Class9 Tsunami:the killer waves. Ten important points from the chapter
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Tsunami waves, which hit India for the first time on Sunday wreaking havoc across the southern coastline, are a known phenomenon in the Pacific Ocean region, which stretches from Chile in Latin America to Japan in East Asia.
The waves are usually triggered by seismic disturbances -- coastal earthquakes, volcanic eruptions or undersea landslides -- that jolt the ocean floor.
Tremors under the sea displace ground surface, sending the water radially outward in concentric circles from its epicenter. The result is a deep wave, stretching from the sea's surface to the floor that travels horizontally at speeds of up to 500 miles per hour and reaches heights of 50 to 100 feet.
The tsunamis that hit India on Sunday were caused by a massive earthquake on the Indian Ocean near Sumatra in Indonesia. Similar waves have hit six other countries, claiming thousand of lives.
The waves travel faster in deep water, rising further as they approach shore. In open sea, Tsunamis are only about a metre high, but when they reach a shoreline, they can be taller than a house and weigh millions of tonnes.
Though the bottom of the wave is slowed down by the sharp elevation of the ocean floor near the coast, its top part keeps moving at the original speed. As a result, vast quantity of water piles up and finally crashes over the shore with amazing force, thus causing massive destruction.
Graph: How quake in Indonesia affected India
The first sign of an approaching Tsunami is the sea tide receding from the shore, which leaves a large part of the sea floor exposed. The ocean water then flows towards the shore faster than before, resulting in high waves. This phenomenon is repeated several times before the Tsunami itself hits the land.
The Tsunamis can travel long distance without losing the original power. Neither islands nor reefs diminish their speed or power.
Usually the waves do not dissipate until they strike a continental landmass, and some waves even travel back and forth across the ocean for over a week.
Japan is one of the two Asian countries -- the other being Indonesia to be frequently hit by Tsunami waves. In fact, the word tsunami is derived from the Japanese language and is used by Japanese to describe very long, low seismic sea waves.
Tsunamis are common in the Pacific Ocean, as there is significant movement of the earth's tectonic plates in the region.
Fortunately, tsunamis of a serious nature are known to occur only once in a decade in the region. But when they occur, they leave a trail of destruction, causing heavy damage to life and property. The major ones -- the Prince William Sound, Alaska, in March 1964, and the one generated off the coast of Chile in 1960 caused havoc over large distances. Hundreds of people died when a huge wave travelling at a speed of 750 km per hour smashed Japan in 1960, following a series of quakes in Chile on the other side of the Pacific Ocean.
In the Indian Ocean, which was hit by a quake on Sunday, the Indo-Australian plate is being subducted by the east margin of the Eurasian plate. Therefore, most Tsunamis generated in this area are propagated towards the southwest shores of Java and Sumatra, rather than into the Indian Ocean.
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