Chapter 12: The Oceans
This chapter explores the “hidden” geography beneath the sea. While the ocean surface looks uniform, the floor is as rugged as the land, featuring mountains, plains, and deep trenches.
1. Relief of the Ocean Floor
The ocean floor is divided into four distinct zones based on depth and distance from the shore:
- Continental Shelf: The shallow, gently sloping area bordering the continents.
- Depth: Up to 200 metres.
- Importance: Home to the world’s richest fishing grounds (e.g., Grand Banks) and sources of oil and gas.
- Continental Slope: A sudden, steep drop-off at the edge of the shelf.
- Deep Sea Plain (Abyssal Plain): An undulating plain that covers about 2/3 of the ocean floor. It is the flattest place on Earth.
- Ocean Deeps (Trenches): Long, narrow, and very deep depressions. The Mariana Trench in the Pacific is the deepest (over 11,000 metres).
2. Salinity of the Ocean
Salinity refers to the amount of salt (mostly sodium chloride) dissolved in 1,000 grams of water. The average salinity is 35 parts per thousand (
353535).
- Higher Salinity: Found in areas with high evaporation and low rainfall (e.g., Red Sea).
- Lower Salinity: Found in areas with heavy rainfall, melting ice, or where large rivers bring in fresh water (e.g., Baltic Sea, Bay of Bengal).
3. Temperature of Ocean Water
- Surface water is warmest at the Equator and coldest at the Poles.
- The temperature drops rapidly with depth. Below 2,000 metres, the water is always near freezing (
0°C to
2°C).
4. Ocean Currents
These are “rivers” of water flowing through the ocean. They are caused by the Earth’s rotation (Coriolis Force), prevailing winds, and differences in water density.
- Warm Currents: Flow from the Equator toward the Poles (e.g., Gulf Stream, Kuroshio).
- Cold Currents: Flow from the Poles toward the Equator (e.g., Labrador, Canary, Benguela).
10 Revision Questions with Mnemonics
Q1: What are the four zones of the ocean floor?
- A: Continental Shelf, Slope, Deep Sea Plain, and Ocean Deeps.
- Mnemonic: “Shelf Slopes to Plain Deeps” (Shelf, Slope, Plain, Deeps).
Q2: Which ocean zone is most important for the fishing industry?
- A: The Continental Shelf (sunlight reaches the bottom, allowing plankton to grow).
- Mnemonic: “Shelf is for Seafood.”
Q3: What is the average salinity of the ocean?
- A: 35 parts per thousand.
- Mnemonic: “Three-Five keeps the ocean alive.”
Q4: Does the Red Sea have high or low salinity?
- A: High (due to extreme heat/evaporation and no rivers).
- Mnemonic: “Red is Hot and Salty.”
Q5: What is the deepest trench in the world?
- A: The Mariana Trench.
- Mnemonic: “Mariana is the Maximum depth.”
Q6: How do currents move in the Northern Hemisphere?
- A: To the Right (Clockwise) due to Coriolis Force.
- Mnemonic: “North is Normal” (Clockwise like a clock).
Q7: Name the warm current that keeps Western Europe warm.
- A: The North Atlantic Drift (an extension of the Gulf Stream).
- Mnemonic: “North Atlantic is a Nice Amenity.”
Q8: What happens when a warm current meets a cold current?
- A: It creates thick Fog and is excellent for fishing (e.g., near Newfoundland).
- Mnemonic: “Warm + Cold = World Class Fishing.”
Q9: Why is the Baltic Sea less salty?
- A: Low evaporation and lots of fresh water from rivers and melting ice.
- Mnemonic: “Baltic is Bland” (less salt).
Q10: What is the Coriolis Force?
- A: A force caused by Earth’s rotation that deflects winds and currents.
- Mnemonic: “Rotation causes Deflection.”
Quick Summary Table: Ocean Currents
| Current Type | Origin | Impact on Coast | Example |
|---|---|---|---|
| Warm | Equator | Brings rain and warmth | Gulf Stream |
| Cold | Poles | Brings dry air and fog | Peru Current |
Understanding ocean currents is often the trickiest part for students, but with the right mnemonics, you will never forget which is which.
The “Golden Rule” of Currents
- Warm Currents: Always flow Away from the Equator (seeking the poles).
- Cold Currents: Always flow Toward the Equator (seeking the heat).
Mnemonics for Major Ocean Currents
1. Cold Currents (The “B-C-L-P” Rule)
Think of these as the “Cool Blue” currents.
- Benguela (Africa)
- Canary (Europe/Africa)
- Labrador (N. America)
- Peru / Humboldt (S. America)
- Mnemonic: “Big Cold Lions Prowl.”
2. Warm Currents (The “G-K-B” Rule)
Think of these as the “Glowing Red” currents.
- Gulf Stream (Atlantic)
- Kuroshio (Japan)
- Brazil (S. America)
- Mnemonic: “Giant Kettles Boil.”
10 Revision Questions with Mnemonics
Q1: Is the Kuroshio current Warm or Cold?
- Answer: Warm.
- Mnemonic: “Kuroshio is Koel” (Wait, that sounds like cool, but remember: Kuroshio = Keen on heat). Alternatively: “Kuroshio is a Hot Show.”
Q2: What is the effect of the Labrador Current on the USA’s NE coast?
- Answer: It brings Icebergs and freezing temperatures.
- Mnemonic: “Labrador is a Low-temperature dog.”
Q3: Which cold current flows along the West coast of South America?
- Answer: The Peru (or Humboldt) Current.
- Mnemonic: “Peru is Purely freezing.”
Q4: Name the warm current that keeps the UK ports ice-free in winter.
- Answer: The North Atlantic Drift.
- Mnemonic: “North Atlantic Drift = No Actual De-icing” (needed).
Q5: What is the “Coriolis Force” effect on currents?
- Answer: It turns them Clockwise in the North and Anti-clockwise in the South.
- Mnemonic: “North is Right, South is Left” (NRL – like the Rugby League).
Q6: Is the Benguela current Warm or Cold?
- Answer: Cold (off the coast of SW Africa).
- Mnemonic: “Benguela is a Brr-guela.”
Q7: Which current is responsible for the dry conditions of the Atacama Desert?
- Answer: The Cold Peru Current (Cold currents bring dry air).
- Mnemonic: “Cold = Cactus” (Dry/Desert).
Q8: What happens at the meeting point of the Gulf Stream and Labrador current?
- Answer: Intense Fog and rich Fishing (Grand Banks).
- Mnemonic: “Foggy Fish” (Both start with F).
Q9: Is the Canary current Warm or Cold?
- Answer: Cold.
- Mnemonic: “Canary is a Chill-bird.”
Q10: Where do Warm currents originate?
- Answer: Near the Equator.
- Mnemonic: “Equator is the Engine” (it heats the water).
Summary Table for Long-Term Memory
| Current | Temperature | Region | Mnemonic |
|---|---|---|---|
| Gulf Stream | Warm | USA / Atlantic | “Gulf is Glowing” |
| Labrador | Cold | Canada | “Labrador is a Lump of Ice” |
| Kuroshio | Warm | Japan | “Kuroshio Keeps it Hot” |
| Oyashio | Cold | Japan | “Oyashio is Oh-so-cold” |
| Benguela | Cold | S. Africa | “Benguela Brings the Chill” |
