This MCQ module is based on: Waves, Tides & Tsunamis
TOPIC 26 OF 29
Waves, Tides & Tsunamis
🎓 Class 11
Social Science
CBSE
Theory
Ch 13 — Movements of Ocean Water
⏱ ~28 min
🌐 Language: [gtranslate]
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13.1 Why the Ocean Is Never Still
Stand on a sea-shore at sunrise and you will notice three different kinds of motion playing out at the same time. The water rushes towards your feet and slides back — that is a wave. The whole shoreline slowly creeps higher up the sand over hours, then retreats — that is a tide. And somewhere out beyond the breakers, vast rivers of water are flowing across the sea, carrying warm or cold water from one continent to another — those are ocean currents. The ocean is never still: its water is dynamic. Its physical characters such as temperature, salinity and density, together with external forces of the Sun, the Moon and the wind, drive both horizontal and vertical motions in every ocean basin on the planet.
📖 Three Movements of Ocean Water — at a Glance
- Waves — horizontal motion of energy, not water. Wind transfers energy to the surface and sets it rolling.
- Tides — vertical rise and fall of sea level, once or twice a day, caused mainly by the gravitational pull of the Moon and the Sun.
- Ocean currents — continuous, river-like flow of large volumes of water in a definite direction across an ocean basin.
The horizontal motion of the ocean is represented by waves? and currents — water (or its energy) is shifted across the surface from one place to another. The vertical motion is represented by tides, by the upwelling of cold water from the deep, and by the sinking of dense surface water at high latitudes. In ocean currents, real water particles travel for thousands of kilometres along a definite path. In waves, by contrast, the water itself does not move forward — only the wave train moves. A floating bottle bobs up and down as the wave passes; it does not get carried to the horizon.
Waves
Energy moves horizontally across the surface. Water particles only travel in small circles. Wind is the master cause.
Tides
Vertical rise and fall of sea level, once or twice a day, due to the gravitational pull of the Moon and Sun, plus centrifugal force.
Currents
Real water flows long distances in a definite direction, like a river inside the ocean. Driven by wind, gravity and Coriolis force.
Upwelling & Sinking
Cold deep water rises to the surface; dense surface water sinks at high latitudes. These are vertical motions too.
13.2 Waves — Energy Walking Across the Sea
Waves? are actually energy, not the water as such, that moves across the ocean surface. As a wave passes, water particles only travel in a small circle and return to almost the same spot. Wind provides the energy that creates the wave; the energy is finally released on shorelines as the wave breaks. The motion of the surface water seldom disturbs the deep, stagnant water at the bottom of the ocean. As a wave approaches a beach it slows down, because friction begins between the moving water and the sea floor. When the depth of water becomes less than half the wavelength, the wave loses its balance and breaks — collapsing into surf.
🌬️ How Wind Builds a Wave
Most waves are caused by the wind driving against water. When a breeze of about two knots or less blows over calm water, small ripples form. As the wind speed increases the ripples grow into rolling waves; eventually white caps appear at the breaking crests. Once formed, waves can travel thousands of kilometres before rolling ashore, breaking and dissolving as surf. They keep absorbing energy from the wind and growing larger as they travel.
The size and shape of a wave reveal its origin. Steep waves are fairly young and have probably been formed by a local wind. Slow and steady waves originate from far away — possibly from another hemisphere altogether. The maximum wave height that the open sea can produce is determined by three things: the strength of the wind, how long it blows, and the distance over which it blows in a single direction. The open oceans, with the longest unbroken stretches of wind, naturally hold the largest waves.
Waves travel because the wind pushes the water body forward in its course while gravity pulls the crests downward. The falling water in turn pushes the former troughs upward, and the wave moves to a new position. Beneath the surface, the actual motion of the water is circular: things floating on the wave are carried up and forward as the crest approaches, and down and back as the trough passes. The wave shape moves on; the water does not.
Characteristics of a Wave
Anatomy of a Wave — Crest, Trough, Length and Height
📖 Seven Properties of a Wave (Definitions)
- Wave crest and trough — the highest and lowest points of a wave respectively.
- Wave height — the vertical distance from the bottom of a trough to the top of a crest.
- Wave amplitude — one-half of the wave height.
