Atoll Order

Atoll Order: the Darwin-Dana-Daly theory of atoll formation for Cambridge IGCSE Marine Science

This free game is part of our Cambridge iGCSE Marine Science (0697) course for exams in EXAM YEARS 2027-2029.

Keep a coral reef alive for 40 million years

Atoll Order is a 3D game about one of the most elegant ideas in marine science: how a ring of coral reef, an atoll, can sit in the middle of a deep ocean when reef-building corals only grow in shallow water. It covers the Cambridge IGCSE Marine Science (0697) point on describing the Darwin-Dana-Daly theory of atoll formation, and the point on explaining how environmental factors affect the distribution of corals.

You look at a tropical volcanic island with a quarter cut away, like a slice taken out of a cake, so you can see the water, the volcanic rock and the coral limestone in cross-section. A clock counts down from 40 million years ago to today, and your reef geologist, Talia, explains each new idea the first time you meet it.

Fringing reef, barrier reef, atoll

First you choose where coral larvae should settle. Bare rock in shallow, clear, sunlit water is right; loose sand, deep rock on the slope and the muddy water at a river mouth are traps, and each one comes with a short explanation. The corals spread round the shore to form a fringing reef.

Then the island starts to sink. This subsidence happens because the volcano's huge weight bends the Earth's crust down, and the hot rock beneath it cools and contracts. You hold GROW to build the reef upwards and keep its top in the sunlit water: let the reef sink into the dark and it drowns, grow it into the air and it dries out. While you fight to stay in the light, the island shrinks inside your reef, a lagoon opens up and you have a barrier reef. Keep going and the volcano's peak goes under, leaving a ring of reef round a lagoon: an atoll.

Dana, Daly and the drill core

• Darwin worked out the idea on the voyage of HMS Beagle and published it in 1842: the island sinks and the reef grows upwards to stay in shallow, sunlit water.
• Dana backed it up with evidence: deep bays on islands with barrier reefs are river valleys that were drowned as the land sank. In the game you see the valleys flood.
• Daly added sea-level change. In the ice ages so much water was locked up in ice sheets that sea level fell by over 100 m. Exposed reefs died and were worn down by waves, then grew up again as the sea rose. You survive two ice ages and have to grow fast enough to keep up with the rising sea.

Finally you drill down through the reef you built and take samples: a weathered layer from an ice age, old coral limestone and, at the bottom, volcanic rock. Coral limestone deep underground must have formed near the surface and then sunk. That is what scientists found when they drilled about 1,400 m into Enewetak (Eniwetok) Atoll in 1952.

What decides where corals grow

The game builds in every factor in the specification. Abiotic factors: reef-building corals need warm water, high light intensity (their zooxanthellae photosynthesise), clear water and a hard substrate. A tropical storm washes mud off the island and the sunlit layer gets thinner; a marine heatwave bleaches the corals so they stop growing. Reef water is clear because it is low in nutrients, which are held within the organisms living there. Biotic factors: crown-of-thorns starfish eat coral polyps, so you tap them off the reef, and parrot fish graze seaweed off the coral and grind coral into the sand that builds sand islands on the atoll.

A game takes about 5-10 minutes, ends with a recap of the theory and your best score is saved, so it works well as a starter, a revision task or homework alongside the lesson.