Tides, Waves & Tsunamis – NDA Geography Notes

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. Watch the waves, walls of moving water that travel across the ocean and crash on the shore. And occasionally, 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 are caused by completely different forces and behave in completely different ways. Understanding the difference between them is important for NDA.

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Tides: What They Are and What Causes Them

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. [NDA 2007-II]

How they form: The Moon’s gravitational pull draws water on the near side of the Earth toward it, creating a bulge (high tide). On the far side of the Earth, a second bulge forms due to the centrifugal effect of the Earth-Moon system. 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.

What determines the timing of tides? The rotation of the Earth. The Moon and Sun create the tidal bulges, but Earth’s rotation carries each location through these bulges and troughs, determining when high and low tide occur. [NDA 2010-I]

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Moon’s Tidal Effect vs Sun’s Tidal Effect

★ IMPORTANT This is one of the most precisely tested facts in this chapter and requires careful attention to two separate statements. [NDA 2007-II]

The Moon’s tidal effect on Earth is GREATER than the Sun’s, about 2.2 times stronger, despite the Sun being far more massive. This is because tidal force depends on the gradient of gravity (how rapidly the pull changes) across the Earth’s diameter. The Moon, being much closer, creates a far steeper gradient.

The Moon’s total gravitational pull at Earth’s surface is LESS than the Sun’s: this is true in terms of absolute gravitational force. But total pull and tidal effect are different things.

NDA 2007-II tested this as an Assertion-Reason question: – Assertion: “The tidal effect of Moon on Earth is less than that of Sun”: FALSE (Moon’s tidal effect is greater) – Reason: “Moon’s gravitational pull at Earth’s surface is less than Sun’s”: TRUE – Answer = (d) A false, R true: the Reason is correct but the Assertion is wrong. [NDA 2007-II]

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Types of Tides

Spring Tides

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

This alignment is called Syzygy and occurs at two Moon phases:

• New Moon (Moon between Sun and Earth = Syzygy Conjunction)
• Full Moon (Earth between Sun and Moon = Syzygy Opposition)

★ IMPORTANT Three NDA-confirmed facts about spring tides: Spring tides = greatest difference in sea level at high and low tides [NDA 2019-I] | Spring tides occur at Syzygy Conjunction AND Syzygy Opposition: both correct [NDA 2023-I] | Exceptionally high and low tides at New Moon or Full Moon when Sun, Moon and Earth are aligned = Spring tides [NDA 2011-I]

Equinoctial Spring Tides: the highest spring tides of the year occur when spring tides coincide with a solar equinox (March and September). These are the strongest tides of the year, occurring every 6 months.

Neap Tides

When the Moon is at 90° to the Sun-Earth line: called Quadrature: the gravitational pulls of the Sun and Moon partially cancel each other. This creates the weakest tides, the lowest high tides and the highest low tides (smallest tidal range).

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

Two confirmed NDA facts about neap tides: Neap tides occur every 14–15 days (roughly every half lunar cycle) | Tidal range is small because the gravitational forces of the Moon and Sun are in quadrature: both correct. [NDA 2024-I]

Quadrature causes Neap tides: NOT Spring tides. [NDA 2023-I]

Tide Type	Position	Moon Phase	Tidal Range	NDA Tag
Spring Tides	Syzygy (alignment)	New Moon + Full Moon	Maximum	[NDA 2011-I
Neap Tides	Quadrature (90°)	First Quarter + Last Quarter	Minimum	[NDA 2024-I]

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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 successive high tides is approximately 12 hours as an approximate answer. [NDA 2010-I]

More precisely, the interval 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]

Tidal Bore: when a very large tidal surge pushes up a funnel-shaped estuary, it creates a tidal bore. Famous examples: Bay of Fundy, Canada (world’s highest tidal range) and the Hooghly River in India.

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Ocean Waves

Waves are surface disturbances caused mainly by wind. The wind blows across the ocean surface, transferring energy to the water.

Key principle: 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 forward with the wave.

What Determines Wave Size

Three factors determine wave height:

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

All three confirmed correct in NDA 2025-II.

Energy of a wave is proportional to the square of its wave height. Doubling wave height quadruples its energy. [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]

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.

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Tsunamis

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

The word tsunami comes from Japanese: “tsu” = harbour, “nami” = wave. Tsunami = “harbour wave.” It is NOT a Latin word.

Causes of tsunamis:

1. Underwater earthquakes (most common; typically above magnitude 7.5 on Richter scale)
2. Submarine landslides
3. Volcanic eruptions
4. Meteorite impacts in the ocean (very rare)

Tsunamis are NOT caused by hurricanes. Hurricanes are atmospheric storms and cannot displace large volumes of ocean water.

Behaviour in Deep Ocean

In the deep ocean, a tsunami is almost undetectable:

• Wave height ≈ only 30–60 cm (less than a metre)
• Wavelength ≈ enormous: 100–200 km between crests
• Speed ≈ very fast: 700–900 km/h (like a jet aircraft)

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

What Happens in Shallow Water: Wave Shoaling

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

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

NDA 2013-I tested three statements about tsunamis approaching shore: [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 are enormous destructive waves; the gentle rise is only in the deep ocean)

Answer: 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.

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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.

Areas affected: Indonesia | Sri Lanka | India (Tamil Nadu, Andhra Pradesh coasts, and Andaman and Nicobar Islands) | Thailand | Maldives | East Africa.

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.

Response: The disaster led to the establishment of the Indian Ocean Tsunami Early Warning System and the Indian National Centre for Ocean Information Services (INCOIS) in Hyderabad.

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