Class 6 Social Science Chapter 2: Oceans and Continents

What colour is the Earth?

Spin a globe gently and watch the colours go by. Or look at a photograph of the Earth taken from space. One colour wins easily, and it is blue. That blue is water. Almost three-fourths of the Earth's surface is covered by it, which is why the planet looks the way it does from far off. The first astronauts to see the whole Earth at once gave it a name out of affection: the blue planet.

The other colour

Look again at the same photograph and a second colour appears. Brown, in patches, with green among it. That is land, and it takes a little over one-fourth of the surface. Put the two together and you have the whole surface: roughly three parts water to one part land. There is a third colour too, white, in swirls across the picture. Those are great masses of cloud, and we shall come back to them.

Oceans, and their smaller arms

The largest bodies of water on the Earth are called oceans. They are not all there is to water, though. Look closely at a world map and you will find narrower arms reaching into the land. Each has a name of its own: a sea here, a bay there, a gulf somewhere else. These are extensions of the oceans rather than separate things. The ocean simply reaches inland and the map gives that part its own label.

Landmass and continent

Land gets two words, and they are not the same. Any large body of land is a landmass. A large continuous stretch of land is a continent. Notice the word continuous, because it is doing the work. A continent is one unbroken expanse, not a scattering of pieces. Every continent is a landmass, but a landmass may be far too small to be called a continent.

Not shared equally

Water and land are not spread evenly over the Earth. Look at two maps drawn as if you were hovering above the North Pole and above the South Pole. They will not look alike. One view is crowded with land and the other is mostly water. The Northern Hemisphere holds more land than the Southern does, and the Southern therefore holds more water. That single fact shapes a great deal, from climate to where people have settled.

Reading a polar map

A polar map is an unfamiliar sight at first, because the lines look wrong. The circles running round and round are the parallels of latitude. The lines radiating out from the centre like spokes are the meridians of longitude. They are the same grid you met in the last chapter, only drawn from above a pole instead of from the side. Once you recognise the grid, the map becomes readable.

Are the oceans separate?

Look at either polar map and ask whether the blue areas stop anywhere. They do not. You can trace a path through water from one ocean into the next without ever crossing land. The oceans are connected, and that is worth holding on to. The names we give them are our own arrangement for talking about one enormous, continuous body of water.

Most of the water is undrinkable

The oceans hold most of the water on the planet, but almost none of it is any use for drinking. Seawater is salty, and most land animals cannot drink it. Humans cannot. Neither can a field of wheat. So the great blue expanse that makes our planet look so well supplied is, for our purposes, very nearly unusable.

Where the freshwater is

Freshwater is the water that is not salty, and it is a very small share of the planet's total. It is held in five places worth remembering. Glaciers hold a great deal of it, frozen. Rivers carry it. Lakes store it. Some is in the atmosphere, as vapour and cloud. And a large amount lies underground, out of sight, where it is called groundwater.

Why there is a water crisis

Here is a question worth sitting with. If the planet has so much water, why do we hear so much about water scarcity? The answer is in the two facts above. Almost all the water is salty, and the freshwater is a thin slice of what is left. That slice is not spread evenly either, and it can be used faster than it is replaced, or spoilt by waste. Abundance and usefulness are not the same thing.

Saving what we have

Because the usable share is so small, saving water is not a small matter. Think about where it is used at home, at school and in your village or town. A tap left running while you brush. A leaking pipe nobody has reported. Water from washing vegetables poured away rather than given to a plant. Rainwater running off a roof into a drain instead of into a tank. Each of these is small, and there are a great many of them.

Why any of this matters

Oceans and continents are not just features to be named and marked on a map. They shape the climate of the whole Earth. What the climate does affects every plant and every animal, and so it affects people too. You can see their mark in our history, in our trade, in our food and in our stories. The rest of this chapter follows that thread.

Scenario:

A class is told that almost three-fourths of the Earth's surface is water. Rohit says that cannot be right, because the world map on the wall looks mostly land. Their teacher hands out a globe instead. The children cover it with small paper squares and count how many fall on water and how many on land. The water squares run to roughly three for every one on land. Sana sees why the wall map misled them. It is drawn flat, and flattening a sphere stretches some parts more than others. The globe and the figure agree, and it was the map that was distorting the answer.

Analysis:

Rohit read the proportion off a flat map, which cannot show a sphere without stretching it. Counting squares on a globe is what reconciles the figure with what the eye sees.

Activity:

Cover a globe or a ball with small paper squares. Count those on water and those on land, and write the ratio you get.

