# Build a Coral Reef Model

Part of the Oceanography Merit Badge Digital Resource Guide: [Introduction & Overview](https://merit-badge.university/merit-badges/oceanography/guide/index.md)

*7. Hands-On Investigation*

> **7b.** Make a series of models (clay or plaster and wood) of a volcanic island. Show the growth of an atoll from a fringing reef through a barrier reef. Describe the Darwinian theory of coral reef formation.

This option is about slow change over a very long time. Your model should show how a coral reef can begin around a volcanic island and end up as a ring-shaped atoll after the island sinks or erodes away.

## Darwin's Basic Idea

Charles Darwin proposed that coral reefs can grow upward while the volcanic island beneath them slowly subsides. In his model:

1. a volcanic island rises above sea level
2. corals grow around the shore, forming a **fringing reef**
3. the island slowly sinks, but the reef keeps growing upward, forming a **barrier reef** with a lagoon between reef and island
4. eventually the island disappears below sea level, leaving a ring-shaped **atoll** around a lagoon

The key idea is that reef growth and island subsidence happen together over time.

Video: [Coral Reefs: Types and Formation (video)](https://youtu.be/mPA9Ze16lGw?si=pVw56hFgRlnPJtro)

Video: [How Coral Reefs are Formed (video)](https://youtu.be/anDSRfSY7LQ)

## What Your Model Series Should Show

Instead of one model, think of this as three stages.

### Stage 1: Fringing reef

Show coral growing close to the shoreline of the volcanic island.

### Stage 2: Barrier reef

Show the island lower than before, with a wider lagoon between the island and reef.

### Stage 3: Atoll

Show the island gone from above sea level, leaving a reef ring and central lagoon.

**What to Label on the Models** — Make the science easy to see

- [ ] volcanic island
- [ ] coral reef
- [ ] lagoon
- [ ] changing sea level relationship
- [ ] which stage is fringing reef, barrier reef, or atoll

## Building Tips

Clay works well because you can shape the island and reef line clearly. A board base helps keep each stage stable. Use different colors for land, reef, and water so your counselor can see the transition quickly.

> **Tip**
>
> Do not make all three models look almost the same. Exaggerate the changes a little so the progression from fringing reef to atoll is obvious at a glance.

## What to Explain When You Present It

Your counselor will likely care as much about your explanation as the model itself. Be ready to say:

- why coral reefs start near shallow sunlit water
- why the reef can keep growing upward
- why the lagoon becomes more obvious in the barrier reef stage
- why the final atoll can remain after the island is gone from view

Next, the focus shifts from long-term reef growth to short-term measurements you can record and graph.

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