Geomorphic features of atoll islands
Regardless of island size or shape, and despite their low
absolute elevation, atoll islands possess subtle but recognizable variations in surface topography which reflect
both geomorphic processes and sediment types. Three natural landform units can often be distinguished: a highgravel ridge or ridges on the ocean or windward side;
a low-sand flat ridge or ridges on the lagoon or leeward
side; and a central depression or flat between the two. This
asymmetric topographic form is also present on islands
comprising a single sediment type, either sand or gravel.
However, on islands located in environments where there
is no clear windward or leeward side, islands are commonly basin shaped with a high peripheral ridge of
roughly equal elevation encircling the whole island. Classic examples of this topographic form occur on the circular
patch reefs within lagoons of Maldivian atolls.
On most atoll islands the highest natural elevation is the
wave-emplaced ocean side or peripheral beach ridge,
which commonly reaches 3–4 m above MSL. On some
islands, however, the beach ridge is topped by a dune of
fine wind-blown sand, although the presence of sand
dunes is not a usual feature on most atoll islands. Where
dunes do occur they reach higher elevations than waveformed ridges, on Cocos (Keeling) Islands, up to 5.5 m
above MSL on Home Island, 7 m on West Island, and
11 m on South Island (based on surveys reported by
Woodroffe and McLean (1994)).
Excluding sand dunes, the highest elevation on atoll
islands, and often the greatest and most complex relative
relief, results from human activity rather than natural
processes. From French Polynesia in the Pacific to the
Laccadive–Maldive archipelago in the Indian Ocean,
crop cultivation, especially for taro and banana, is universally carried out in flat-floored depressions or pits
excavated down to the island water table. Spoil dug
from the pit is dumped around the pit edge building
a spoil bank above the level of the surrounding land.
The resulting topography is frequently chaotic, especially where groups of pits and banks occur in close
proximity to one another.
Sediments, soils, and groundwater
Atoll islands are made up of calcareous sediments, primarily the skeletal remains of organisms living on the
adjacent reef or in the lagoon. Invariably, the coarsest
materials comprise whole or broken clasts of coral, typically of sizes from a few centimeters to a few decimeters. In contrast to this single taxa dominance in the
gravel and rubble fraction, sand-sized sediments may
include small coral fragments, but more commonly comprise a mix of coralline algae, mollusks, foraminifera,
and Halimeda. Both the calcareous and textural characteristics of the sediments have a major influence on soil
properties. Not surprisingly, atoll soils tend to be light
colored, shallow, alkaline and coarse textured with no
clay. Fertility is highly dependent on the organic matter
content primarily from vegetation and leaf litter, and
moisture retention is generally low. Atoll soils are also
excessively well drained; water from rainfall quickly
percolates to the water table. Fresh water generally tops
the groundwater lens within atoll islands, beneath which
is the denser saline water originating from the surrounding ocean and lagoon. Traditionally, the fresh water lens
has been used as the primary source of potable water by
atoll communities.
Processes of formation and change on atoll islands
Islands accumulate on atoll reefs where sediment is
available and where wave processes are focused, often
as a result of wave refraction around the reef. Frequently, island shape mimics that of the reef, several
examples from Tuvalu being cited by Richmond
(1993). Whether or not islands will form on atoll reefs
is also dependent on the strength of wave energy.
A west to east traverse across South Maalhosmadulu
atoll in the Maldives shows that continual high wave
energy on the western reef rim sweeps sediment right
across the reef platform and islands only occur on the
broadest reef flats. In contrast, on the eastern atoll rim
where wave energy levels are lower, islands occur on
most of the available reef platforms. The more equal
and lowest magnitude wave energy of the central atoll
ensures mobile sediments are trapped on reef tops, the
resulting islands occupying a large proportion of the
available reef surface (Kench et al., 2006).
Catastrophic storms have also been regarded as
a primary mechanism in the initiation of atoll islands and
especially motu as recognized in the classifications of
MacNeil (1972) and Richmond (1993) outlined earlier.
