Class 7 Geography

Water: Ocean

Unlike the calm waters of a pond or lakes, ocean water keeps moving continuously. These movements can be classified into:


When water on the surface of the ocean rises and falls alternately, they are called waves. They are formed when winds scrape the ocean floor.

When we throw a ball in the ocean, it gets washed back to the shore by waves. The stronger the wind blows, the bigger the wave becomes. During a storm, the wind blows at a very high speed forming huge waves. These may cause tremendous destruction. The following can shift large amounts of ocean water:

Due to the above, a huge tidal wave called tsunami is formed. It may be as high as 15 metres. The highest tsunami ever measured was 150 m high. These waves travel at a speed of 700 km/h.


The rhythmic rise and fall of water twice in a day is called a tide.

High Tide: This occurs when water covers much of the shore by rising to its highest level.

Low Tide: There is a low tide when water falls to its lowest level and recedes from the shore.

Tides are caused by the strong gravitational pull exerted by the sun and the moon on the earth's surface. The rise and fall of water due to tides is used for generating electricity at some places. The water of the earth closer to the moon gets pulled under the moon's gravitational force and causes a high tide.

Types of Tides

Spring Tide

spring tide

During the full moon and new moon days, the sun, the earth and the moon are in the same line and the tides are the highest. These tides are called spring tides.

Neap Tide

neap tide

When the moon is in its first and third quarters, the ocean waters get drawn in diagonally opposite directions by the gravitational pull of the sun and the earth. This results in low tides, called neap tides.

Advantages of High Tide:

Ocean Current

They are streams of water flowing continuously on the ocean's surface in definite directions. They influence the temperature conditions of the area. They may be warm or cold.

Warm Current

They originate near the equator and move towards the poles. The Gulf Stream is a warm current. They bring warm temperature over land surface.

Cold Current

They carry water from polar or higher latitudes to tropical or lower latitudes. For example, the Labrador Ocean Current. They result in cold temperature over the land surface.

Effect of the meeting of warm and cold currents:


Tsunami or the harbor wave brought large scale destruction in the Indian Ocean on 26 December 2004. It was due to an earthquake of magnitude 9.0 on the Richter scale. The epicenter of this earthquake was near the western boundary of Sumatra. There was a sudden movement of sea floor causing earthquake, as the Indian plate went under the Burma plate. The ocean floor was displaced by about 10 to 20 m and huge mass of ocean water filled the gap that was created by the displacement.

Tsunami travelled at a high speed of about 800 km/h. The Indira Point in the Andaman and Nicobar Islands got submerged after the tsunami. Some of the islands in the Indian Ocean got completely washed away. The destruction killed more than 10000 people and affected more than one lakh houses. Coastal areas of Andhra Pradesh, Tamil Nadu, Kerala, Puducherry and the Andaman and Nicobar Islands were the worst affected areas in India.

Early warning systems are in place which can give a warning of tsunami three to four hour in advance. Such warning systems were in place in the Pacific Ocean but not in the Indian Ocean because tsunamis are rare in the Indian Ocean. The first indication that tsunami is approaching is the rapid withdrawal of water from the coastal region, followed by a destructive wave.


Terrarium: It is the artificial enclosure for keeping small house plants.

Salinity: It is the amount of salt in grams present in 1000 grams of water.

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