- Wave period — the time interval between two successive wave crests (or troughs) passing a fixed point.
- Wavelength — the horizontal distance between two successive crests.
- Wave speed — the rate at which the wave moves through the water, measured in knots.
- Wave frequency — the number of waves passing a given point during one second.
| Term | Symbol idea | What it tells us |
|---|---|---|
| Wave height (H) | Vertical | How "tall" the wave is — energy carried per unit length |
| Wavelength (L) | Horizontal | Distance between two crests — used in the breaking rule (depth < L/2) |
| Period (T) | Seconds | How often crests arrive at a fixed point |
| Frequency (f) | Per second | 1 ÷ Period; counts of waves per second |
| Speed (c) | Knots | How fast the wave-shape travels (in deep water, c ≈ L/T) |
Wave Breaking, Swash and Backwash
As a wave reaches shallower water near the coast, friction slows the lower part of the wave but not the top. The crest gets thinner, leans forward, and finally tumbles down — the wave breaks. The water that rushes up the beach after a wave breaks is called the swash?. The water that drains back down the slope into the next wave is called the backwash?. The continuous see-saw of swash and backwash is what shapes a beach — building it up when swash is stronger, dragging sand seaward when backwash is stronger.
🌊 Tsunami — A Wave Born in the Earth, Not the Wind
Most waves are wind-driven, but a tsunami? is something different. A tsunami is generated by a sudden vertical disturbance of the sea floor — usually a submarine earthquake, but also a volcanic eruption or a massive landslide. In the open ocean a tsunami has a very long wavelength (sometimes hundreds of kilometres) and very small height (under 1 metre), so ships at sea barely feel it. But it travels at jetliner speeds — up to 800 km/h. As it approaches a coast and the water grows shallow, the wavelength shrinks while the energy is conserved, so the wave height rises dramatically — sometimes to 10–30 metres — devastating low-lying coastal areas. The Indian Ocean tsunami of 26 December 2004 killed more than 230,000 people from Indonesia to Somalia.
LET'S EXPLORE — A Wave on a Pond
L3 Apply
NCERT suggests this Project: visit a lake or pond, throw a stone, and watch waves spread out. What do you actually see — does any water travel from the centre to the edge, or does only the wave shape spread?
What you see is exactly the principle of all ocean waves. The stone disturbs a small patch of water, and the disturbance spreads outward as circular ripples. If you drop a leaf on the surface a metre away from the impact, watch carefully: as the wave passes, the leaf bobs up, then forward, then down, then back — it traces a small circle and returns to roughly the same spot. It is not swept to the edge of the pond. Only the energy travels outward; the water itself stays. This is precisely how an ocean wave moves: a bottle floating in the open sea moves up-and-forward, down-and-back, up-and-forward as each crest passes — but is not carried out across the ocean.
13.3 Tides — The Vertical Pulse of the Sea
The periodical rise and fall of sea level, once or twice a day, mainly due to the attraction of the Sun and the Moon, is called a tide?. The movement of water caused by meteorological effects — winds and atmospheric pressure changes — is called a surge. Surges are not regular like tides. The study of tides is very complex, both spatially and temporally, because tides have great variations in frequency, magnitude and height from one place and one day to the next.
📖 Definition — Tide
A tide is the regular vertical rise and fall of sea level produced mainly by the gravitational pull of the Moon, and to a smaller extent the Sun, on the waters of the Earth — once or twice a day. Storm surges, which look similar, are caused by weather (winds and low pressure) rather than gravity, and are not rhythmic.
What Causes a Tide? Gravity and Centrifugal Force
The Moon's gravitational pull (to a great extent) and the Sun's gravitational pull (to a lesser extent) are the major causes of tides. Another factor is centrifugal force, which is the force that acts to counterbalance gravity as the Earth–Moon pair revolves around their common centre of mass. Together, the gravitational pull and the centrifugal force are responsible for creating two major tidal bulges on the Earth — one on the side facing the Moon and one on the opposite side.
On the side of the Earth facing the Moon, the Moon's pull is stronger than the centrifugal force — so a tidal bulge forms towards the Moon. On the opposite side, the Moon's pull is weaker (because that side is farther from the Moon) and the centrifugal force is dominant — so a second bulge forms away from the Moon. The "tide-generating" force is the difference between these two forces — gravitational attraction of the Moon and centrifugal force. On the surface of the earth, the horizontal tide-generating forces are more important than the vertical forces in creating the tidal bulges.
