A tsunami begins above an undersea earthquake, landslide or volcanic eruption along the ocean floor, explains Lisa Gardiner of the National Earth Science Teachers Association. In the case of an earthquake, when the movement along a fault moves the seafloor upward, water also pushes upward and becomes a tsunami wave.
Undersea landslides and volcanic eruptions can also produce enough force that water above them is displaced, becoming a train of tsunami waves. When the waves approach land, they become higher in the shallow coastal waters. National Geographic News adds that a tsunami can also be generated by a giant meteor impact with the ocean.