Maharashtra Board Class 8 Geography Chapter 5 Ocean Currents PDF Download

Ocean Currents

Let's Recall

When does any material flow?

What happens exactly when it flows?

Which anomalies in the material is responsible for the initiation of the flow?

Try This

Materials required: A large metal tray, water, plastic sequins, spirit lamp, etc.

Note: The following activity should be carried out by students under supervision of teachers. Focus on observation.

Keep the large metal tray on a stand. Fill it with water. After the water becomes still, leave the sequins in them. After sometime, the sequins will start floating in the water and become still too.

Observe all these things. After sometime, light the spirit lamp and place it below one corner of the tray. Observe what happens. See figure 5.1

On the basis of observation, discuss in the class and put forth your opinions regarding the experiment. Consider the following questions for the same.

What did you understand initially by observing the sequins?

When the temperature of water started increasing, what changes did you see?

Observe the movement of the sequins.

What conclusion can be drawn from the same?

Where can such processes take place on the earth's surface?

What are those processes and why do they happen?

Note: In the experiment the heat is given by the spirit lamp. Please keep in mind that in reality, the source of heat for oceans is the sun.

Geographical Explanation

It will occur to you that as the temperature of water increases, the plastic sequins move from one place to another. As the temperature rises, the density of water decreases and it becomes lighter. And, therefore, the water having lower temperature which is heavier replaces the water with higher temperatures. After sometime, the sequins start moving in a circular motion. There is movement of these sequins because of the flow of water.

Teacher's Note

Ocean water moves in circles just like the air moves in your classroom when you open a window. The sun heats ocean water, and then cooler water moves to take its place.

Exam Trick

Remember: Hot water is light and goes up. Cold water is heavy and goes down. This creates circular motion.

Points to Remember

Ocean currents are caused by differences in water temperature.
Hot water has less density than cold water.
Cold water replaces hot water from below.
This creates circular movement of water.
The sun is the main source of heat for oceans.

Can You Tell?

A very strange incident occurred in the Pacific Ocean in the year 1992. A cargo ship sailed towards America from Hong Kong. While travelling through the Pacific Ocean, near the Hawaii Islands, a container full of toys fell into the ocean and broke. Around 28000 rubber toys started floating on the ocean. This incident occurred on 10th January, 1992. Now a strange thing happened. After around 10 months, on 16th November, 1992, some of these toys reached the coast of Alaska. Some of them crossed the Bering Strait and moved upto the Arctic ocean by the year 2000. Some of them also floated to Atlantic Ocean from the Arctic. Some of these reached the eastern coast of America in 2003 and some of the toys had even reached the European coast by 2007. From the Hawaii Islands, some toys took the route to Australia!

Why Did The Toys Travel In This Way?

The region from sea level to the depth of 500 m. is considered to be the surface water. Sunlight can reach till this depth. The movements in this layer occur mainly due to differences in temperature and salinity. The planetary winds give speed to the ocean currents.

Horizontal (Surface) Ocean Current

The flow on the surface of the ocean moves only 10% of the oceanic water. The surface flow is considered up to 500 metres of depth. The discharge of water in the oceans is measured in Sverdrup unit. It is equivalent to 1 million cu.m./second discharge. The horizontal flow of ocean water occurs as warm and cold currents. These currents flow from the equator to the poles and from the poles to the equator. These currents are pushed to long distances by the planetary winds. As a result, the ocean waters flow from equator to both the poles and vice versa. You have studied the map given in figure 5.4 earlier. Study the map again and answer the following questions.

What are the major types of ocean currents?

What do you call the currents flowing from the equator to the poles?

Teacher's Note

Ocean currents are like rivers in the ocean. They move water from hot places to cold places. Wind helps push these water rivers across the ocean.

Exam Trick

Remember: Warm currents come FROM equator TO poles. Cold currents come FROM poles TO equator. This makes a circular pattern.

Points to Remember

Surface ocean currents move only the top 500 meters of ocean.
Surface currents are created by temperature and salinity differences.
Planetary winds push ocean currents long distances.
Warm and cold currents flow in opposite directions.
Ocean currents create a circular pattern in the ocean.

Geographical Explanation

The ocean water can be divided into two parts based on depth-surface water and deep water.

The upper layer extends upto 500 meters from the sea level.

Below 500 m. depth.

Make Friends With Maps

What do you call the currents flowing from the poles to the equator?

When the currents are moving in a circular manner, what difference is visible in their direction in Northern and Southern Hemisphere respectively?

What might happen at the places where these currents meet?

When two different types of currents meet along the coast then what type of human settlements and occupations are seen.

Teacher's Note

In India, the monsoon winds change direction with the seasons. This changes how ocean currents move in the Indian Ocean from summer to winter.

Exam Trick

Remember: Indian Ocean currents move clockwise in summer and opposite in winter. This is because of monsoon winds that change direction.

Points to Remember

Earth's rotation affects how ocean currents move.
In Northern Hemisphere, currents move clockwise.
In Southern Hemisphere, currents move counterclockwise.
Monsoon winds change ocean current direction in Indian Ocean.
Coastline shape changes the direction of currents.

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