Revision summary
A tsunami is a train of long ocean waves caused by sudden motion of the seafloor. Most begin on subduction megathrusts; landslides and volcanoes can also trigger them. In deep water they are fast and low; near shore they flood. The 2004 Sumatra quake devastated the Andamans, Tamil Nadu, Sri Lanka, the Maldives and coasts as far as Africa. Warning systems work only with drills, and mangroves take some of the blow.
Model answer
Introduction
A tsunami is not a high wind sea. It is a series of long waves set off when the ocean floor suddenly moves and shoves the whole water column. In deep water the wave is fast and low. Near shore it slows, stands up, and floods. That is why a fishing village can look calm ten minutes before it disappears.
Body
How and where they form
Most tsunamis start at subduction zones, where one plate dives under another and the locked interface slips in a megathrust earthquake. The 2004 event began off Sumatra on the Sunda megathrust. Similar belts ring the Pacific: Japan, Chile, Alaska, Kamchatka. A undersea landslide, a volcanic flank collapse, or a rare meteor can do the same job. Krakatoa in 1883 coupled explosion and wave. Not every sea quake makes a tsunami; the slip must lift or drop the seafloor.
Waves cross oceans at jet-airliner speeds. The 2004 train hit the Tamil Nadu and Kerala coasts, the Andaman and Nicobar Islands, Sri Lanka, the Maldives and East Africa hours after Sumatra. India’s Makran coast (Pakistan–Iran) is another local source for Gujarat and Oman. The Sunda trench is the neighbour India must watch to the southeast.
Consequences
- First is drowning: more than two lakh dead in 2004, with the Andamans and the Tamil fishing coast among the Indian wounds. Then come saline fields, broken boats, ruined tourism, and trauma. A tsunami does not look like a curling surf poster. It can arrive as a fast flood or as the sea withdrawing first, which is a warning if people have been taught to run inland.
Consequences are worse where mangroves and coral were already cut, because those buffers take some energy. Early warning — INCOIS buoys and tide gauges after 2004 — saves lives only if a siren and a drill exist on the beach. Japan’s 2011 Tohoku wave showed that even a prepared country can lose a nuclear plant to a wave taller than the wall.
The map of formation is the map of convergent plate edges. The map of death is the map of low, crowded coasts in the same ocean basin.
Flow diagram
flowchart LR
subgraph source [Subduction sources]
SU[Sunda Sumatra 2004]
MK[Makran]
JP[Japan trench]
end
SU --> AN[Andaman Nicobar]
SU --> TN[Tamil Nadu Kerala]
SU --> SL[Sri Lanka]
SU --> MV[Maldives]
SU --> AF[East Africa]
MK --> GJ[Gujarat coast]
Conclusion
Tsunamis are long waves from sudden seafloor change, usually a megathrust quake. They grow deadly on shallow coasts. The 2004 Indian Ocean disaster, from Sumatra to Africa, is the example India cannot treat as ancient history.
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Can the Arabian Sea produce one?
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