Title:

Four glasses and a spoon

What You Notice:

The three full glasses look like plenty until the fourth step. Seeing the usable share shrink to part of a spoonful makes the phrase water scarcity mean something, which reading the figure alone rarely does.

Safety:

Pour the leftover water onto a plant rather than down a drain, and mop up any spills so nobody slips.

The emblem of the Indian Navy carries the motto Sam noh Varunah, a request that Varuna be favourable to us. Varuna is a Vedic deity linked with water — with the oceans and the sky alike.

The first astronauts to see the whole Earth at once gave it an affectionate nickname. They called it the blue planet, and the name has stuck ever since.

The word groundwater means exactly what it says: freshwater held underneath the ground, out of sight. It is one of the five places freshwater is stored.

How the Earth's surface divides — Three parts water, one part land — Picture the surface cut into four. Three of those parts are ocean and one is land. That is why the planet looks blue.

Where freshwater hides — GRLAG — Glaciers, Rivers, Lakes, Atmosphere, Groundwater. Five places, and all of them together are only a tiny share of the planet's water.

Five names on the map

Open a world map and you will find five oceans marked. The Pacific Ocean, the Atlantic Ocean, the Indian Ocean, the Southern Ocean — also called the Antarctic — and the Arctic Ocean. Those five names are worth learning properly, because almost everything else in this chapter hangs off them. Find each one on a map with a finger before you go further.

Which is biggest?

The five are nothing like equal in size. The Pacific is the largest by a long way. The Atlantic comes second. The Indian Ocean is third, the Southern Ocean fourth, and the Arctic Ocean is the smallest of them all. You can see this on a map without measuring anything. The Pacific takes up so much room that most world maps have to split it, showing part at each edge of the page.

The lines are ours

Look at the map again and notice the boundaries drawn between the oceans. Now ask what those lines are made of. Nothing. They are conventions, agreed on so that we have a way of talking about different parts of one enormous body of water. The natural world pays no attention to them. Seawater flows steadily across every one of those lines, and it always has.

Life that crosses the lines

Because the water moves between the oceans, so does the life in it. Many plant and animal species turn up in several oceans at once. A whale does not belong to the Atlantic in the way a person belongs to a country. It swims where the food is. This is one of the clearest ways of seeing that the five oceans are five names rather than five separate places.

Marine flora

The plant life of a place is called its flora, and marine means of the sea. So marine flora is the plant life of the oceans. It includes algae, which are often so tiny that a single one cannot be seen, and it includes every kind of seaweed. There is a great deal of it, and later in the chapter we shall see just how much the whole planet depends on it.

Marine fauna

The animal life of a place is its fauna. Marine fauna is the animal life of the oceans, and it is enormous. Thousands of species of colourful fish. Dolphins and whales. Sharks, turtles, starfish and penguins. And in the deep water, where no sunlight reaches, creatures so strange and so little known that new ones are still being found.

From the surface to the deep

An ocean is not one habitat but many stacked on top of each other. The sunlit surface layer is bright, warm and busy. Below it the light fades. Deeper still it is completely dark and very cold. Each of those layers has its own life forms, suited to that depth and found nowhere else. So the variety in the ocean is not only across its width but all the way down.

What bounds the Indian Ocean

The Indian Ocean has clear limits, and they are easy to remember from a map. Asia lies to its north. Africa is to the west. Australia is to the east. And to the south it runs into the Southern Ocean. Hold those four in mind and you can place the Indian Ocean on any world map without hunting for a label.

The two seas beside India

Closer to home, two parts of the Indian Ocean lie on either side of our country. To the west is the Arabian Sea. To the east is the Bay of Bengal. They are not separate from the Indian Ocean; they are arms of it, reaching up on each side of the Indian peninsula. Much of India's coast, its ports and its fishing belong to one or the other.

Oceans send the rain

Remember the white swirls in the photograph of the Earth from space. Those are great masses of cloud, and they form over the ocean. When they drift over the continents they bring rain. The monsoon rains India waits for every summer begin over the ocean. Without that rain there would be no agriculture worth the name, and life here would suffer badly. This is the oceans at their most generous.

And sometimes the storm

The same oceans also raise storms. A cyclone is a violent event with extreme rainfall or very strong winds, and it forms over the sea before moving towards land. When a cyclone reaches a coast it can cause widespread damage to homes, crops, boats and lives. The ocean that waters our fields in one season can flatten a coastal town in another.

