Water Oceans Notes in English Class 11 Geography Chapter-12 Book-Fundamental of Physical Geography

 

hydrologic cycle

Water is a cyclic resource that can be used and reused again and again. In the hydrological cycle, water evaporates from the oceans into the atmosphere, falls on the earth as rain and then returns to the oceans via rivers. This process has been going on for billions of years and is the basis of life for every organism on Earth.

Features of the water cycle

• Movement process: Water continuously moves in gaseous, liquid, and solid forms between the oceans, the atmosphere, the surface, and living organisms.

• Distribution of water: 71% of the water is found in the oceans, but it is not drinkable because it is salty. The remaining water is found in glaciers, underground water, lakes, rivers and the atmosphere.

• Causes of water crisis:  The distribution of water is uneven; some places have water in abundance, while some areas have acute shortage of it. Increasing population and increasing demand for water have made the water crisis more severe. Moreover, pollution of river water further complicates this problem.

relief of the ocean floor

The oceans are a large part of the Earth's outer crust, with a complex and diverse geology (subsurface) beneath the surface. The oceans cover a large portion of the Earth's surface and are divided into five major ocean systems:

1. Pacific Ocean

2. Atlantic Ocean

3. Indian Ocean

4. Southern Ocean

5. Arctic Ocean

6. These oceans also include seas, bays, estuaries and other smaller water bodies.

features of the ocean floor

• The ocean floor is typically located at a depth of 3 to 6 km below the sea floor. 

• Its landforms are more varied and complex than the features present on land, such as mountains, plains and depressions. 

• Major structures on the ocean floor include the world's largest mountain ranges (such as the Mid-Ocean Ridge), the deepest trenches (such as the Mariana Trench), and vast flat areas (mica plains).

division of the ocean floor

The ocean floor can be divided into four major parts:

1. Continental Shelf

The continental shelf is the extended edge of each continent, surrounded by shallow seas and bays. It is the shallowest part of the ocean, where the sea floor slopes very slightly (averaging less than 1°). The shelf ends with a steeper zone called the shelf cavity.

• The oceanic shelf varies in width, with an average width of about 80 km. However, in some places, such as the coast of Chile and Sumatra, the shelf can be very narrow or disappear altogether. 

• The Siberian Shelf is the widest in the Arctic Ocean, with a width of about 1,500 km. The depth of the shelf can range from 30 m to 600 m. 

• The shelf is a major area for sediment accumulation, where sediments brought from land by rivers, glaciers and wind are deposited. Over time, these sediments become sources of fossil fuels such as petroleum and natural gas.

2. Continental slope 

The continental slope is the part that connects the continental shelf to the ocean basin. It begins where the surface of the shelf abruptly turns into a slope.

• The continental slope is located in areas with a gradient of 2° to 5°, with depths ranging from 200 metres to 3,000 metres. 

• This region represents the termination point of the continents. Landforms such as deep canyons and trenches are found in this sloped region, which makes it geologically unique.

3. Serious Sea Plain

Deep sea plains are gently sloping areas of the ocean basin. These are considered to be the smoothest and flattest landforms in the world.

• The deep sea plains have a depth of 3,000 to 6,000 metres. 

• These plains are covered with fine particles, such as clay and silt, which give them a flat surface. 

• This area is considered to be the flattest part of the oceans.

4. Ocean Trench

Ocean trenches are the deepest parts of the oceans. These are narrow and deep basins with steep sides, which are 3 to 5 km deeper than the surrounding ocean floor.

• Oceanic trenches are located at the base of the continental slope and near island arcs. 

• These are the deepest areas compared to other parts of the oceans and are associated with active volcanoes and areas of strong earthquakes.

• These troughs play an important role in the study of plate tectonics, as they help in understanding the movement of plates. 

• So far 57 ocean trenches have been discovered, out of which 32 are located in the Pacific Ocean, 19 in the Atlantic Ocean and 6 in the Indian Ocean.

relief miniatures

Apart from the major features of the ocean floor mentioned above, some small but important shapes are prominently found in different parts of the oceans. 

1. Mid-ocean ridges

• Mid-ocean ridges are made up of two chains of mountains located within the oceans. There is a deep crack (depression) between these chains.

• Oceanic mountain ranges reach heights of about 2,500 metres, and in some places they reach the ocean surface, such as Iceland, which is part of the Mid-Atlantic Ridge. 

• These mountain ranges are formed by the cooling of lava at the bottom of the oceans and are the result of the separation of the Earth's plates.

2. Sea dune

• Sea dunes are mountains with pointed tops that rise from the bottom of the ocean and do not reach the surface of the ocean. They are formed due to volcanic activities.

• Sea dunes are formed as a result of volcanic activity and can range in height from 3,000 to 4,500 metres. 

• For example, the Emperor Sea Dune is an extension of the Hawaiian Islands in the Pacific Ocean.

3. The flattest submerged valleys

• Marine canyons are deep submerged valleys often compared to the Grand Canyon of the Colorado River.

• Marine valleys begin at the mouths of large rivers and extend into the sea, cutting across continental shelves and slopes. 

• A prime example of these is the Hudson Canyon, which is considered the world's most famous and largest marine canyon.

4. The Sunken Island

• These are flat-topped dunes inside the sea. These are submerged mountains, which are formed by the process of slow sinking. 

• There are more than 10,000 such sea dunes and submerged islands in the Pacific Ocean.

