Water (Oceans) — NCERT Solutions

Answer:

The deepest trenches of the major oceans are:

The Mariana Trench in the Pacific Ocean is the deepest point on Earth.

Answer:

Concept: Thermal stratification of ocean water.

A thermocline is a distinct layer in the ocean where the temperature decreases rapidly with increasing depth. It acts as a boundary between the warm, sunlit surface water (epipelagic zone) and the cold, deep water below.

• Above the thermocline: water is warm due to solar heating.
• Within the thermocline: temperature drops sharply.
• Below the thermocline: water is uniformly cold.

The thermocline is most pronounced in tropical and subtropical regions and is less distinct in polar areas.

Answer:

Thermal Layers Encountered (from surface to depth):

1. Surface Layer (Epipelagic Zone): The topmost layer, warmed by solar radiation. Temperature ranges from 20°C to 25°C. It extends to about 200 m depth.

2. Thermocline Layer: A transitional zone where temperature decreases rapidly with depth. It lies between 200 m and 1,000 m depth.

3. Deep Water Layer: Below 1,000 m, the temperature is uniformly very cold (near 0°C–3°C) and remains relatively constant.

Reasons for Temperature Variation with Depth:

• Solar radiation heats only the surface layers; sunlight cannot penetrate beyond ~200 m.
• Deeper waters are not reached by solar energy, so they remain cold.
• Cold, dense water sinks and accumulates at depth, maintaining low temperatures.
• Mixing by wind and waves is limited to the surface layer only.

Answer:

Concept: Chemical composition of ocean water.

Salinity is defined as the total amount of dissolved salts (in grams) present in 1,000 grams of sea water. It is expressed in parts per thousand (‰).

$$\text{Salinity} = \frac{\text{Weight of dissolved salts (g)}}{\text{Weight of sea water (g)}} \times 1000 \text{ (‰)}$$

• The average salinity of ocean water is approximately 35‰ (i.e., 35 g of salt per 1,000 g of sea water).
• The most abundant salt in sea water is sodium chloride (NaCl), which constitutes about 77.7% of all dissolved salts.
• Salinity varies with evaporation, precipitation, river inflow, and ice formation.

Answer:

Introduction: The hydrological cycle (water cycle) is the continuous movement of water through the Earth's systems — atmosphere, land, and oceans. All its elements are closely interrelated and interdependent.

Elements and their Interrelationship:

1. Evaporation: Solar energy heats the surface water of oceans, lakes, and rivers, converting liquid water into water vapour. This water vapour rises into the atmosphere. Transpiration from plants also adds water vapour (together called evapotranspiration).

2. Condensation: As water vapour rises, it cools and condenses around dust particles to form tiny water droplets, leading to the formation of clouds and fog. Condensation is directly dependent on the water vapour supplied by evaporation.

3. Precipitation: When water droplets in clouds combine and become heavy enough, they fall as precipitation — rain, snow, sleet, or hail. Precipitation is the outcome of condensation and returns water to the Earth's surface.

4. Runoff and Infiltration: Precipitated water either flows over the land surface as runoff (into rivers and streams, eventually reaching the ocean) or seeps into the ground as infiltration (replenishing groundwater).

5. Groundwater Flow: Infiltrated water moves slowly through soil and rock, eventually discharging into rivers, lakes, or the ocean, completing the cycle.

Conclusion: Thus, evaporation → condensation → precipitation → runoff/infiltration → back to evaporation form a continuous, interlinked loop. Energy from the Sun and gravity are the driving forces of this cycle, ensuring the constant recycling of water on Earth.

Answer:

Introduction: The temperature of ocean water is not uniform; it varies both horizontally (from equator to poles) and vertically (from surface to depth). Several factors influence this distribution.

Factors Influencing Ocean Temperature Distribution:

1. Latitude:
The most important factor. Solar insolation is maximum at the equator and decreases towards the poles. Hence, surface temperature is highest near the equator (~27°C) and decreases progressively towards the poles (~−2°C near polar regions). Isotherms (lines of equal temperature) generally run parallel to latitudes.

2. Unequal Distribution of Land and Water:
The Northern Hemisphere has more landmass, which heats and cools faster, causing greater seasonal variation in ocean temperatures. The Southern Hemisphere, being more oceanic, shows less variation.

3. Prevailing Winds:

• Onshore winds push warm surface water towards the coast, raising coastal temperatures.
• Offshore winds push surface water away, causing cold deep water to upwell (upwelling), lowering temperatures.
• Trade winds and westerlies redistribute heat across ocean surfaces.

4. Ocean Currents:

• Warm currents (e.g., Gulf Stream, Kuroshio) raise the temperature of the regions they flow through.
• Cold currents (e.g., Labrador Current, Benguela Current) lower the temperature of adjacent areas.

5. Depth:
Temperature decreases with increasing depth. The surface layer is warm due to solar heating. The thermocline marks a zone of rapid temperature decrease. Below ~1,000 m, temperature is uniformly cold (0°C–3°C).

6. Enclosed Seas vs. Open Oceans:
Enclosed or semi-enclosed seas (like the Red Sea, Persian Gulf) have higher temperatures due to limited water exchange and high evaporation, while open oceans have more moderate temperatures.

Conclusion: The temperature distribution of oceans is thus a result of the interplay of insolation, latitude, winds, currents, and depth. These factors together determine the thermal structure of the world's oceans, which in turn influences climate, marine life, and ocean circulation.