What are tsunamis? How and where are they formed? What are their consequences? Explain with examples.

What are tsunamis? How and where are they formed? What are their consequences? Explain with examples. [10 marks, 2025]

A tsunami is a series of long sea waves produced when a large volume of ocean water is displaced suddenly. It is mainly a tectonic and coastal hazard, not a tidal phenomenon; therefore, the term “tidal wave” is misleading.

Formation of tsunamis:

Sudden seabed displacement → movement of the water column → waves spread across the ocean → waves rise near the coast → inundation

Its main causes are:

1. Undersea earthquakes
2. Volcanic eruptions
3. Submarine or coastal landslides
4. Meteorite impacts (extremely rare)

E.g., the 2022 Tonga volcanic eruption generated tsunami waves across the Pacific. Of these, undersea earthquakes at subduction zones trigger the overwhelming majority of tsunamis, because a rupturing fault can heave a long stretch of the seabed in seconds; volcanic eruptions and submarine landslides act less often, and meteorite impacts remain a rare, textbook case.

Tsunamis become more destructive near the coasts — as they enter shallow waters:

1. friction with the sea floor slows them;
2. the water gets compressed into a smaller depth;
3. wave height increases;
4. large volumes of water rush inland.

This process is called shoaling. In the deep ocean the same wave travels at the speed of a jet aircraft yet stands less than a metre high, so ships rarely notice it; shoaling converts that speed into destructive height only as the coast approaches.

Tsunamis occur mainly near tectonically active plate boundaries, especially subduction zones:

1. the Pacific Ring of Fire—Japan, Indonesia, the Philippines, Alaska and Chile;
2. the Indian Ocean margins, particularly the Sumatra–Andaman region;
3. some parts of the Mediterranean and Caribbean.

Low-lying coasts, islands, deltas and funnel-shaped bays are especially vulnerable because waves can penetrate farther inland or become concentrated. The Sumatra Andaman margin generated the catastrophic Indian Ocean tsunami of 2004, which devastated coasts from Indonesia to India and Sri Lanka.

Consequences of tsunamis:

1. Human consequences. E.g., destruction of settlements.
2. Economic consequences. E.g., damage to ports.
3. Environmental consequences. E.g., coastal erosion, salinisation of agri land, etc.
4. Cascading consequences. E.g., failure of power systems.
5. Transboundary consequences. E.g. affect countries far from the source.

Because tsunami waves cross entire ocean basins within hours, their consequences are transboundary — the 2022 Tonga eruption sent waves to distant Pacific coasts far from the source volcano.

To conclude, tsunamis are infrequent but extremely destructive hazards produced mainly by sudden seabed displacement in tectonically active oceans.