Tides, Waves & Tsunamis – NDA Geography Notes

Introduction

Stand at the seashore and watch the water. Twice a day the sea rises and covers the beach — and twice a day it retreats. These rhythmic rises and falls are tides. Walk further and watch the waves — walls of moving water that travel across the ocean and crash on the shore. And occasionally, in regions near tectonic plate boundaries, a great earthquake sends a wave of enormous energy racing across the ocean at the speed of a jet aircraft — a tsunami.

These three phenomena — tides, waves, and tsunamis — are all movements of water but they are caused by completely different forces and behave in completely different ways. Understanding the difference between them is important both for NDA and for understanding how the ocean responds to the forces acting on it.

Understand the Topic

Part 1 — Tides

What are Tides?

Tides are the regular rise and fall of sea level caused by the gravitational pull of the Moon and the Sun on the Earth’s oceans. The Moon has a stronger tidal effect than the Sun despite the Sun being far more massive — because the Moon is much closer to the Earth. Tidal force depends on the difference in gravitational pull across the diameter of the Earth — and the Moon’s closeness makes its tidal gradient much steeper. [NDA 2007-II]

What causes tides?

The Moon’s gravitational pull pulls the water on the near side of the Earth toward it — creating a bulge of water (high tide) on that side. On the far side of the Earth, a second bulge forms due to the centrifugal effect of the Earth-Moon system rotating around a common centre. Between the two bulges, water is lower — creating low tides.

As the Earth rotates, different parts of the ocean pass through these bulges and troughs — experiencing two high tides and two low tides each day in most places.

Tidal effect of Moon vs Sun:

The assertion that “the tidal effect of Moon on Earth is less than that of Sun” is FALSE — the Moon’s tidal effect is GREATER than the Sun’s (about 2.2 times stronger), despite the Sun being far more massive. The gravitational pull of the Moon at Earth’s surface IS less than the Sun’s — but tidal force depends on the gradient of gravity across the Earth’s diameter, not just the total pull. [NDA 2007-II]

The statement that “Moon’s gravitational pull at Earth’s surface is less than Sun’s” — this is TRUE. [NDA 2007-II]

So in the NDA 2007-II question: Assertion = FALSE (Moon’s tidal effect IS greater than Sun’s); Reason = TRUE (Moon’s gravitational pull is less than Sun’s — but this does not correctly explain the assertion because the assertion itself is false). Answer: (d) A false, R true.

Movements of tides are mostly determined by:
The rotation of the Earth determines the timing and pattern of tides as different parts of the ocean pass through the tidal bulges. The Moon and Sun create the bulges; Earth’s rotation moves the ocean through them. [NDA 2010-I]

Types of Tides

Spring Tides:

When the Sun, Moon, and Earth are in a straight line (in alignment), the gravitational pulls of the Sun and Moon combine — reinforcing each other. This creates the strongest tides — the highest high tides and the lowest low tides.

This alignment is called Syzygy — and occurs:

• During New Moon (Moon between the Sun and Earth — conjunction)
• During Full Moon (Earth between the Sun and Moon — opposition)

Spring tides refer to the greatest difference in sea level at high and low tides. [NDA 2019-I]

Spring tides occur when Sun, Earth, and Moon are in alignment — at both syzygy conjunction (New Moon) AND syzygy opposition (Full Moon). [NDA 2023-I]

Quadrature — when Moon is at 90° to the Sun-Earth line — causes NEAP tides, not spring tides. [NDA 2023-I]

The tide produced when Moon and Sun pull Earth in the same direction = Spring Tide. [NDA tested via CDS]

Exceptionally high and low tides at New Moon or Full Moon when Sun, Moon and Earth are approximately aligned = Spring tides. [NDA 2011-I]

Highest spring tides occur at Full Moon or New Moon in association with the equinox — called equinoctial spring tides. These occur twice a year (March and September) and are the strongest tides of the year. [NDA tested via CDS]

Neap Tides:

When the Moon is at 90° to the Sun-Earth line — called quadrature — the gravitational pulls of Sun and Moon partially cancel each other. This creates the weakest tides — the lowest high tides and the highest low tides.

Neap tides occur at First Quarter and Last Quarter of the Moon cycle.

Neap tides occur every 14–15 days — TRUE (roughly every half lunar cycle). Small tidal range because gravitational forces of Moon and Sun are in quadrature — TRUE. [NDA 2024-I]

Equinoctial Spring Tides:

These occur at intervals of every 6 months — when spring tides coincide with a solar equinox. [NDA tested via CDS]

Frequency of Tides

In most places there are two high tides and two low tides every day — a semi-diurnal tidal pattern.

