Structure of Ocean Floor Class 8 Geography Notes Maharashtra State Board
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. We classify the various landforms on the earth based on altitudes. A similar classification can be done for landforms submerged under the water.
The Relief of Ocean Floor:
Land submerged below oceanic water is called the ocean floor. The relief of the ocean floor is decided upon by the depth of the sea floor and the shape of the land there. The average depth of the oceans is around 3700 meters. 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.
The land near the coast and submerged under the sea is called the 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 on others. Its depth is upto 200 meters below sea level. The continental shelf is very important from the point of view of humans. Extensive fishing grounds are found on the continental shelf. As this part is shallow, the sunlight reaches its bed. Algae, plankton, etc. grow here. This is fish food. Natural gas, mineral oil, and various minerals can be obtained by mining the continental shelf. For example, Mumbai High is located on the continental shelf of the Arabian Sea. It is a source from where we obtain mineral oil and natural gas.
After the extent of the continental shelf is over, the slope of the sea bed becomes steeper. This is called a continental slope. The depth of the slope is from 200 m to 3600 meters. In some places, it is more. The continental slope is narrow. The lower boundary of the continental slope is considered to be the boundary of continents.
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
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, and 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, the Chagos Plateau in the Indian Ocean.
Marine Deeps and Marine Trenches:
On the ocean bed, some landforms 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 distances are called trenches. The trenches are thousands of meters deep from the sea level. The Mariana Trench, in the Pacific Ocean, is the deepest in the world. Its depth is around 11034 meters. 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. Earthquakes and volcanic eruptions occurring in the ocean bed give rise to tsunamis in the nearby coastal areas.
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:
1. 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.
2. 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 seawater.
3. Besides, some human-induced material is also found here. This includes sewage, solid waste, radioactive 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.
Mean Sea Level:
The 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 meters high and the Mariana Trench is -11034 meters. For survey purposes in India, the height of sea level at Chennai is considered to be zero and the elevation of any place in India is measured concerning this.
The deposition of many materials coming from land keeps occurring in the seas and oceans. This is in the form of natural deposits and sediments. But, man disposes of unwanted materials in the sea. This poses a hazard to the ocean bed and the sea water. Also, it is harmful to marine life. We must keep in mind that the biodiversity found in seas and oceans is higher than on the land.
While studying the ocean floor, we must keep in mind the age of the ocean bed. By studying the deposits on the ocean bed, it occurred to scientists that the deposits on the ocean floor are not older than 200 million years. The maximum age of the rocks on the continents is supposed to be 3200 million years. Then where have the deposits on the sea bed which are older than 200 million years gone?
This made the scientists restless. Then they started the study of the rocks along with the deposits. This made them realize that the rocks are also not older than 200 million years. It was inferred that the ocean floor is very young compared to the earth’s surface. Now this is unanimously accepted. This research was then used in the study of the concept of plate tectonics.
Detailed Maharashtra State Board Class 8 Geography Notes Structure of Ocean Floor are particularly useful for answering essay questions.