Two Tidal Bulges — Why Most Coasts See Two High Tides Each Day
The size of each tidal bulge is shaped not only by gravity, but by the local geometry of the coast as well. Tidal bulges on wide continental shelves have greater height, because the shallowing water amplifies the rise. When tidal bulges hit mid-oceanic islands they become low, because the deep ocean around them does not concentrate the energy. The shape of bays and estuaries along a coastline can also magnify the intensity of tides — funnel-shaped bays greatly change tidal magnitudes by squeezing the rising water into ever-narrower channels. When a tide is channelled between islands or into bays and estuaries, the moving water is called a tidal current.
🇨🇦 Tides of the Bay of Fundy, Canada
The highest tides in the world occur in the Bay of Fundy in Nova Scotia, Canada, where the tidal bulge reaches 15–16 metres. Because there are two high tides and two low tides every day (a 24-hour period), each tide must come in within about a six-hour period. As a rough estimate, the tide rises about 240 cm an hour (1,440 cm divided by 6 hours). Walk down a beach with a steep cliff alongside (which is common there), and you must watch the tide carefully — if you walk for an hour and then notice the tide is coming in, the water will be over your head before you get back to where you started.
Types of Tides
Tides vary in their frequency, direction and movement from place to place and also from time to time. They may be grouped into different types based on their frequency in 24 hours, or based on their height in relation to Sun-Moon-Earth positions.
Tides Based on Frequency
Semi-diurnal Tide
The most common pattern — two high tides and two low tides each day. The successive high or low tides are approximately of the same height.
Diurnal Tide
There is only one high tide and one low tide during each day. The successive high and low tides are approximately of the same height.
Mixed Tide
Tides having variations in height. Generally observed along the west coast of North America and on many islands of the Pacific Ocean.
Tides Based on Sun, Moon and Earth Positions — Spring & Neap
The height of the rising water (high tide) varies appreciably depending on the position of the Sun and the Moon with respect to the Earth. Spring tides and neap tides come under this category.
🌕 Spring Tide — Higher than Average
When the Sun, the Moon and the Earth are in a straight line (a syzygy), the gravitational pulls of the Sun and Moon add up. The height of the tide is greater than usual. These are called spring tides? and they occur twice a month — once on the full-moon period and once during the new-moon period. (The name has nothing to do with the season "spring" — it comes from the older sense of "spring up".)
🌓 Neap Tide — Lower than Average
Normally, there is a seven-day interval between spring tides and neap tides?. At neap tide, the Sun and Moon are at right angles to each other (first and third quarter moons), and the forces of the Sun and Moon tend to counteract one another. The Moon's attraction, though more than twice as strong as the Sun's, is diminished by the counteracting force of the Sun's gravitational pull — so the high tide is lower and the low tide is higher than average.
Spring Tide vs Neap Tide — The Sun-Moon-Earth Geometry
Perigee, Apogee, Perihelion and Aphelion — Tidal Range Variations
Once in a month, when the Moon's orbit is closest to the Earth (perigee), unusually high and low tides occur. During this time the tidal range (difference between high tide and low tide) is greater than normal. Two weeks later, when the Moon is farthest from the Earth (apogee), the Moon's gravitational force is limited and tidal ranges are less than their average heights.
When the Earth is closest to the Sun (perihelion), around 3 January each year, tidal ranges are also much greater, with unusually high and unusually low tides. When the Earth is farthest from the Sun (aphelion), around 4 July each year, tidal ranges are much less than average. So the highest tides of all years are seen when perigee, perihelion and a new or full Moon line up — three accidents of geometry stacking together.
Tidal Range — How Much Bigger Are Spring & Perigee Tides?
Schematic comparison of average daily tidal range at four well-known coasts. Bay of Fundy (15–16 m bulge per NCERT) holds the world record.
