunamis can traverse the oceans at speeds of up to 500 miles per hour and can be as high as 100 feet tall when they reach land. The majority of tsunamis on Earth occur in the Pacific Ocean's "Ring of Fire," which is an area where earthquakes and volcanoes are a common occurrence.