How the Great Barrier Reef was
formed
Thirteen
thousand years ago
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. 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 2:
Hills became islands with fringing reefs and
eventually disappeared under many meters of coral
as the sea level continued to rise.
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.
|
|
Figure 3: Cross section of
coastal north Queensland 13,000 years
ago.
|
Sea-level was
approximately 60 meters lower than present time.
|
|
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.
|
Figure 4:
Cross section of a fringing reef around a
continental island.
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.
|
A view across
Hinchinbrook channel to Hinchinbrook Island with
mangrove forests in the foreground.
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Last updated - December 18, 2008
Copyright ©1996-2003 Australian Institute of
Marine Science
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