Maharashtra Board Class 8 Geography Chapter 4 Structure Of Ocean Floor PDF Download

Structure Of Ocean Floor

Think About It

If the classification of landforms on land can be done on the basis of altitude and size, then how can the landforms submerged under water be classified?

Can You Tell?

Continents and oceans are a part of the lithosphere and hydrosphere respectively.

Continents and oceans lie on plates.

During low tides, the level of ocean water goes down and the land below the water near the coast is exposed.

Ships wreck when they strike against the rocks in the ocean.

If the above points are correct, then select the most appropriate option from the following:

The surface of the earth is occupied by land and water.

There is land below the ocean too.

Even if the water has the same level, the land submerged is not even.

The level of water and land is uneven.

Discuss in the class regarding the choice of your options. Know from your teachers about the correct option.

Geographical Explanation

We know that the proportion of water and land is uneven on the earth. Around 71% of the surface is covered by water but there is land even below this water. But like water, it is not at the same level.

Can You Tell?

Try to name the landforms shown in figure 4.1.

What parameters were used for classifying the landforms on the earth?

What parameters were used for naming the landforms below water?

Geographical Explanation

The Relief Of Ocean Floor

Land submerged below oceanic water is called the ocean floor. The relief of ocean floor is decided upon by the depth from the sea floor and the shape of the land there.

The average depth of the oceans is around 3700 metres. The ocean bed is also uneven like the land on the continents. The ocean floor relief consists of all submerged landforms. The structure of the ocean floor differs from ocean to ocean. We will look at the sequence of landforms and the details of major landforms on the ocean floor. As we go away from the coast, the structure of the ocean floor changes. Study the explanation below and figure 4.2 together.

We classify the various landforms on the earth on the basis of altitudes. A similar classification can be done for landforms submerged under the water.

Teacher's Note

The ocean floor under your local beach has many landforms just like mountains on land. When fishermen go to sea, they must know these underwater shapes.

Exam Trick

Remember: Ocean floor = underwater mountains and valleys. Just like hills in your village, oceans also have high and low places under water.

Points To Remember

Ocean floor has many landforms like mountains and plains.
The average depth of oceans is 3700 metres.
Water covers 71% of earth but land is below water too.
Ocean floor is uneven like land on continents.

Continental Shelf

The land near the coast and submerged under the sea is called continental shelf. This is the shallowest part of the ocean bed. It is also called submerged coastland. Its slope is gentle.

The extent of the continental shelf is not uniform everywhere. It is narrow along the coasts of some continents while it is broad for hundreds of kilometers at others. Its depth is upto 200 meters below the sea level.

The continental shelf is very important from the point of view of humans. Extensive fishing grounds are found in the continental shelf. As this part is shallow, the sunlight reaches its bed. Algae, plankton, etc. grow here. This is food for fish. Natural gas, mineral oil and various minerals can be obtained by mining the continental shelf. For example, Mumbai High located on the continental shelf of the Arabian Sea. It is a source from where we obtain mineral oil and natural gas.

Teacher's Note

The continental shelf near India is like a farmer's field near the coast. Just like a farmer grows rice in his field, fishermen catch fish in shallow continental shelf water.

Exam Trick

Remember: Continental shelf = fishing zone. Shallow water = sunlight reaches = plants and fish grow = fishing.

Points To Remember

Continental shelf is near the coast and is very shallow.
Its depth is up to 200 metres below sea level.
Sunlight reaches the continental shelf.
Fish and plants grow here in large numbers.
We get mineral oil and natural gas from here.

Continental Slope

After the extent of continental shelf is over, the slope of the sea bed becomes steeper. This is called continental slope. The depth of the slope is from 200 m to 3600 metres. In some places, it is more. The continental slope is narrow. The lower boundary of continental slope is considered to be the boundary of continents.

Teacher's Note

Continental slope is like the steep hill near your village where you cannot walk easily. The ocean floor goes down suddenly here.

Exam Trick

Remember: Continental slope = steep drop. After shelf ends, ocean floor goes down fast like a waterfall.

Points To Remember

Continental slope comes after continental shelf.
Its depth is 200 metres to 3600 metres.
The slope is very steep and narrow.
This is the boundary of continents.

Abyssal Plains

Beyond the continental slope lie the abyssal plains which are the flat part of the sea bed. The abyssal plains consist of various submerged landforms like hills, plateaus, etc.

Teacher's Note

Abyssal plains are like flat land in your village. After the steep hill (continental slope), the ground becomes flat again deep under the ocean.

Exam Trick

Remember: Abyssal = very deep and flat. Like the bottom of a very deep well.

Points To Remember

Abyssal plains are flat areas of ocean floor.
They are very deep under the water.
They have some hills and plateaus inside them.
They come after continental slope.

