Chapter 11: Islands and Coral Reefs
This chapter explores the various types of islands and the fascinating biological construction of coral reefs. Islands are not just random landmasses; they have distinct geological origins.
1. Types of Islands
Islands are classified into two main groups based on their proximity to continents:
A. Continental Islands
These were once part of a mainland and are now separated by shallow seas (the continental shelf).
- Individual Islands: Located near the shore (e.g., Newfoundland, Madagascar).
- Archipelagos (Island Groups): Clusters of islands (e.g., British Isles, Malay Archipelago).
- Festoons or Island Arcs: Curving chains of islands often marking the edge of a continental shelf (e.g., Japanese Islands, Aleutian Islands).
B. Oceanic Islands
These are small, isolated islands located in the deep ocean, with no past connection to any continent.
- Volcanic Islands: The peaks of underwater volcanoes (e.g., Hawaii, St. Helena, Canary Islands).
- Coral Islands: Formed by the organic accumulation of coral polyps (e.g., Maldives, Marshall Islands).
2. Coral Reefs
Coral reefs are built by Coral Polyps (tiny marine organisms). When they die, their calcium carbonate skeletons accumulate to form massive structures.
Conditions for Coral Growth:
- Warm Water: Temperature between 20°C and 30°C.
- Shallow Water: Depth not exceeding 180 feet (sunlight is needed for photosynthesis).
- Clear, Salty Water: Silt and fresh water (from river mouths) kill corals.
- Food: They need oxygenated water with plenty of microscopic organisms.
3. Types of Coral Reefs
- Fringing Reef: Attached to the shore, growing outward (the simplest type).
- Barrier Reef: Separated from the shore by a wide, deep Lagoon (e.g., Great Barrier Reef of Australia).
- Atoll: A circular or horseshoe-shaped reef surrounding a central lagoon, with no central island.
10 Revision Questions with Mnemonics
Q1: What are the three main types of coral reefs?
- A: Fringing Reef, Barrier Reef, and Atoll.
- Mnemonic: “Fab Bright Atolls” (Fringing, Barrier, Atoll).
Q2: What temperature is ideal for coral growth?
- A: Between 20°C and 30°C.
- Mnemonic: Corals love the “Twenty-Thirty” rule.
Q3: Why don’t corals grow near the mouths of large rivers like the Amazon?
- A: Rivers bring Fresh Water and Silt (mud), both of which kill coral polyps.
- Mnemonic: Corals hate “Muddy Water” (Silt + Fresh).
Q4: What is a “Lagoon”?
- A: A shallow body of water separated from the open sea by a barrier reef or an atoll.
Q5: What is the difference between a Continental and an Oceanic island?
- A: Continental islands sit on the Shelf (connected to mainland); Oceanic islands rise from the Deep (usually volcanic or coral).
- Mnemonic: Continental = Connected; Oceanic = Outlier.
Q6: What is the deepest a coral can grow?
- A: Roughly 180 feet (60 meters) because they need sunlight.
- Mnemonic: “Sunlight at One-Eighty keeps the coral healthy.”
Q7: What is an Island Festoon?
- A: An archipelago arranged in a curve or arc (e.g., Japan).
- Mnemonic: “Festoon is a Spoon” (curved shape).
Q8: Give an example of a famous Barrier Reef.
- A: The Great Barrier Reef in Australia (over 1,200 miles long).
Q9: How is an Atoll different from a Fringing Reef?
- A: A Fringing Reef is stuck to the land; an Atoll is a ring with nothing in the middle but water.
- Mnemonic: “Atoll is a Bowl” (hollow center).
Q10: What is the primary material of a coral reef?
- A: Calcium Carbonate (Limestone) from the skeletons of polyps.
- Mnemonic: “Coral Bone is Calcium Stone.”
Quick Summary Table: Coral Comparison
| Type | Connection to Land | Water Body |
|---|---|---|
| Fringing | Direct/Close | None or narrow channel |
| Barrier | Separated | Wide, deep Lagoon |
| Atoll | None (Island submerged) | Central Lagoon |
