How the Great Barrier Reef was formed Thirteen thousand years ago

Figure 2: Hills became islands with fringing reefs and eventually disappeared under many meters of coral as the sea level continued to rise.

Sea level rose steadily from about 20,000 years ago until about 6,000 years ago, after which it approached its present height. Thus, sea level would have been about half way between its old height and its present height about 13,000 years ago.

What happened to the hills?

As sea level rose, the lower lying coastal plains became flooded and the hills became islands. Corals settled and grew on hard surfaces around these islands. When these corals died, new corals settled upon them and grew. These coral skeletons were cemented together by calcareous algae and yet other calcareous algae formed sand that filled in the spaces amongst this framework.

Figure 3: Cross section of coastal north Queensland 13,000 years ago.

Over time, these processes formed fringing reefs around the islands. As sea level continued to rise, small hills were submerged by the sea and reefs grew completely over them.

Reefs grow best where there is lots of water movement. Hence, fringing reefs are usually better developed on the windward sides of islands than on the leeward (sheltered) sides. Once sea level has risen to cover a low hill, material eroded from the reef is carried from its windward side to the more sheltered side, where it builds up. Thus, although corals grow best on the windward side of reefs, the reefs themselves grow fastest on their leeward side.

Figure 4: Cross section of a fringing reef around a continental island.

The taller hills were not inundated by the rising sea level. They became what is known today as continental islands because they are often extensions of mountain ranges on the mainland. Most of these islands have fringing reefs.

Rocky coastline of Magnetic Island, a continental island 8 km off the North Queensland coast.

The dark patches at the top of the photograph are coral colonies surrounded by sand fill that has been deposited in the shallow lagoon between the reef and the shore.

  • Most of the bays on Magnetic Island have fringing reefs with the leeward (sheltered) side of the island supporting mangrove forests.
  • Sea-level was approximately 60 meters lower than present time.
A view across Hinchinbrook channel to Hinchinbrook Island with mangrove forests in the foreground.

This drawing shows how coral build reefs around islands.

The rising and falling tides and wave action usually deposits sand between the reef and the shore.

Most continental islands have rocky headlands, mountain peaks and sandy beaches. The leeward side of these islands usually have mangrove forests.

Leeward side of Hinchinbrook Island, a large continental island off the Nth Queensland coast.