Most tsunamis are caused by earthquakes at destructive plate boundaries. Here, an oceanic plate is subducted into the mantle beneath a continental plate. This movement causes friction, which in turn causes the plates to stick. Energy accumulates, like that of a compressed spring. When the energy exceeds the friction, the plates snap back into position. This movement thrusts the water above, causing a wave to form. The waves can travel large distances. When the waves reach shallower water, the following happens:
A sign that a tsunami is approaching a coast is water retreating. Shortly after this happens, the waves reach the shore. This is actually the trough of the wave following behind.
The main impact is coastal flooding. The force of such large waves can damage and destroy coastal property as well as endanger lives.