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How the Great Barrier Reef was formed 

By Tanya Murphy, Dive Instructor

The Great Barrier Reef is one of the most biodiverse ecosystems on the planet with more than 600 species of coral and around 10% of the world’s fish species. But why is there so much coral and marine life at the Great Barrier Reef compared to other parts of the ocean?

It all starts with coral, which provides the habitat and food chain for all the Reef’s other creatures to thrive. Coral is an animal which needs sunlight and warmth to grow, and so it thrives best in shallow waters.


Between Victoria and Bundaberg on Australia’s East Coast, the ocean gets very deep quite close to the coast, so there aren’t many areas shallow enough for corals to grow. North of Bundaberg on the other hand, there is a large shallow shelf extending outward from the coast, which averages about 30-60m deep and is called the continental shelf.

Up until about 10,000BC, our planet was in an ice age which lasted more than two million years and which made sea levels dramatically lower than they are today. During that time the continental shelf was dry land. Local indigenous tribes have detailed accounts of how their ancestors used to live on this land, which they now call “sea country”. In fact, they can point out specific parts of the Reef and describe what sort of vegetation and geography it used to have, and who lived there, 12,000 years ago!

When the ice age ended, the sea levels rose and covered the shelf, forming a shallow lagoon. Corals started to grow on the chain of submerged hills along the outer edge of the shelf, where it was shallowest, and they could get more sunlight.

Corals always grow towards the surface, trying out-compete each other to reach as much sunlight as possible. Corals grow slowly - some species just a few millimetres per year - but they can live for thousands of years and grow tens of metres. If a hard coral dies, it leaves behind a hard limestone skeleton, and new corals grow on top. Thus, over thousands of years the corals continue building up until they are nearly touching the surface - and that is how coral reefs are formed! Some reefs create spectacular cliff-face drop-offs, overhangs, caverns and canyons as they grow, creating a magical landscape for us to explore.

Thus, each submerged hill and mound on the continental shelf has turned into a coral reef. There are more than 3000 Reefs which form the Great Barrier Reef. Most of them are in a chain close to the outer edge of the continental shelf but “mid-shelf” reefs also exist, as well as “fringing reefs” which are reefs growing around islands or mainland areas. The reefs closer to the outer edge of the continental shelf tend to be famous for having the clearest water and greatest diversity of marine life.

So, when you travel with Sunlover Reef Cruises, just 45 minutes from Cairns you can enjoy beautiful snorkelling on the fringing reef at Fitzroy Island. But if you want to experience clearer water, spectacular canyons and drop-offs, and a richer variety of marine life, be sure to visit one of our outer reef sites, such as Moore Reef!