What a tsunami is

A tsunami is a different kind of danger, and it does not begin with weather at all. It is a very large and very powerful wave, set off when the sea floor is jolted by an earthquake or shaken by a volcano erupting under water. The disturbance on the sea floor pushes the water above it, and that push travels outward. A tsunami can cross thousands of kilometres and then flood coastal areas, causing enormous damage.

December 2004, and what was learnt

On 26 December 2004 an earthquake in Indonesia sent a tsunami across the Indian Ocean. India and thirteen other countries were struck. More than two lakh lives were lost. Within India the worst damage fell on the Andaman and Nicobar Islands, and on the coasts of Tamil Nadu and Kerala. Such tsunamis are rare, but they are very destructive. The lesson taken from that day was that they can often be detected before they reach a coast.

Warning, and disaster management

So many countries now work together on early warning systems. There is an Indian Ocean Tsunami Warning System, and India is one of the countries contributing to it. A warning sent by radio travels far faster than a wave, which is what makes the whole idea work. More broadly, events that cost lives and property are handled under disaster management, and India has a National Disaster Management Authority for disasters of every kind.

Scenario:

A fishing village on the coast of Tamil Nadu holds a meeting. The panchayat wants to know what the village should do when the sea threatens it. An older fisherman remembers 26 December 2004. A tsunami crossed the Indian Ocean that day, from an earthquake in Indonesia. India and thirteen other countries were struck. More than two lakh people died. A younger member argues that the village already watches the weather for cyclones, so it is prepared. The teacher at the meeting puts two questions to the class. Would watching the weather have helped that December morning? And what else would the village need?

Analysis:

Watching the weather would not have helped, and that is the heart of it. A cyclone is a weather event that builds over the sea. A village can see it coming in the sky and in the forecasts. A tsunami is not weather at all. It begins when the ocean floor moves in an earthquake or eruption, and the sky above may be perfectly clear. So the village needs two kinds of preparation, not one. For cyclones, weather warnings and safe shelters. For tsunamis, a warning that comes from far away. The useful fact is that a tsunami takes time to cross thousands of kilometres. It can often be detected before it arrives. That is why many countries collaborate on early warning systems, including the Indian Ocean Tsunami Warning System, to which India contributes. A radio message travels far faster than a wave. Beyond both, India handles such events through the National Disaster Management Authority.

Activity:

Draw two columns in your notebook, headed Cyclone and Tsunami. Under each, write where it begins, whether the sky gives a warning, and one action the village should take. Then underline the single fact that makes an early warning system possible.

On 26 December 2004 a powerful tsunami struck India and thirteen other countries around the Indian Ocean. Over two lakh lives were lost.

Countries around the Indian Ocean now share an early warning system for tsunamis, and India is one of the contributors to it.

India has its own National Disaster Management Authority, set up to deal with disasters of every kind rather than one sort alone.

The five oceans by size — Pacific, Atlantic, Indian, Southern, Arctic — Learn them in that order and you have both the names and the sizes at once, largest first and smallest last.

The two seas beside India — A to the west, B to the east — Arabian Sea on the west of India, Bay of Bengal on the east. The first letters run in the same order as the alphabet, west to east.

A question with more than one answer

How many continents are there? It sounds like a question with one right answer, and it is not. Depending on how you choose to count, you may list anything from four to seven. This is not a muddle in the textbooks. It happens because a continent is defined as a large continuous stretch of land, and deciding where one continuous stretch ends and another begins turns out to involve a judgement.

America: one or two?

North America and South America are generally treated as two continents. Look at a map, though, and you will see they are joined by a narrow neck of land. Seen as a single landmass, they can just as reasonably be called one continent. Neither view is wrong. The two names are useful, and so is the single one, depending on what you are trying to say.

Europe and Asia

Here the case is sharper. Europe and Asia are generally counted as two continents, yet the map makes it perfectly clear that they form one unbroken landmass. Nothing separates them. They are counted separately for historical and cultural reasons, because Europe's development ran differently from Asia's. Geologists, who care about the rock rather than the history, often treat the whole thing as one continent and call it Eurasia.

Africa and Eurasia

The same argument can be pushed one step further. Africa and Eurasia are usually counted as two, but they are joined at one corner, and sometimes they are treated as a single continent. Put all these choices together and you get the range: four continents at the fewest, seven at the most.

The four counts

It is worth seeing the counts laid out. Four gives Africa-Eurasia, America, Antarctica and Australia. Five gives Africa, America, Antarctica, Australia and Eurasia. Six gives Africa, Antarctica, Australia, Eurasia, North America and South America. Seven gives Africa, Antarctica, Asia, Australia, Europe, North America and South America. In practice the list of seven is the one almost everyone uses, and it is the one to learn.

