In the movies, any big surge of water from the oceans -- the result of an underwater earthquake or a crashing asteroid -- is often called a tidal wave. But it’s another case of Hollywood not quite getting things right. A tidal wave is a predictable event. But a giant wave created in a violent event is known as a tsunami.
A tidal wave is produced by the daily tides, which are caused by the gravitational pull of the Moon and Sun. As the tide rolls in and out, it produces a rise and fall in sea level that stretches across thousands of miles.
Tidal waves are most pronounced in narrow bays or in rivers along the coast. As the tide forces its way into these inlets, it can raise the water level by several feet in just a few hours. The highest “tidal waves” are found in the Bay of Fundy, in the Canadian province of New Brunswick, where the water level can rise with the tide by 50 feet.
A tsunami, on the other hand, occurs when some event disturbs the ocean. An earthquake, for example, can jiggle or displace the ocean floor, perhaps triggering an underwater landslide. The water above such an event rises or falls, creating a surface wave that can travel at hundreds of miles an hour.
A tsunami wave isn’t necessarily very tall in the open ocean -- a few inches to a few feet different from the level of the ocean around it. But it can be very long, so when it approaches land, the water piles up -- creating devastation like that seen in the Indian Ocean in December of 2004.