occupied by islands, and equally rarely are no islands present. In the Maldives there are about 1,200 islands in its
21 administrative atolls, while in the Tuamotus most of
the 75 atolls have over 20 islets each, with about 280 islets
on the atoll of Raroia. Wiens (1962) has shown that the
number of islands on atolls and the area they occupy
decrease westward across the Pacific from the Tuamotu
archipelago to the Caroline Islands.
Atoll islands are small and often cover only a small
fraction of the available reef space. For instance, on the
five main atolls in Tuvalu (Nanumea, Nui, Nukufetau,
Funafuti, and Nukulaelae), there are 113 islands that
occupy just 1–18% of the possible reef platforms. Here
the average island size is 13 ha, but the majority are tiny
with fifty-one <1 ha, another twenty-seven <5 ha, and
only four >100 ha, the largest being 220 ha. Similarly,
in the Maldives on South Malhosmadulu atoll there are
53 islands with a mean size of 10 ha and range from
<1 to 46 ha. On this atoll the average proportion of reef
platform occupied by islands is 28%, with a range from
<1 to 71%. In both Tuvalu and the Maldives frequency
distributions of island size and proportion of available reef
occupied by islands are strongly skewed toward the lower
values, a common situation throughout the atoll world.
Within atolls, islands are unevenly distributed around
the reef rim to windward and leeward, particularly in the
Pacific where the trade winds dominate. In many cases
islands are concentrated along the eastern (windward)
reef, with a tendency toward the southeast sector in the
Marshall Islands and the northeast in the Tuamotus. Frequently, these windward islands are narrow linear features
which tend to be closer to the lagoon shore than to the
ocean reef edge. Along the western side (leeward) of
Pacific atolls the reef is often discontinuous with fewer
and more compact islands. Islands are rare within lagoons
of Pacific atolls. In the Indian Ocean the distribution of
reefs and islands is different. In the Laccadive–Maldive
archipelago, atoll rims are typically more open, and
islands occupy reefs both around the eastern and western
margins as well as within the central lagoons. This more
regular distribution of islands is primarily a consequence
of the seasonal reversal in monsoonal conditions.
Motu
In coral reef literature, the Polynesian word motu has
a more restricted meaning than simply “island” or “islet,”
although usage has not been consistent. In Stoddart and
Steers’ (1977) classification, reef islands with a high seaward shingle ridge or ridges and a lower sand area to leeward are called motu, to distinguish them from pure sand
cays or pure shingle cays. Such motu are typically associated with high-energy environments on the windward rim
of atolls. Other authors equate motu exclusively with
coarse-grained sediments, coral gravel, and cobbles, to
separate them from simple sand cays. On the other hand,
Richmond (1993) uses the more general meaning of motu
to mean any atoll islet or island. Note that in New Zealand
Maori language motu is not limited to an islet or island, but
can also refer to any land, clump of trees, ship, or anything
else that is separated or isolated.
Classification of atoll islands
There have been several classifications of reef islands (see
Coral Cay Classification and Evolution) and these can be
equally applied to islands on atolls. However, there are
two well-known classifications specifically for atolls.
The first by MacNeil (1972) who classified atoll islets
“according to the way they are formed” noting that most
atoll islands in the northern Marshall Islands are accumulations of sediments around obstructions of rock, the most
common obstructions being platforms of raised reef, storm
rubble tracts, and beach rock. Hence, three main types are
distinguished:
Type 1 islets are formed on platforms or eroded remnants of raised reef
Type 2 islets are formed on rubble tracts of storm-cast
debris
Type 3 islets are formed from consolidated sand bars or
beach rock
Special islets, including combinations of the above, were
recognized as the fourth type.
More recently, Richmond (1993) distinguished four
types of atoll islets based on morphology, sediment and
rock characteristics, and position on reef rim from observations in western Kiribati, Tuvalu, and the Cook Islands:
Type 1 islets are analogous to sand cays and occur primarily adjacent to reef passages on the leeward rim or
atop lagoon patch reefs
Type 2 islets are typically U-to-boomerang shaped and
develop on high energy complex bends of the atoll rim
Type 3 islets are narrow, elongated gently curved to sinuous in plan shape, and are composed primarily of several parallel ridges which are often separated by
a central depression.
Type 4 islets are complex landforms developed around
cemented rubble and reef flat deposits and exhibit
a wide variety of shapes.
This last class of islets corresponds closely to the first two
islet types of MacNeil’s classification above.
In addition to these scientific classifications, most atoll
states and communities have local words or terms to
describe the range of typical islands in their territories.
Thus, in the Maldives, island classification is based on
the presence or absence of vegetation, typical sediments,
and relative size. Large vegetated sand islands are known
as raa and smaller ones as fushi. Huraa are vegetated shingle or rubble islands. Unvegetated islands include finolhus
for sand cay/banks and huraagandu or huraalette for shingle cay/banks. Throughout much of Polynesia, prefixes
or suffixes provide further descriptions of atoll motu
such as Motu-iti = small island, Motu-one = sand island,
Motu-loa = long island, and Tua-motus = islands on the
ocean’s back.