Ebb, Flow and Tidal Bore
The time between a high tide and the next low tide, when the water level is falling, is called the ebb. The time between a low tide and the next high tide, when the water level is rising, is called the flow or flood. In some funnel-shaped estuaries, the flow rushes upstream so violently that it forms a single steep wall of water — a tidal bore?. Famous examples include the Hooghly estuary near Kolkata, the Bay of Fundy in Canada, the Bristol Channel in England, and the Pororoca on the Amazon River in Brazil — where surfers have ridden the bore for over thirty kilometres at one stretch.
SOURCE — From the NCERT Textbook
L2 Understand
NCERT writes: "The Moon's attraction, though more than twice as strong as the Sun's, is diminished by the counteracting force of the Sun's gravitational pull." Explain in your own words what this sentence is describing.
This sentence describes the situation during a neap tide. The Moon, although small, is so close that its tide-raising effect is more than twice as strong as the Sun's. But during the first and third quarter moons, the Sun lies at right angles to the Moon as seen from Earth — so the Sun's gravitational pull tries to raise water in a perpendicular direction, partly cancelling the Moon's pull. The result is that the Moon's bulge is reduced (not eliminated), and high tides on these days are noticeably lower than at full or new Moon. So neap tides are not "no tides" — they are simply weaker tides where the two pulls work against each other instead of together.
Importance of Tides
Because tides are caused by Earth-Moon-Sun positions which are known accurately, tides can be predicted well in advance — months and even years ahead. This predictability helps navigators and fishermen plan their work. Tidal flows are of great importance in navigation: many large harbours lie near rivers and within estuaries which have shallow "bars" at the entrance — the rising tide deepens the channel just enough for ships and boats to enter the port. Tides are also helpful in desilting sediments and in removing polluted water from river estuaries — the daily ebb-and-flow flushes the river mouth like a giant pulse-pump. Tides are also used to generate electrical power, in Canada, France, Russia and China; in India, a 3 MW tidal power project at Durgaduani in the Sundarbans of West Bengal is under way.
⚓ Why Big Ports Read Tide Tables
Calcutta-Haldia, Mumbai, Kandla and Mangalore — every major Indian port publishes a tide table. Captains schedule the entry of large container ships only at the high tide so that the keels do not scrape the silt bar at the harbour mouth. A miscalculation can ground a 200-metre-long ship — and unloading it can cost a million dollars per day of delay.
Navigation
High tide deepens river-mouth bars, letting ocean-going ships pass safely into estuarine ports.
Fishing
Predictable tide tables help fishermen choose the safest, most productive hours to put out and to return.
Tidal Power
Canada, France, Russia and China generate electricity from tidal flows. India's 3 MW Durgaduani project (Sundarbans) is underway.
Estuary Cleansing
Daily ebb-and-flow desilts sediments and flushes polluted water out of river estuaries.
THINK ABOUT IT — Why Two Tides a Day, Not One?
L4 Analyse
If only one side of the Earth faces the Moon at any time, common sense says only one bulge — and so only one high tide a day. Yet most coasts of the world record two high tides and two low tides each day. How is this possible?
The key is that there are two bulges, not one. On the Moon-facing side, the Moon's gravity pulls the water towards itself — first bulge. On the side opposite to the Moon, the centrifugal force of the rotating Earth-Moon system is greater than the Moon's pull (because that side is farther from the Moon, so its pull is weaker). This produces a second bulge away from the Moon. As the Earth rotates once on its axis in 24 hours, every coastline passes under both bulges and therefore experiences two highs and two lows each day. This is the semi-diurnal pattern, the most common kind of tide on Earth.
🎯 Competency-Based Questions — Waves & Tides
Case Stem. A coastal navigator is preparing the entry plan for a 9-metre-draught cargo ship into the Hooghly estuary near Kolkata. The chart says the bar at the river mouth is only 7 metres deep at low water but 12 metres deep at high water. The captain consults the tide table: today is a full moon in early January, the moon is at perigee, and a tidal bore of 1.5 m height is expected on the flood. Use this scenario for Q1–Q4.
Q1. Why will today's high tide be especially high?
L3 ApplyAnswer: (b). A full moon places Sun-Earth-Moon in a line (spring tide). Perigee places the Moon closest to Earth that month. Early January is near perihelion, when the Earth is closest to the Sun. All three boost the gravitational pull on water — together they produce the highest tides of the year, just what the captain needs to clear a 9-metre bar.
