The meteor strike that occurred billions of years ago is the rarest cause of a tsunami. The most common triggers are underwater volcanoes, earthquakes and landslides. A wave is basically formed when the geography of the ocean floor suddenly and drastically changes, either rising or falling. For example, in the case of the 2004 Indonesian tsunami, the action of one tectonic plate being forced under another caused a huge earthquake which deformed the ocean floor. This action causes millions of gallons of water to be displaced abruptly, causing a movement wave.
A tsunami wave moves in much the same way that ripples do from a pebble dropped into a pond. Spreading outward from the epicenter, the waves move at a rapid pace, reaching speeds of 590 miles per hour in the open ocean. The waves slow as the tsunami approaches the shallower waters near land. The wave speed also slows as the energy from the initial event dissipates.
Starting from the point of origin, the wave is not very large visually. It is made up of a chain of small waves, generally no larger than 1 meter tall and almost invisible in the open ocean. The wave front varies in size depending on the severity of the original event and can be hundreds of miles in length. The largest waves to actually impact land in recorded history were around 121 feet in height.
A tsunami is not just a single wave, but a chain of multiple waves known as a wave train. Unlike tidal waves caused by wind blowing surface water, only tsunami waves are caused by the movement of the whole water column from sea bed to surface. The waves become more pronounced near land. As the first waves slow down, they cause the faster moving waves behind them to grow larger and build more power. This produces the rising surges of water that rush deeply inland.