The World's Foundations; or, Geology for Beginners · Ch. XXX, part 1 · Unit 5: Part III: Fire, coral and caves
34. Coral: polyps, reefs and atolls
Goal: Your child can explain how coral polyps build reefs, why a coral island rises above the water, and what an atoll is.
You'll need
- The book, open to this chapter
- A piece of coral or a sea-shell, and some sand
- White vinegar
- Strips of paper and tape (or dandelion stalks in season)
- A globe, atlas or free online map
- A notebook for a "rock journal" and a pencil
What's in the book
The first part of Chapter XXX (pages 291 to 298) describes the soft coral polyp and its hard lime skeleton, explains that reef-building corals live only from low tide down to about 120 feet, and shows how waves heap broken coral into islands, which plants, insects and birds then reach. It describes the ring-shaped atoll with its lagoon, and the Maldive Islands, made of atolls of atolls.
The lesson, step by step
A question to start
SayCan tiny soft animals, too small to see well, build an island?
Read together
Read pages 291 to 298, up to the paragraph that begins "In addition to atolls". Look at the picture of the coral island. Stop on:
An Atoll is simply a ring of land — or rather of coral — surrounded by the deep ocean, with a lake or ' lagoon ' of shallow salt water in the middle.
Polyp and skeleton
In plain words: the coral polyp is a soft, jelly-like animal that takes lime from the sea water and builds a hard skeleton under and around itself. If you have coral, look at its tiny holes with a magnifying glass: each held a polyp.
The polyps stop, the waves go on
Polyps cannot live above water, so they build only up to low tide. Then the waves take over, breaking off coral and heaping it up into an island, and grinding it into white sand.
Our yellow sand is flinty in nature, but the sand of a coral-island is made chiefly of lime.
Test your sand and a shell with vinegar. Which fizzes? (Lime fizzes; most yellow sand does not.)
Life arrives
Read the paragraph about how seeds, tree-trunks with insects and lizards, and birds come to a new coral island. Ask your child to list them in order.
Atolls of atolls
The book explains the Maldives with a picture:
A child's chain of dandelion stalks would best illustrate this.
Make small paper rings, link them into a chain, and join the chain into one big ring. Each small ring is an atoll with its own lagoon; the big ring encloses a great lagoon. Find the Maldives in the Indian Ocean on a map.
Tell it back
Ask your child to tell the life story of a coral island, from the first polyps to the first coconut tree.
What we know now
Reef-building corals have tiny algae living inside them that need sunlight to make food for the coral. That is why reef corals grow only in shallow, clear, sunny water, just the depths the book gives.
Afterwards
Notice it this week: test any sand you can find (sandbox, beach, aquarium gravel) with vinegar. Is it flint sand or lime sand?
Answers to the book's problems
The polyps build their lime skeletons up to low-tide level; the waves then break off coral and pile it higher, and grind it to sand; seeds, insects, lizards and birds arrive, and plants grow. An atoll is a ring of coral land around a shallow lagoon, with deep ocean outside.
Worked out for this site from the scan. If a number in your copy differs, trust the book.