48
ATOLL ISLANDS (MOTU)
21 administrative atolls, while in the Tuamotus most of
the 75 atolls have over 20 islets each, with about 280 islets
on the atoll of Raroia. Wiens (1962) has shown that the
number of islands on atolls and the area they occupy
decrease westward across the Pacific from the Tuamotu
archipelago to the Caroline Islands.
Atoll islands are small and often cover only a small
fraction of the available reef space. For instance, on the
five main atolls in Tuvalu (Nanumea, Nui, Nukufetau,
Funafuti, and Nukulaelae), there are 113 islands that
occupy just 1–18% of the possible reef platforms. Here
the average island size is 13 ha, but the majority are tiny
with fifty-one <1 ha, another twenty-seven <5 ha, and
only four >100 ha, the largest being 220 ha. Similarly,
in the Maldives on South Malhosmadulu atoll there are
53 islands with a mean size of 10 ha and range from
<1 to 46 ha. On this atoll the average proportion of reef
platform occupied by islands is 28%, with a range from
<1 to 71%. In both Tuvalu and the Maldives frequency
distributions of island size and proportion of available reef
occupied by islands are strongly skewed toward the lower
values, a common situation throughout the atoll world.
Within atolls, islands are unevenly distributed around
the reef rim to windward and leeward, particularly in the
Pacific where the trade winds dominate. In many cases
islands are concentrated along the eastern (windward)
reef, with a tendency toward the southeast sector in the
Marshall Islands and the northeast in the Tuamotus. Frequently, these windward islands are narrow linear features
which tend to be closer to the lagoon shore than to the
ocean reef edge. Along the western side (leeward) of
Pacific atolls the reef is often discontinuous with fewer
and more compact islands. Islands are rare within lagoons
of Pacific atolls. In the Indian Ocean the distribution of
reefs and islands is different. In the Laccadive–Maldive
archipelago, atoll rims are typically more open, and
islands occupy reefs both around the eastern and western
margins as well as within the central lagoons. This more
regular distribution of islands is primarily a consequence
of the seasonal reversal in monsoonal conditions.
Motu
In coral reef literature, the Polynesian word motu has
a more restricted meaning than simply “island” or “islet,”
although usage has not been consistent. In Stoddart and
Steers’ (1977) classification, reef islands with a high seaward shingle ridge or ridges and a lower sand area to leeward are called motu, to distinguish them from pure sand
cays or pure shingle cays. Such motu are typically associated with high-energy environments on the windward rim
of atolls. Other authors equate motu exclusively with
coarse-grained sediments, coral gravel, and cobbles, to
separate them from simple sand cays. On the other hand,
Richmond (1993) uses the more general meaning of motu
to mean any atoll islet or island. Note that in New Zealand
Maori language motu is not limited to an islet or island, but
can also refer to any land, clump of trees, ship, or anything
else that is separated or isolated.
Classification of atoll islands
There have been several classifications of reef islands (see
Coral Cay Classification and Evolution) and these can be
equally applied to islands on atolls. However, there are
two well-known classifications specifically for atolls.
The first by MacNeil (1972) who classified atoll islets
“according to the way they are formed” noting that most
atoll islands in the northern Marshall Islands are accumulations of sediments around obstructions of rock, the most
common obstructions being platforms of raised reef, storm
rubble tracts, and beach rock. Hence, three main types are
distinguished:
Type 1 islets are formed on platforms or eroded remnants of raised reef
Type 2 islets are formed on rubble tracts of storm-cast
debris
Type 3 islets are formed from consolidated sand bars or
beach rock
Special islets, including combinations of the above, were
recognized as the fourth type.
More recently, Richmond (1993) distinguished four
types of atoll islets based on morphology, sediment and
rock characteristics, and position on reef rim from observations in western Kiribati, Tuvalu, and the Cook Islands:
Type 1 islets are analogous to sand cays and occur primarily adjacent to reef passages on the leeward rim or
atop lagoon patch reefs
Type 2 islets are typically U-to-boomerang shaped and
develop on high energy complex bends of the atoll rim
Type 3 islets are narrow, elongated gently curved to sinuous in plan shape, and are composed primarily of several parallel ridges which are often separated by
a central depression.
Type 4 islets are complex landforms developed around
cemented rubble and reef flat deposits and exhibit
a wide variety of shapes.
This last class of islets corresponds closely to the first two
islet types of MacNeil’s classification above.
In addition to these scientific classifications, most atoll
states and communities have local words or terms to
describe the range of typical islands in their territories.
Thus, in the Maldives, island classification is based on
the presence or absence of vegetation, typical sediments,
and relative size. Large vegetated sand islands are known
as raa and smaller ones as fushi. Huraa are vegetated shingle or rubble islands. Unvegetated islands include finolhus
for sand cay/banks and huraagandu or huraalette for shingle cay/banks. Throughout much of Polynesia, prefixes
or suffixes provide further descriptions of atoll motu
such as Motu-iti = small island, Motu-one = sand island,
Motu-loa = long island, and Tua-motus = islands on the
ocean’s back.
48
ATOLL ISLANDS (MOTU)