Chapter Notes

Water (Oceans)

Can you imagine life without water? It's often said that "water is life" because it is a fundamental part of every living thing on Earth. Our planet is incredibly lucky to be a "water planet," with an abundant supply of water on its surface, which is why we call it the 'Blue Planet'. This is a rare feature in our solar system, as there is no water on the sun or any other planet.

Hydrological Cycle

Water is a cyclic resource, meaning it can be used and reused continuously. It moves in a constant cycle from the ocean to the land and back to the ocean.

The hydrological cycle is the continuous movement of water on, in, and above the Earth. This cycle has been operating for billions of years and is essential for all life. The circulation of water happens within the Earth's hydrosphere in its three different forms: liquid, solid (ice), and gaseous (water vapor). It involves a constant exchange of water between the oceans, atmosphere, land, and living organisms.

Components and Processes of the Water Cycle:

• From Oceans to Atmosphere: Water enters the atmosphere from the oceans through processes like Evaporation, Evapotranspiration, and Sublimation.
• In the Atmosphere: Once in the atmosphere, water vapor undergoes Condensation (forming clouds) and returns to the Earth's surface as Precipitation (rain, snow, etc.).
• On Land:
  • Some water is stored as ice and snow. When it melts, it becomes snowmelt runoff that flows into streams.
  • Water that flows over the land's surface is called surface runoff, which contributes to stream flow and freshwater storage. Some of this water seeps into the ground through infiltration.
  • Water that seeps underground is stored as groundwater. This water can eventually be discharged back to the surface through springs.

Distribution of Earth's Water:

• About 71% of all water on Earth is in the oceans.
• The rest is freshwater found in glaciers, icecaps, groundwater, lakes, rivers, and the atmosphere.
• Nearly 59% of the water that falls on land evaporates and returns to the atmosphere. The rest becomes surface runoff or infiltrates the ground.

Relief of the Ocean Floor

The oceans are located in the great depressions on the Earth's surface. Geographers have divided the world's oceans into five main bodies: the Pacific, the Atlantic, the Indian, the Southern, and the Arctic oceans.

The ocean floor is not flat; it has a complex and varied landscape, much like the continents. It features the world's largest mountain ranges, deepest trenches, and vast plains. These features are formed by tectonic, volcanic, and depositional processes. A large part of the ocean floor lies 3 to 6 km below sea level.

Divisions of the Ocean Floors

The ocean floor can be divided into four major divisions, along with several minor relief features.

Continental Shelf

The continental shelf is the submerged, extended edge of a continent.

• Characteristics: It's the shallowest part of the ocean, with a very gentle slope (a gradient of 1° or less). It ends at a steep drop-off called the shelf break.
• Width and Depth: The average width is about 80 km, but it varies greatly. It is very narrow along the coasts of Chile and Sumatra but extends up to 1,500 km in the Siberian shelf in the Arctic Ocean. The depth can range from 30 m to 600 m.
• Significance: Continental shelves are covered in sediments brought by rivers, glaciers, and wind. Over long periods, these massive sedimentary deposits become the source of fossil fuels.

Continental Slope

The continental slope connects the continental shelf to the deep ocean basins.

• Characteristics: It begins at the shelf break, where the ocean floor drops off sharply. The gradient is much steeper, ranging from 2° to 5°.
• Depth: The depth varies between 200 m and 3,000 m.
• Features: Canyons and trenches are often found in this region. The continental slope marks the true end of the continents.

Deep Sea Plain

Deep sea plains, also known as abyssal plains, are the vast, gently sloping areas of the deep ocean basins.

• Characteristics: These are the flattest and smoothest regions on the entire planet.
• Depth: They are typically found at depths between 3,000 m and 6,000 m.
• Composition: They are covered with fine-grained sediments like clay and silt.

Oceanic Deeps or Trenches

These are the deepest parts of the oceans.

• Characteristics: Trenches are steep-sided, narrow basins that are about 3 to 5 km deeper than the surrounding ocean floor.
• Location and Significance: They are often found near continental slopes and island arcs and are associated with active volcanoes and strong earthquakes. Because of this, they are very important for studying plate movements.
• Distribution: So far, 57 deeps have been explored: 32 in the Pacific Ocean, 19 in the Atlantic Ocean, and 6 in the Indian Ocean.

Minor Relief Features

Beyond the major divisions, the ocean floor has several other significant features.

• Mid-Oceanic Ridges: This is a vast underwater mountain system. It consists of two chains of mountains separated by a large depression. Some peaks are so high (up to 2,500 m) that they rise above the ocean's surface. [!example] Iceland is an example of a part of the Mid-Atlantic Ridge that is visible above sea level.

• Seamount: A seamount is an underwater mountain with a pointed top that rises from the seafloor but does not reach the water's surface. They are volcanic in origin and can be 3,000-4,500 m tall. [!example] The Emperor seamount, an extension of the Hawaiian Islands in the Pacific, is a good example.