The interval between two high tides is approximately 12 hours — approximate answer. [NDA 2010-I]

More precisely, the interval between two successive high tides is 12 hours 26 minutes — because the Moon advances in its orbit while the Earth rotates, requiring the Earth to rotate a little extra to bring the same point back under the Moon. [NDA 2019-I]

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Tidal Bore

A tidal bore is a wave created when a very large tidal surge pushes up a funnel-shaped estuary. Famous examples occur in the Bay of Fundy (Canada — the world’s highest tidal range) and the Hooghly River in India.

Part 2 — Waves

What are Ocean Waves?

Ocean waves are surface disturbances caused mainly by wind. The wind blows across the ocean surface, transferring energy to the water. This energy travels across the ocean as waves.

Key point: In an ocean wave, it is the energy that moves forward — not the water itself. Water particles move in circular orbits and return to their original position. A floating object bobs up and down as a wave passes but does not travel with the wave.

What determines wave size?

1. Wind speed — faster wind = larger waves
2. Wind duration — longer blowing time = larger waves
3. Fetch — the distance of open water over which wind blows; longer fetch = larger waves

The height of waves is determined by wind speed, wind duration, and fetch — all three are correct. [NDA 2025-II]

The energy of a wave is proportional to the square of its wave height. [NDA 2025-II]

Waves retain most of their energy as they travel across the deep ocean. [NDA 2025-II]

All three statements about sea waves are correct: [NDA 2025-II]

1. Height determined by wind speed, duration, fetch — TRUE
2. Energy proportional to square of wave height — TRUE
3. Retain most energy crossing deep ocean — TRUE

Wave terminology:

• Crest = highest point
• Trough = lowest point
• Wavelength = distance between two successive crests
• Wave height = vertical distance from trough to crest
• Fetch = distance of open water over which wind blows

Part 3 — Tsunamis

What is a Tsunami?

A tsunami is a series of very long, fast-moving ocean waves generated by sudden large-scale disturbances on the ocean floor — mainly:

• Underwater earthquakes (most common cause; typically above magnitude 7.5 on Richter scale)
• Submarine landslides
• Volcanic eruptions
• Meteorite impacts in ocean (very rare)

The word tsunami comes from Japanese — “tsu” = harbour, “nami” = wave. Tsunami means “harbour wave.” [NDA tested via CDS]

Tsunamis are NOT caused by hurricanes — hurricanes are atmospheric storms and cannot generate tsunamis. Tsunamis need a sudden displacement of a large volume of ocean water. [NDA tested via CDS]

The statement “Tsunami is a Latin word” is FALSE — it is Japanese. [NDA tested via CDS]

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Behaviour of Tsunamis in Deep Ocean

In the deep ocean, a tsunami is almost undetectable:

• Wave height = only about 30–60 cm (less than a metre)
• Wavelength = enormous — 100–200 km between crests
• Speed = very fast — about 700–900 km/h (equivalent to a jet aircraft)

Ships at sea rarely notice a tsunami passing beneath them because the wave is so spread out and the height so small relative to its length.

Tsunamis are small in open ocean yet may be over 30 m high when reaching the coastline — TRUE. [NDA tested via CDS]

This is explained by: Tsunamis have long wavelength and travel at high speed across open ocean. As they approach shore, wavelength decreases and wave height increases — TRUE, and this correctly explains why. [NDA tested via CDS]

What Happens When a Tsunami Approaches Shore

As a tsunami enters shallow water near the coast, wave shoaling occurs:

1. The wave slows down — because the ocean floor creates friction
2. The wavelength decreases — waves pile up behind each other
3. The wave height increases dramatically — the energy compressed into a shorter distance grows taller
4. The wave can reach heights of 30 metres or more at the coastline

As tsunami leaves deep water and travels toward shallow water: [NDA 2013-I]

1. Speed considerably reduced — TRUE
2. They attain enormous height — TRUE
3. They appear as gentle rise and fall of sea — FALSE (in shallow water they appear as enormous destructive waves)
   Answer: (a) 1 and 2 only

The Pacific coasts are most vulnerable to tsunamis — because the Pacific is surrounded by the Ring of Fire with frequent large underwater earthquakes. [NDA tested via CDS]

Tsunami waves are generated by undersea earthquakes exceeding 7.5 on the Richter scale. [NDA tested via CDS]

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The 2004 Indian Ocean Tsunami

On December 26, 2004, a massive underwater earthquake of magnitude 9.1–9.3 struck off the northern coast of Sumatra, Indonesia. The earthquake displaced the ocean floor and sent tsunami waves across the Indian Ocean.

The tsunami struck Indonesia, Sri Lanka, India (Tamil Nadu, Andhra Pradesh coasts, and Andaman and Nicobar Islands), Thailand, Maldives, and East Africa.

Total death toll: over 230,000 people — one of the deadliest natural disasters in recorded history.

Indira Point — the southernmost tip of India on Great Nicobar Island — was partially submerged by the 2004 tsunami.

This disaster led to the establishment of:

• Indian Ocean Tsunami Early Warning System
• Indian National Centre for Ocean Information Services (INCOIS) in Hyderabad