The World's Foundations; or, Geology for Beginners · Ch. XXX, part 2 · Unit 5: Part III: Fire, coral and caves
35. Coral: barrier reefs and the sinking sea floor
Goal: Your child can explain fringing reefs, barrier reefs and atolls, and how a slowly sinking island could turn one into the next.
You'll need
- The book, open to this chapter
- A bowl, a lump of modeling clay for the island, and a strip of a different-colored clay for the reef
- A pitcher of water
- Paper and pencils
- A notebook for a "rock journal" and a pencil
What's in the book
The second part of Chapter XXX (pages 298 to 304) describes fringing reefs close to shore and barrier reefs farther out, including the great reef off Australia, with coral reaching thousands of feet deep. It explains the theory of a slowly sinking Pacific sea floor, which turns a fringing reef into a barrier reef and then an atoll, and discusses how fast coral grows.
The lesson, step by step
A question to start
SayReef corals can only live in shallow water. So how can coral rock go down thousands of feet?
That is the puzzle of this lesson.
Read together
Read pages 298 to 304, to the end of the chapter. Stop on:
All islands in mid-ocean are in reality hill-tops or mountain-summits.
Three kinds of reef
Fringing reef: joined to the shore. Barrier reef: farther out, with a channel of water between it and the land. Atoll: a ring with no island left in the middle. Draw all three as seen from above.
Sink the island
Make a clay mountain in the bowl, with its top poking up as an island. Press a thin ring of the other clay around its shore: a fringing reef.
Now pour in a little water: the "island sinks". Build the reef ring a little higher, so it stays at the surface. Pour again, build again. Soon the reef stands apart from the shrinking island (barrier reef), and at last the island is under water and only the ring is left: an atoll.
Slow and fast coral
Read the two examples: a ship's anchor only thinly crusted after fifty years, and an islet in the Maldives covered by new reef within a few years. The book also says coral grew three feet in six months in an experiment in Madagascar. Ask: What does this tell us about measuring how old a reef is?
Tell it back
Ask your child to explain the sinking-island theory using the clay model, and why it solves the puzzle of deep coral.
What we know now
This theory, which the book does not name, was first put forward by Charles Darwin. It was proved in 1952, when drillers on Enewetak Atoll in the Pacific bored down through more than 4,000 feet of coral rock before reaching the old volcano underneath. The sea floor sinks because old ocean crust cools and settles as it moves away from where it was formed. The Great Barrier Reef off Australia stretches about 1,400 miles.
Afterwards
Notice it this week: look at a satellite map of the Pacific with an adult and find a ring-shaped atoll.
Answers to the book's problems
The island slowly sank while the polyps kept building upward to stay near the surface; the dead coral below became limestone, so coral rock now reaches thousands of feet down though living reef coral grows only in shallow water. Growth speed varies so much (an anchor barely crusted in fifty years, three feet in six months elsewhere) that a reef's thickness cannot tell its age.
Worked out for this site from the scan. If a number in your copy differs, trust the book.