Q2. The captain receives a warning that a tidal bore is expected. What is a tidal bore, and why is the Hooghly especially prone to it?
L4 AnalyseA tidal bore is a single steep wall of water that races upstream during the flood tide. It forms when a large incoming tide is forced into a shallow, funnel-shaped estuary. The Hooghly mouth narrows and shallows rapidly as it goes inland, squeezing the rising tide into a vertical wall — sometimes more than a metre tall. Famous bores also occur in the Bay of Fundy, the Bristol Channel and the Amazon (Pororoca). The captain must time the entry so the ship is not caught broadside by the bore.
Q3. Two weeks later the same ship arrives at the same bar at high tide, but the high tide is much lower. Why?
L5 EvaluateTwo weeks after a spring tide is a neap tide. The Moon is now at first or third quarter, lying at right angles to the Sun, so the two pulls partially cancel each other. High tide is lower than average and low tide is higher than average. Additionally, the Moon may have moved from perigee to apogee (farthest from Earth), reducing its gravitational effect further. The bar may now be too shallow to admit the ship — the captain may have to wait two more weeks for the next spring tide.
HOT Q. Imagine an undersea earthquake suddenly strikes 200 km off the coast. Sketch in five steps how a tsunami forms, travels and devastates a coastline — naming the energy source and the change in wavelength & height as the wave nears land.
L6 CreateHint: (1) Earthquake on the sea floor lifts or drops a huge column of water — the energy source is tectonic, not wind. (2) The displaced water spreads outward as a wave with very long wavelength (often 100+ km) but small height (under 1 m) in the open ocean. (3) The wave races at jetliner speed (~800 km/h), almost unnoticed by ships. (4) As it approaches the coast and water shallows, friction slows the front of the wave; energy is conserved, so the wave height shoots up — sometimes 10–30 m. (5) A wall of water sweeps inland, drowning low-lying areas — the 26 December 2004 Indian Ocean tsunami killed 230,000+ people. The ratio "depth < ½ wavelength" defines where the wave begins to break — for a 100-km wavelength, that means anywhere shallower than 50 km of ocean.
⚖️ Assertion–Reason Questions — Class 11
Options:
(A) Both A and R are true, and R is the correct explanation of A.
(B) Both A and R are true, but R is NOT the correct explanation of A.
(C) A is true, but R is false.
(D) A is false, but R is true.
(A) Both A and R are true, and R is the correct explanation of A.
(B) Both A and R are true, but R is NOT the correct explanation of A.
(C) A is true, but R is false.
(D) A is false, but R is true.
Assertion (A): A floating bottle in the open sea is not carried away by passing waves but bobs up and down at almost the same spot.
Reason (R): Waves are energy moving across the ocean surface, not water itself; water particles only travel in small circles as a wave passes.
Answer: (A) — Both statements are true and R precisely explains A. The wave shape moves; the water does not. Surfers, swimmers and floating objects experience exactly this circular orbital motion — up-and-forward as the crest approaches, down-and-back as the trough passes.
Assertion (A): Most coasts of the Earth experience two high tides and two low tides every day.
Reason (R): The combined effect of the Moon's gravitational pull and the centrifugal force of the rotating Earth-Moon system creates two tidal bulges — one on the side facing the Moon and one on the opposite side.
Answer: (A) — Both true and R is the correct explanation. As the Earth rotates once in 24 hours, every coastline passes under both bulges, producing the semi-diurnal pattern. Diurnal and mixed tides occur where local geometry distorts the basic two-bulge pattern.
Assertion (A): A tsunami in the open ocean is barely felt by ships, but causes devastation on the coast.
Reason (R): A tsunami has a very long wavelength and small height in deep water, but as it approaches the shallow coast its wavelength shrinks and the wave height rises sharply because energy is conserved.
Answer: (A) — Both statements true; R is the correct explanation of A. In the open ocean a tsunami may be only 1 m high but 100 km long, hidden under a moving ship. As it shoals near the coast the leading edge slows, the rear of the wave piles up behind it, and the height grows tens of times. This is why mid-ocean ships in 2004 reported nothing unusual until news arrived from devastated coasts.
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Class 11 Geography — Fundamentals of Physical Geography
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