Page 19 - Coral Reef Teachers Guide

Coral Forest Teacher’s G u i d e What and Where are the Coral Reefs?

CORAL REEF FORMATION TYPES OF REEFS

Today’s coral reefs have accumulated during the last There are three major types of coral reefs: fring‐
10,000 years since the last glacial periods of the Pleis‐ ing reef, barrier reef, and atoll. In tropical areas,
tocene epoch. As glacial ice melted and sea levels fringing reefs grow directly from the shorelines
and temperatures rose, present‐day reefs began to of continents and islands. Barrier reefs are found
form. further out, separated from the shore by a stretch
of protected water, the lagoon. Atolls are offshore
Hard corals provide the main structural framework.
ring‐like coral formations that surround a shallow,
Other organisms, such as coralline algae and proto‐
central lagoon.
zoans, bind and cement everything together with
Darwin’s Theory of Atoll Formation: In 1842,
sheetlike growth that stabilizes the reef. Sand and
Charles Darwin provided the theory of atoll forma‐
sediments are created by boring organisms, such as
tion which offers the most widely accepted expla‐
sponges and bivalves (i.e. clams, oysters); green cal‐
nation of coral reef formation today.
cified algae (Halmedia) which has calcium carbonate
plates that drop off; and, grazers, such as parrotfish The theory is best understood in terms of reef for‐
and sea urchins, which attack the coral for food, ex‐ ma‐ tion on a tropical island. A tropical volca‐
tracting nutrition from the polyps, breaking down their nic island furnishes the shallow underwater base
limestone bases, and excreting the waste as sand on which the coral grows. Eventually, the island
(Figure 1‐8). becomes sur‐ rounded by a fringing reef which is
separated in places from the island by only a shal‐
DID YOU KNOW? The calcium carbonate from the
low, narrow strip of water (Figure 1‐9a).
sand, shells, and coral maintains the pH balance in the
ocean which in turn maintains life as we know it. If the island sinks gradually i nto the water, then a
chan‐ nel develops between the land and the coral
forming a barrier reef (Figure 1‐9b). A similar
process can occur with larger land masses due to
the shifting of crustal plates. The Great Barrier Reef
is the best ex‐ ample of this.

If the island continues to sink slowly enough be‐
neath the surface of the water, coral growth is able
to keep pace and the reef survives as an atoll (Fig‐
ure 1‐9c). Rather than being a closed ring, the atoll
usually consists of numerous tiny islands separated
by channels. These channels allow for water ex‐
change between the open sea and the lagoon.

Atolls are found in deep, clear water throughout
remote areas of the Indo‐Pacific region. Located in
the Marshall Islands in the Pacific, Kwajalein,
the world’s largest atoll, is almost 80 miles long
(a) (129km).
(b) DID YOU KNOW? Darwin’s theory of atoll forma‐
Figures 1-8. A source of tion was very controversial in his time. Most people
sand: (a) parrotfish did not believe that land could sink. They did not
grazing tail-up on hard under‐ stand, as we do today, that the Earth is a
coral, and (b) close-
up of the parrotfish’s dynamic mass, constantly moving and changing its
“beak.” (Illustrations: form.
Wendy Weir)

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