Water (Oceans)

(b) Hydration
Reason: Hydration is a chemical process (chemical weathering) where minerals absorb water. Evaporation, Condensation, and Precipitation are the physical processes driving the water cycle.

(b) 200–2,000m
Reason: The continental slope connects the continental shelf (shallow) and the ocean basin. It drops steeply from the shelf break (approx 200m) down to the deep ocean plains.

(c) Oceanic Deep (Trench)
Reason: Oceanic Deeps (Trenches) are major relief features, being the deepest parts of the oceans. Seamounts, Atolls, and Guyots are considered minor relief features.

(b) 1,000 gm
Reason: Salinity is defined as the weight of dissolved salts per 1000 parts of seawater. It is usually expressed as parts per thousand (ppt or ‰).

(b) Arctic Ocean
Reason: The Arctic Ocean is the smallest and shallowest of the world’s five major oceans.

Earth is called the Blue Planet because nearly 71% of its surface is covered with water (oceans, seas, lakes). From space, the oceans reflect blue light, giving the planet its characteristic colour.

The continental margin is the zone of transition between the continent and the deep ocean floor. It consists of the Continental Shelf, Continental Slope, Continental Rise, and Deep Ocean Trenches. [Image of continental margin diagram]

• Pacific Ocean: Mariana Trench (11,034 m) – deepest in the world.
• Atlantic Ocean: Puerto Rico Trench (8,380 m).
• Indian Ocean: Sunda Trench (Java Trench) (7,450 m).

The thermocline is a distinct layer in a large body of water (like an ocean) where the temperature changes more rapidly with depth than it does in the layers above or below. It typically lies between 100m and 1000m depth, acting as a boundary zone.

Layers:

1. Epilimnion (Surface Layer): Warm, thin layer (approx 500m thick).
2. Thermocline: Rapid cooling layer (500m – 1000m).
3. Hypolimnion (Deep Layer): Very cold layer extending to the bottom.

Why it varies: Water is heated by the sun from the top. Sunlight cannot penetrate deep into the ocean (photic zone is shallow). Furthermore, cold water is denser and sinks to the bottom.

Salinity is the total content of dissolved salts in seawater. It is calculated as the amount of salt (in grams) dissolved in 1,000 grams (1 kg) of seawater. The average salinity of the oceans is 35% (35 parts per thousand).

[Image of hydrological cycle diagram]
The hydrological cycle is a continuous circulation of water in the Earth-atmosphere system.

• Evaporation: Solar energy converts water from oceans/lakes into vapour.
• Condensation: Rising vapour cools and turns into clouds (liquid/solid).
• Precipitation: Water returns to earth as rain or snow.
• Runoff & Infiltration: Rainwater flows into rivers (Runoff) or seeps underground (Infiltration), eventually returning to the oceans.

This cycle connects the atmosphere, lithosphere, and hydrosphere, ensuring water is neither created nor destroyed, only recycled.

The temperature of ocean water is not uniform and is influenced by:

• Latitude: Surface temperature decreases from the equator ($~27^\circ C$) to the poles ($~0^\circ C$) because insolation decreases poleward.
• Unequal distribution of Land and Water: Oceans in the northern hemisphere (more land contact) are warmer than those in the southern hemisphere due to heat transfer from land.
• Prevailing Winds: Winds blowing from land to sea (offshore) drive warm surface water away, causing upwelling of cold water from below (lowering temperature). Onshore winds pile up warm water (raising temperature).
• Ocean Currents: Warm currents (e.g., Gulf Stream) raise the temperature of cold areas, while cold currents (e.g., Labrador) lower the temperature of warm areas.

(Guidance for Student Activity)

(i) Ocean Floor Relief on Map:
Consult an atlas and mark the following on a world map:

• Pacific: Mariana Trench, East Pacific Rise.
• Atlantic: Mid-Atlantic Ridge (S-shaped).
• Indian: Sunda Trench.

(ii) Mid-Oceanic Ridges in Indian Ocean:
Identify the inverted ‘Y’ shape ridge system:

• Central Indian Ridge (North).
• Splits into Southwest Indian Ridge (towards Africa).
• Splits into Southeast Indian Ridge (towards Australia).
• Carlsberg Ridge in the Arabian Sea.