🌊 Chapter 13: Ocean Water
🌐 Why is the Earth called the Blue Planet?
- Oceans cover 71% of Earth's total surface.
- When viewed from space, Earth appears blue, hence the name Blue Planet.
💧 Water Cycle
- The water cycle operates continuously, moving water between oceans, atmosphere, and land.
- Steps:
- Evaporation – Water evaporates from oceans → forms clouds.
- Condensation & Precipitation – Water vapor condenses → rains on Earth.
- Runoff – Water flows back into oceans via rivers.
- 🌟 Significance: Keeps the lithosphere, hydrosphere, and atmosphere connected.
🔢 Mathematical Representation:
RF (Rainfall)=RO (Runoff)+ET (Evapotranspiration)\text{RF (Rainfall)} = \text{RO (Runoff)} + \text{ET (Evapotranspiration)}RF (Rainfall)=RO (Runoff)+ET (Evapotranspiration)
🌊 Ocean Features
1️⃣ Ocean Trenches / Troughs
- Definition: Long, narrow, deep depressions in the ocean floor with steep slopes.
- Deepest Trench: Mariana Trench, Pacific Ocean – 11,033 m.
2️⃣ Continental Frontier
- Outer boundary of continents submerged in the sea.
3️⃣ Sublimation
- Direct conversion of solid → gas (e.g., ice → water vapor).
4️⃣ Continental Shelf
- Submerged part of a continent under the sea.
- Depth: up to 200 m
- Width: few km → 1000 km (average ~80 km)
- Ends at continental slope (steep descent).
5️⃣ Continental Slope
- Connects continental shelf → deep ocean floor.
- Slope: 2–5°
- Depth: 200–300 m
6️⃣ Deep Oceanic Plain / Abyssal Plain
- Flat, wide areas after continental slope.
- Slope: < 1°
- Depth: 3000–6000 m
- Covers ~40% of ocean floor
- Composition: Fine sediments like clay & silt
7️⃣ Seamounts / Nital Hills
- Submerged mountains >1000 m above ocean floor.
- Flat-topped: Guyot
- Most located in Pacific Ocean
- Formed by volcanic activity
8️⃣ Submarine Canyons
- Deep, narrow gorges on continental slopes or ocean plains.
- Longest: Bering, Pribilof, Gemchug (Pacific Ocean)
- Famous: Hudson Canyon (Atlantic Ocean)
9️⃣ Marine Mound
- Submerged mountain with pointed peaks, does not reach ocean surface.
- Height: 3000–4500 m
- Example: Emperor Seamounts, Pacific Ocean
✅ Exam Tip: Memorize the key features in order: Shelf → Slope → Abyssal Plain → Seamount → Submarine Canyon → Marine Mound
🧂 Salinity of Ocean Water
- Sea water is salty due to the salts dissolved in it.
- Definition: Salinity = grams of salt in 1000 g (1 kg) of sea water.
- Usually expressed in ‰ (parts per thousand).
⚡ Factors Affecting Salinity
- Supply of Water:
- Cold water → lower salinity
- River mouths → low salinity due to freshwater input
- Evaporation:
- Less at poles & higher latitudes → lower salinity
- More at Tropic of Cancer & Capricorn → higher salinity
- Ocean Currents:
- Cold currents → lower salinity
- Warm currents → higher salinity
🌐 Horizontal Distribution of Salinity
- Open Oceans:
- Highest salinity → Tropic of Cancer & Capricorn (high evaporation)
- Lower salinity → Equator (high rainfall)
- Lowest salinity → Poles (ice melting dilutes water)
🌡️ Temperature Distribution of Ocean Water
- Sea water is heated by solar radiation, so its temperature varies by depth and location.
- Layers of Ocean Water by Temperature:
- First Layer (Surface Layer):
- Depth: ~0–500 m
- Temperature: 20 °C → 25 °C
- Warmest layer, directly heated by the Sun
- Second Layer (Thermocline / Heat Gradient):
- Depth: 500–1000 m
- Temperature decreases rapidly with depth
- Third Layer (Deep Layer):
- Depth: >1000 m → ocean floor
- Temperature: ~0 °C (Antarctic surface water)
- Receives heat indirectly through circulation
🔹 Temperature Gradient vs Salinity Gradient
- Both affect water density → denser water sinks, lighter water rises → drives ocean currents.
- This stratification maintains vertical layering of ocean water.
✅ Exam Tip: Remember:
- 🌡️ Thermocline = temperature drops rapidly
- 🧂 Halocline = salinity rises rapidly
- Both layers exist around 500–1000 m depth and control ocean circulation