5. Coral Islands

• These are small islands formed by coral reefs in warm oceans. They are surrounded by deep water on all sides. Sometimes they can be surrounded by clear, salty or very deep water.

ocean water temperature

The temperature of the oceans varies with place and depth. Ocean water is heated by the sun's heat. Water heats up and cools more slowly than land. The temperature of ocean water varies due to various natural causes. These factors affect the distribution of temperature in different parts of the oceans.

• Effect of latitude:  Near the equator, the temperature of surface water is high due to abundance of solar energy, whereas as we move towards the poles, this temperature gradually decreases due to lack of solar energy.

• Distribution of land and water:  The oceans of the Northern Hemisphere receive relatively more heat because they are surrounded by more land, while the oceans of the Southern Hemisphere remain relatively colder because they are surrounded by less land.

• Southern winds:  Southern winds move the warm ocean water away from the coast, resulting in the rise of colder water from below and a decrease in the ocean temperature. In contrast, onshore winds deposit warm water on the coast, increasing the temperature of coastal areas.

• Ocean currents:  Warm ocean currents raise the temperature of cold areas, such as the Gulf Stream, which warms the west coast of Europe. Cold currents lower the temperature of warm areas, such as the Labrador Current, which cools the east coast of North America.

• Enclosed seas and open seas:  Enclosed seas located in low latitudes have higher temperatures than open seas, while enclosed seas located in high latitudes have lower temperatures than open seas.

Vertical and horizontal distribution of temperature:

The temperature of the oceans varies with depth from the surface and with latitude. It depends on the Sun's heat, the depth of the water, and the spatial spread of the oceans.

Vertical distribution (temperature with depth)

1. Temperature gradient (thermocline):

• The thermocline is the zone between the surface water and deeper layers, where the temperature falls rapidly. 

• This zone begins at a depth of 100–400 metres below the sea surface and extends for several hundred metres. 

• Below the thermocline, temperatures can drop to as low as 0°C.

2. Three layers of temperature

• First layer (superficial warm layer):  The surface layer has a depth of about 500 m, and its temperature ranges from 20°C to 25°C. This layer persists throughout the year in tropical regions, while in mid-latitudes it forms only in summer.

• Second layer (thermocline layer):  The depth of the middle layer ranges from 500 to 1,000 meters, and in this layer the temperature falls rapidly with increasing depth.

• Third layer (cold deep layer):  The deep layer extends to the ocean floor. In the Arctic and Antarctic regions, the temperature of this layer remains almost the same from the surface to the depth, which is around 0°C.

Horizontal distribution (temperature with latitude)

1. Surface temperature

• The average surface temperature of the oceans is about 27°C, which decreases as we move from the equator towards the poles. 

• The temperature drops by an average of 0.5°C at each latitude.

2. Temperature rate

• The surface temperature of the oceans is about 22°C at 20° latitude, about 14°C at 40° latitude, and decreases to about 0°C near the poles.

3. Northern and Southern Hemispheres

• The average ocean temperature of the Northern Hemisphere is about 19°C, while the average temperature of the Southern Hemisphere is about 16°C. This difference is due to the uneven distribution of land and water.

4. Important Facts

• The temperature of the upper surface of the oceans is high due to direct receipt of heat from the sun. 

• The temperature decreases with increasing depth, but this decrease is not the same at every place. 

• The temperature falls rapidly up to a depth of about 200 metres, after which the fall slows down.

Salinity of ocean water

Ocean water is full of mineral salts. Salinity is a measure of the amount of salt dissolved in ocean water.

• Salinity refers to the amount of salt dissolved in seawater per 1,000 grams (1 kg) of water. It is measured as PPT (Parts Per Thousand) or percentage (%). 

• For example, if the salinity of water is 35 PPT, it means that 35 grams of salt is dissolved in 1,000 grams of water.

• Salinity is an important property of ocean water, which affects the composition and properties of sea water. 

• In the salinity range, 24.7% is considered the highest limit of saline water.

Factors affecting the salinity of ocean water

The salinity of the oceans depends on many natural processes and factors. These factors change the amount of salt dissolved in ocean water.

• Evaporation and precipitation: The salinity of ocean water increases due to more evaporation, while salinity decreases when there is more precipitation.

• Coastal and polar effects: In coastal areas, the salinity of ocean water decreases due to fresh water coming from rivers. Whereas, in polar regions, salinity increases due to ice formation, while it decreases when ice melts.

• Wind: Wind moves water from one area to another, causing changes in salinity.

• Ocean currents: Warm and cold currents affect the distribution of salinity.

• Temperature and Density: Temperature and density are directly related to salinity. Any change in temperature or density also changes the salinity of ocean water.

Horizontal distribution of salinity

• The salinity of the oceans is affected by geographic location, evaporation, rainfall, and river runoff. The salinity of the open oceans ranges from 33% to 37%.

• In the Red Sea it can be up to 41% and in the Arctic regions it can be from 0% to 35%. In hot and dry areas salinity reaches 70%. 

• The average salinity of the Atlantic Ocean is 36%, the Pacific Ocean is 33%–35%, and the Indian Ocean is 35%. 

• In the Bay of Bengal, salinity is low due to the river Ganges, whereas in the Arabian Sea, salinity is high due to evaporation. 

• Salinity is high in the Mediterranean Sea, and low in the Black Sea and the Baltic Sea due to rivers.

Vertical distribution of salinity

• The change in salinity with sea depth depends on the sea conditions. 

• Salinity at the surface changes due to ice formation, evaporation, or the addition of fresh water, while it remains constant at depth. 

• Lighter, low-salinity water remains on top, and heavier, higher-salinity water remains at the bottom. 

• Salinity increases rapidly with depth in the "halocline" region. Salinity contributes to water stratification by increasing the density of seawater.