• Submarine Canyons: These are deep valleys, sometimes as large as the Grand Canyon, that cut across continental shelves and slopes. They often extend from the mouths of large rivers. The Hudson Canyon is the most famous example.

• Guyots: A guyot is a seamount with a flat top. Scientists believe they were once active volcanoes that rose above the surface, were eroded flat by waves, and then gradually subsided back below sea level.

• Atoll: An atoll is a low island found in tropical oceans, made of coral reefs that surround a central depression, or lagoon. The lagoon can contain seawater or a body of fresh, brackish, or highly saline water.

Temperature of Ocean Waters

Ocean water is heated by solar energy, just like land, but the process of heating and cooling is much slower for water.

Factors Affecting Temperature Distribution

Several factors influence the temperature of ocean water:

1. Latitude: The surface water temperature decreases as you move from the equator towards the poles. This is because the amount of solar energy (insolation) received decreases with higher latitude.
2. Unequal Distribution of Land and Water: The oceans in the Northern Hemisphere receive more heat because they are in contact with a larger landmass compared to the oceans in the Southern Hemisphere.
3. Prevailing Wind:
   • Winds blowing from land to the ocean (offshore winds) push warm surface water away from the coast, causing cold water from below to rise. This process is called upwelling.
   • Winds blowing from the ocean to the land (onshore winds) pile up warm water near the coast, raising the temperature.
4. Ocean Currents: Warm currents raise the temperature of the water in cold areas, while cold currents lower the temperature in warm areas. [!example] The warm Gulf Stream raises temperatures along the eastern coast of North America and the west coast of Europe. In contrast, the cold Labrador Current lowers temperatures near the northeast coast of North America.

Horizontal and Vertical Distribution of Temperature

Vertical Distribution (With Depth): The temperature of ocean water generally decreases with increasing depth. This change is not uniform and can be described as a three-layer system in middle and low latitudes.

1. The First Layer: This is the top layer of warm oceanic water, about 500 m thick. Temperatures range from 20°C to 25°C. In the tropics, this layer is present year-round, but in mid-latitudes, it develops only during the summer.
2. The Second Layer (Thermocline): Below the first layer is the thermocline, a boundary region where the temperature drops rapidly with increasing depth. This layer is typically 500-1,000 m thick.
3. The Third Layer: This is the deep layer, which is very cold and extends to the ocean floor. Temperatures here approach 0°C. About 90% of the total volume of ocean water is found below the thermocline.

In the Arctic and Antarctic regions, the surface water is already close to 0°C, so there is very little temperature change with depth. Here, there is just one layer of cold water from the surface to the ocean floor.

Horizontal Distribution (Across the Surface):

• The average surface temperature of the oceans is about 27°C.
• It gradually decreases from the equator towards the poles at a rate of about 0.5°C per degree of latitude.
• The average temperature is around 22°C at 20° latitude, 14°C at 40° latitude, and 0°C near the poles.
• The oceans in the Northern Hemisphere are warmer (average 19°C) than in the Southern Hemisphere (average 16°C) due to the greater influence of landmasses.
• The highest temperatures are recorded slightly north of the equator.

Salinity of Ocean Waters

Salinity is the total amount of dissolved mineral salts in seawater. It is calculated as the amount of salt (in grams) dissolved in 1,000 grams (1 kg) of seawater and is expressed in parts per thousand (ppt or %o). Water with a salinity above 24.7 %o is considered 'brackish water'.

Factors Affecting Ocean Salinity

1. Evaporation and Precipitation: In areas with high evaporation and low precipitation, salinity increases. In areas with low evaporation and high precipitation, salinity decreases.
2. Freshwater Flow: In coastal regions, the inflow of freshwater from rivers lowers salinity. In polar regions, the freezing of ice increases salinity (as salt is left behind), while the melting of ice decreases it.
3. Wind: Wind can influence salinity by transferring water from one area to another.
4. Ocean Currents: Currents mix water and contribute to variations in salinity.

Horizontal Distribution of Salinity

• The average salinity in the open ocean ranges from 33 %o to 37 %o.
• In landlocked seas with high evaporation, like the Red Sea, salinity can be as high as 41 %o.
• In areas like estuaries or the Arctic, salinity fluctuates seasonally from 0 to 35 %o.
• Atlantic Ocean: Average salinity is around 36 %o. The highest salinity (37 %o) is found between 20°N and 30°N latitude.
• Indian Ocean: Average salinity is 35 %o. The Bay of Bengal has lower salinity due to large amounts of freshwater from rivers, while the Arabian Sea has higher salinity due to high evaporation.
• Pacific Ocean: Salinity varies due to its large size. It decreases in the northern parts because of melted water from the Arctic.

Vertical Distribution of Salinity

Salinity also changes with depth.

• Surface salinity can increase due to evaporation or decrease due to freshwater input.
• At depth, salinity is much more fixed.
• Generally, salinity increases with depth. The zone where salinity increases sharply is called the halocline.
• Higher salinity water is denser, so it tends to sink below lower salinity water. This creates a layered structure in the ocean, known as stratification.