If an asteroid collides with Earth, there is a good chance it will hit an ocean, simply because two-thirds of the Earth's surface are covered by water. In this case, a gigantic explosion occurs and the asteroid is pulverized and vaporized, along with a huge volume of water. This creates a crater in the water surface that quickly fills, generating a series of very dangerous tsunami that radiate across the ocean.

Tsunami (pronounced tsoo-nah-mee) is a Japanese word that can be translated as "harbor wave". A tsunami is a wave train (or series of waves) generated in water by an impulsive disturbance. Apart from impact of meteorites, other physical processes that can generate tsunami are earthquakes, landslides, volcanic eruptions, or even explosions. In other words, a tsunami can be generated by any disturbance that displaces a large water mass from its equilibrium position. Tsunami can savagely cover coastlines, causing devastating damages.

How do tsunami differ from other water waves?  

Tsunami differ from normal wind-generated waves (the waves everyone has observed on the beach) because of their longer periods and wave lengths. In fact, wind-generated waves usually have periods (time between two successive waves) of 5 to 20 seconds and a wavelength (distance between two successive waves) of 100 to 200 meters. A tsunami can have a period in the range of 10 minutes to 2 hours and a wavelength in excess of 500 km.

It is because of their long wavelengths that tsunami behave as shallow-water waves characterized by a very small ratio between the water depth and the wave' wavelength.
These shallow-water waves have constant characteristics: first of all, their speed is proportional to the depth of the water while the rate at which the wave loses its energy is inversely related to its wavelength. This means that, since a tsunami has a very large wavelength, it will lose little energy as it propagates. Hence in very deep water, a tsunami will travel great transoceanic distances at high speeds and with limited energy loss.
For example, when the ocean is 6000 m deep, unnoticed tsunami travel about 890 km/hr, the speed of a jet airplane. And they can move from one side of the Pacific Ocean to the other side in less than one day. As the tsunami crosses the deep ocean, its length from crest to crest may be a hundred miles or more, and its height from crest to trough will only be a few feet or less. They can not be felt aboard ships nor can they be seen from the air in the open ocean. In the deepest oceans, the waves will reach speeds exceeding 970 km/hr.

What happens when the tsunami meets the land?
As we have seen, a tsunami travels at a speed that is related to the water depth. Therefore, as the water depth decreases, the tsunami slows down. On the other hand, the energy transported by the tsunami, which is dependent on both its wave speed and wave height, must remain nearly constant. Consequently, as the tsunami slows down as it enters shallower water, its height grows.
Because of this effect, a tsunami, imperceptible at sea, may grow to be several meters or more in height near the coast. Just like other water waves, tsunami begin to lose energy as they rush onshore (part of the wave energy is reflected offshore, while some other is dissipated through bottom friction and turbulence), but still, they reach the coast with tremendous amounts of energy.

What about the consequences ?

What would happen if a tsunami hit New York?
(Image by Dreamworks)

Many features modify the tsunami as it approaches the shore: first of all, the presence of reefs, bays, entrances to rivers, undersea features and the slope of the beach are very important. Other characteristics that influence the size of the phenomenon are the features of the place where the tsunami first generated (the water depth near the source, the energy of the explosion, etc).
When the tsunami finally reaches the shore, it may appear as a rapidly rising or falling tide, a series of breaking waves, or even a bore (a step-like wave with a steep breaking front). Tsunami may reach a maximum vertical height onshore above sea level, called a runup height, of 30 meters. In the worst cases, consequences can be very bad: tsunami are capable of inundating, or flooding, hundreds of meters inland and the fast-moving waves can crush homes and other coastal structures. Luckily, tsunami rarely become this great, and sometimes they may break far offshore.