First, tsunami are NOT “tidal waves”! They are seismic sea waves that are caused by a major displacement of the ocean due to Earthquakes, Volcanic Eruptions, Submarine landslides or meteor impacts.
Tsunami travel 435 mph across the Pacific ocean basin and rise to maximum height as they rush into shallow water. The wave height is approximately 1-2m high in the open ocean, so they usually aren't visible until they arrive on the coast as a series of waves. Tsunami are the most dangerous when they occur locally because there is no warning time.
Tsunami are formed due to a displacement of ocean water. This most frequently occurs as a result of movement along a subducting plate. NOAA's JetStream website has a great description of how tsunami are generated. The images below are from their site.
|First, two converging plates are placed under compression. Stress begins to build where the two plates meet (1).
|Stress continues to build, with the subducting plate being pulled down (2) while the overriding plate is pushed up (3).
|Both sides further deform as stress continues to build.
|Eventually, the stress is released as an earthquake, and the crust on both sides return to "normal". This means that the bulge that was building on the overriding plate snaps downward. The water above this area is also pulled down, and then rebounds back upwards, generating the waves that form the tsunami.
|The tsunami waves radiate outward from their point of origin, rushing onshore as a series of huge waves.|
The movie below shows the scope of the December 26, 2004 Sumatra tsunami as the waves moved around the world (click image to go to the Quicktime Movie). More animations about this tsunami can be found at the NOAA Center for Tsunami Research.