286
N. Harvey
Reef are fastest in shallow well-lit waters and can be up to 20 mm a-I for massive corals and up to 100 mm a-I for branching corals. There are also numerous soft coral species which are not significant in reef building. However, another important reef builder
is the crustose coralline algae which in some places on the Great Barrier Reef form up
to 80% of reef sediments (Hopley 1982).
The individual reefs within the Great Barrier Reef vary considerably ranging in size
from small reef pinnacles up to reefs of 20 to 30 km in length and more than 125 km 2
and in shape from flat platform reefs to the elongate ribbon reefs of the continental
shelf margin. In some places the reefs are separated by channels of 200 m elsewhere
they may be 20 km apart. The reefs are submerged with some being exposed at very
low tides. There are about 300 reef island sand cays, 87 of these are permanently vegetated. There are also about 600 continental or high islands (Hopley 1982).
\The Great Barrier Reef varies in its form and complexity. In the north the outer reef
is formed by ribbon reefs (Fig. 15.2) up to 27 km long about 500 m wide with narrow
restricted passages. Here the continental shelf is very narrow and some of the outer
reefs may be 30 to 60 km from the coast. Behind the barrier is what looks like open
water which often contains coral patches or "bommies". The water may be 50 m or so
deep compared to the steep drop off beyond the shelf edge reefs. There are a series of
patch reefs occupying the mid shelf area in addition to a number of high or continental islands usually with fringing reefs (Fig. 15.3). Further towards the land maybe
5-10 km there are a series of reefs with reef deposits and reef islands often associated
with mangroves, beach rock, shingle ridges and sand cays. These are called the low
Fig. 15.3. Lizard Island and associate fringing reef in the Cairns section of the Reef. The island is of
continental origin and was used by Captain James Cook as a vantage point to find a navigable break
in the outer barrier (Photo: Nick Harvey; Note: The coloured version of this figure can be found on the
CD, enclosed with this book)
N. Harvey
Reef are fastest in shallow well-lit waters and can be up to 20 mm a-I for massive corals and up to 100 mm a-I for branching corals. There are also numerous soft coral species which are not significant in reef building. However, another important reef builder
is the crustose coralline algae which in some places on the Great Barrier Reef form up
to 80% of reef sediments (Hopley 1982).
The individual reefs within the Great Barrier Reef vary considerably ranging in size
from small reef pinnacles up to reefs of 20 to 30 km in length and more than 125 km 2
and in shape from flat platform reefs to the elongate ribbon reefs of the continental
shelf margin. In some places the reefs are separated by channels of 200 m elsewhere
they may be 20 km apart. The reefs are submerged with some being exposed at very
low tides. There are about 300 reef island sand cays, 87 of these are permanently vegetated. There are also about 600 continental or high islands (Hopley 1982).
\The Great Barrier Reef varies in its form and complexity. In the north the outer reef
is formed by ribbon reefs (Fig. 15.2) up to 27 km long about 500 m wide with narrow
restricted passages. Here the continental shelf is very narrow and some of the outer
reefs may be 30 to 60 km from the coast. Behind the barrier is what looks like open
water which often contains coral patches or "bommies". The water may be 50 m or so
deep compared to the steep drop off beyond the shelf edge reefs. There are a series of
patch reefs occupying the mid shelf area in addition to a number of high or continental islands usually with fringing reefs (Fig. 15.3). Further towards the land maybe
5-10 km there are a series of reefs with reef deposits and reef islands often associated
with mangroves, beach rock, shingle ridges and sand cays. These are called the low
Fig. 15.3. Lizard Island and associate fringing reef in the Cairns section of the Reef. The island is of
continental origin and was used by Captain James Cook as a vantage point to find a navigable break
in the outer barrier (Photo: Nick Harvey; Note: The coloured version of this figure can be found on the
CD, enclosed with this book)