SSC
Earthquakes on the sea floor, undersea landslides and volcanic eruptions under the sea can cause:
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Answer: Option B — Tsunamis
Detailed Solution
A tsunami is a series of huge sea waves caused by a sudden displacement of a large volume of water, usually by an undersea earthquake, but also by undersea landslides, volcanic eruptions or, rarely, meteorite impacts.
Meaning of the word:
"Tsunami" is a Japanese word meaning "harbour wave" (tsu = harbour, nami = wave). They were once wrongly called "tidal waves", although they have nothing to do with tides.
How a tsunami forms:
- Most tsunamis are caused by large earthquakes (usually of magnitude 7.5 or more) at plate boundaries where one plate sinks beneath another.
- The sudden movement of the sea floor lifts or drops the water above it, starting waves that spread out in all directions.
Characteristics of tsunami waves:
- In the deep ocean, tsunami waves are low (often less than a metre high) but extremely long (hundreds of kilometres) and fast, travelling at 500 to 800 km per hour, the speed of a jet aircraft.
- As they approach shallow coastal waters, they slow down and grow much taller, sometimes reaching 10 to 30 metres or more.
Major tsunamis:
26 December 2004: An earthquake of magnitude 9.1 off Sumatra, Indonesia, caused a tsunami that killed about 2.3 lakh people in 14 countries, including over 10,000 in India (mainly Tamil Nadu and the Andaman and Nicobar Islands).
11 March 2011: The Tohoku earthquake in Japan (magnitude 9.0) caused a tsunami that killed about 18,000 people and damaged the Fukushima nuclear power plant.
Tsunami warning in India:
The Indian Tsunami Early Warning Centre (ITEWC), at INCOIS in Hyderabad, was set up in 2007. It provides warnings for India and other Indian Ocean countries.
5 November is observed as World Tsunami Awareness Day, declared by the UN in 2015.
Why not the other options?
- ✗Tides are caused by the gravitational pull of the Moon and the Sun.
- ✗Cyclones are storms formed over warm seas.
- ✗Ocean currents are caused by winds, the Earth's rotation and differences in temperature and salinity.
Quick tip
Remember: Tsunami ("harbour wave") = caused by undersea earthquakes, landslides or volcanic eruptions; India's warning centre = INCOIS, Hyderabad.
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Oceans & Hydrosphere — Preparation
This question tests your understanding of Oceans & Hydrosphere, a key topic in the General Knowledge section. The detailed solution above uses a step-by-step approach to show the reasoning, and where available, a shortcut that helps you solve similar questions in under 90 seconds during the actual exam.
This topic covers the hydrosphere: the oceans and seas of the world, ocean relief features such as trenches and ridges, ocean currents, tides, waves, salinity, and the world's major lakes, straits, gulfs and islands. Questions commonly ask about the largest and deepest ocean (the Pacific, with the Mariana Trench), warm and cold ocean currents, the causes of tides, and which water bodies a strait connects. For example, the Palk Strait separates India and Sri Lanka, and the Strait of Gibraltar connects the Atlantic Ocean and the Mediterranean Sea. Practise the oceans and hydrosphere questions below, written in the style of the general awareness sections of exams such as SSC, banking, railways, CUET GAT, MAH CET and CLAT. Each question comes with a detailed explanation that adds related facts, so every question helps you revise more than one point.