Mountain Ranges And Plateaus

The hills and mountains found on the ocean-bed are called submerged hills and mountains. These hills are hundreds of kilometers wide and thousands of kilometers long. Peaks of some of the submerged hills come above the sea level. They are visible to us as marine islands. Iceland in the Atlantic Ocean, Andaman and Nicobar Islands in the Bay of Bengal are examples.

The summits of some marine islands are flat and extensive. They are called oceanic or submarine plateaus. For instance, Chagos Plateau in the Indian Ocean.

Teacher's Note

Andaman and Nicobar Islands near India are actually mountain peaks under the ocean. Some mountains are so tall that their tops come above water and we can see them as islands.

Exam Trick

Remember: Island = mountain peak above water. If the peak was one metre higher, it would come out of water and be an island.

Points To Remember

Submerged mountains are hundreds of kilometres wide.
Some peaks of mountains come above sea level as islands.
Andaman and Nicobar Islands are mountain peaks.
Submarine plateaus have flat tops.

Marine Deeps And Marine Trenches

On the ocean-bed, there are some landforms which are deep, narrow and steep. They are called marine deeps or trenches. Generally, the shallower ones are called marine deeps while deeper ones and extending for longer distance are called trenches. The trenches are thousands of meters deep from the sea level. The Mariana Trench, in the Pacific Ocean, is the deepest trench in the world. Its depth is around 11034 metres. The mid-oceanic submerged hills and marine trenches are geologically the most active areas of the ocean-bed in the world. There are many active volcanoes here. These areas are also earthquake-prone areas. Earthquakes and volcanic eruptions occurring in the ocean-bed give rise to tsunamis in the nearby coastal areas.

Teacher's Note

Mariana Trench is the deepest place on Earth, much deeper than Mount Everest is tall. If we place Mount Everest at the bottom of Mariana Trench, its peak would still be 2186 metres below water.

Exam Trick

Remember: Trench = valley under ocean. Marine deep = small valley. Trench = very deep long valley. Mariana Trench = deepest.

Points To Remember

Marine deeps and trenches are very deep areas.
Mariana Trench is 11034 metres deep.
These areas have many volcanoes and earthquakes.
Tsunamis happen near these areas.

Marine Deposition

The marine–beds are the deep parts of the world in respective regions. Hence, various deposits are found in these parts. The deposits are as follows:

Pebbles, clay, soil etc. brought by rivers, glaciers, etc. from the continents.

The deposition occurs mainly on the continental shelf. These are called marine deposits.

Lava and ash erupting out of volcanic eruptions is also found here. Fine soil particles are deposited on a large scale. Remains of marine plants and animals are mixed in these deposits. This mixture is made up of fine particles and lies in the form of fine clay. This is around 30% of the deposits. These are called marine oozes. To understand the form of the marine life in the ocean and the availability of minerals on the ocean bed these deposits are very important. Sedimentary rocks are formed due to the depositing of layers over layers of the sediments and the pressure of sea water.

Besides, some human-induced material is also found here. This includes sewage, solid waste, radio-active material, waste chemicals, plastics, etc. These wastes prove hazardous to the hydrosphere. These materials are extremely harmful to the marine life and its environment. Though the polluting components are less, their nuisance value is more.

Teacher's Note

Rivers bring soil from land and drop it on the ocean floor. This is like how a farmer's field gets new soil every time the river floods. Marine oozes are made of dead sea animals and plants which become rock over time.

Exam Trick

Remember: Marine deposits = river soil + dead plants and animals + volcanic ash. Like layers of cake, each layer is a different deposit.

Points To Remember

Marine deposits are made of river soil, volcanic ash and dead sea animals.
Marine oozes are 30% of all deposits.
They help us understand ocean life.
Human waste and plastics are also deposited here.
These waste materials are very harmful to sea life.

Mean Sea Level

Elevation or depth of any place is measured from the sea level. The average of the highest high tides and the lowest low tides is considered as sea level. This average is taken to be zero and altitudes or depths are measured and shown in positive or negative values. For example, Mount Everest is 8848 metres high and Mariana Trench is -11034 metres.

For survey purpose in India, the height of sea level at Chennai is considered to be zero and the elevation of any place in India is measured with reference to this.

Teacher's Note

Sea level in Chennai is like the zero mark on your ruler. All heights in India are measured from this zero point. Mount Everest is 8848 metres above this line, and Mariana Trench is 11034 metres below this line.

Exam Trick

Remember: Sea level = zero. Higher than sea level = positive metres. Lower than sea level = negative metres. Like a number line with zero in the middle.

Points To Remember

Sea level is the average of high and low tides.
Sea level is taken as zero height.
Heights above sea level are positive.
Depths below sea level are negative.
In India, Chennai sea level is the zero point.

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