The Olympic rings

You have almost certainly seen the five interlocking rings that serve as an Olympic symbol. They stand for sportspeople gathering from all over the world, and the number five was picked out five continents that people live on — Africa, America, Asia, Australia and Europe. Notice which one is missing, and why. Antarctica has no permanent population to send a team.

Comparing their sizes

The seven continents are nothing like equal. From largest to smallest they run: Asia, Africa, North America, South America, Antarctica, Europe and Australia. Asia is far ahead of the rest. Australia is the smallest, which is why it is sometimes discussed alongside very large islands. Put Europe and Asia together as Eurasia and the combined landmass dwarfs every other entry on the list.

What the continents leave out

Look carefully at a world map and you will notice that the continents do not account for all the land. Smaller pieces are dotted about, some in mid-ocean, surrounded by water on every side. These are islands. There are lakhs of them on the planet, of every imaginable size, from a rock a boat could circle in a minute to something the size of a country.

Why a continent is not an island

Here is a fair objection. A continent is surrounded by water on all sides too, so why is it not an island? The honest answer is size, and nothing else. Continents are so large that we treat them as a separate category. The line between the two is one we have drawn for convenience, much like the boundaries between the oceans.

The largest island, and our own

The world's biggest island is Greenland. To get a sense of it, you would have to add up the areas of the ten largest states of India to reach the same size. Closer to home, India has more than 1,300 small islands. Two groups matter most: the Andaman and Nicobar group, out in the Bay of Bengal, and the Lakshadweep group, off the western coast.

India in Antarctica

Antarctica is a continent of very cold climate and harsh conditions, most of it under ice. India has been working there since 1981, through the Indian Antarctica Programme. The first Indian station, Dakshin Gangotri, was set up in 1983, and two more have followed. About forty teams of Indian scientists have carried out research there, particularly on how climate and environment have changed over time. The settlement even runs a library and a post office.

What the oceans do for us

Now the larger point of the chapter. The oceans are not scenery. They send rain to the continents as one stage of the Earth's water cycle, a topic Science takes further. Without them there would be no rainfall at all, and the Earth would be a desert. Marine flora makes over half the oxygen in the world, and that is why the oceans are sometimes called the planet's lungs. They regulate climate and they sustain life.

Oceans in human history

People have leaned on the oceans for as long as there have been people near a coast. From early times they have carried people migrating to new regions, and goods of every kind in trade. They have carried military campaigns, and they have fed people through fishing. They have shaped culture too. Almost every coastal people has its tales of the sea: sea gods and goddesses, sea monsters, treasures from the deep, the ocean's dangers and its blessings.

A responsibility, not just a resource

The United Nations has set 8 June aside as World Oceans Day, to remind everyone how much the ocean does in daily life. Scientific studies show how badly human activity has polluted it. Several million tonnes of plastic waste are dumped into the sea each year, and it chokes marine life, and there are other forms of pollution besides. Overfishing, taking fish faster than they can be replaced, is another cause of decline. Protecting the oceans is a collective responsibility.

Scenario:

A class is asked how many continents there are. Answers come back as five, six and seven. Meera says most of the class must be wrong. Their teacher asks each child to say where they drew the line. Those who said six had joined North and South America, and those who said five had also joined Africa and Eurasia. Everybody had counted carefully. The class works out that the disagreement is about what counts as one landmass, since the two Americas touch at a narrow neck. Seven is the count most widely used, and the others are not errors but different choices about where a boundary lies.

Analysis:

Three answers, no arithmetic mistakes. A continent is a line people draw, and the count follows from where they agree to draw it.

Activity:

On a world map, mark where you would separate the continents. Count them and compare your total with two classmates.

Title:

Count the continents four ways

What You Notice:

The land never changes across the four drawings. Only the lines you choose to draw on it change. That is the whole reason the answer to how many continents there are depends on who is counting.

To match the area of Greenland, the largest island on Earth, you would have to add together the ten largest states of India.

India has run an Antarctica programme since 1981. Its first station there, Dakshin Gangotri, was built in 1983, and the settlement even has a library and a post office.

The five Olympic rings were picked to stand for the five continents people live on — Africa, America, Asia, Australia and Europe.

The seven continents by size — Asia, Africa, the two Americas, Antarctica, Europe, Australia — Asia first, Africa second, then North and South America, then Antarctica, with Europe and Australia last.

What makes a continent, not an island — Both are surrounded, only one is huge — Water goes all the way round a continent too. Size is the only thing that keeps it out of the island list.