The hurricane/typhoon hypothesis of island formation
has a long history with strong support from some of the
greatest late nineteenth- to early twentieth century reef scientists based on observations particularly in the Cocos
(Keeling) Islands and Funafuti Atoll. Modern analogues
of the delivery of large masses of coral debris to reef surfaces by tropical storms abound; the massive rubble banks
formed during Typhoon Ophelia at Jaluit atoll in January
1958 and Hurricane Bebe at Funafuti in October 1972 provide two excellent examples. Indeed, as MacNeil (1972)
and many others have noted, without storms most islets
would not be built, and because of storms some islets
become larger, while others are reduced or disappear.
One implication of this is that sediment delivery is episodic, a point stressed by Wood-Jones (1912: 261) who
based on his experience on Cocos (Keeling) proposed
a “law of atoll growth” stating that “the processes of formation take place in fits and starts.”
A more critical assessment of the role of episodic
storms and fair weather periods was developed by
Bayliss-Smith (1988) who modeled the difference in
response between gravel motu and sand cays on Ontong
Java atoll, Solomon Islands. Storms result in a net input
of coarse rubble on motu, but they cause shore erosion
ATOLL ISLANDS (MOTU)
49
Regardless of island size or shape, and despite their low
absolute elevation, atoll islands possess subtle but recognizable variations in surface topography which reflect
both geomorphic processes and sediment types. Three natural landform units can often be distinguished: a highgravel ridge or ridges on the ocean or windward side;
a low-sand flat ridge or ridges on the lagoon or leeward
side; and a central depression or flat between the two. This
asymmetric topographic form is also present on islands
comprising a single sediment type, either sand or gravel.
However, on islands located in environments where there
is no clear windward or leeward side, islands are commonly basin shaped with a high peripheral ridge of
roughly equal elevation encircling the whole island. Classic examples of this topographic form occur on the circular
patch reefs within lagoons of Maldivian atolls.
On most atoll islands the highest natural elevation is the
wave-emplaced ocean side or peripheral beach ridge,
which commonly reaches 3–4 m above MSL. On some
islands, however, the beach ridge is topped by a dune of
fine wind-blown sand, although the presence of sand
dunes is not a usual feature on most atoll islands. Where
dunes do occur they reach higher elevations than waveformed ridges, on Cocos (Keeling) Islands, up to 5.5 m
above MSL on Home Island, 7 m on West Island, and
11 m on South Island (based on surveys reported by
Woodroffe and McLean (1994)).
Excluding sand dunes, the highest elevation on atoll
islands, and often the greatest and most complex relative
relief, results from human activity rather than natural
processes. From French Polynesia in the Pacific to the
Laccadive–Maldive archipelago in the Indian Ocean,
crop cultivation, especially for taro and banana, is universally carried out in flat-floored depressions or pits
excavated down to the island water table. Spoil dug
from the pit is dumped around the pit edge building
a spoil bank above the level of the surrounding land.
The resulting topography is frequently chaotic, especially where groups of pits and banks occur in close
proximity to one another.
Sediments, soils, and groundwater
Atoll islands are made up of calcareous sediments, primarily the skeletal remains of organisms living on the
adjacent reef or in the lagoon. Invariably, the coarsest
materials comprise whole or broken clasts of coral, typically of sizes from a few centimeters to a few decimeters. In contrast to this single taxa dominance in the
gravel and rubble fraction, sand-sized sediments may
include small coral fragments, but more commonly comprise a mix of coralline algae, mollusks, foraminifera,
and Halimeda. Both the calcareous and textural characteristics of the sediments have a major influence on soil
properties. Not surprisingly, atoll soils tend to be light
colored, shallow, alkaline and coarse textured with no
clay. Fertility is highly dependent on the organic matter
content primarily from vegetation and leaf litter, and
moisture retention is generally low. Atoll soils are also
excessively well drained; water from rainfall quickly
percolates to the water table. Fresh water generally tops
the groundwater lens within atoll islands, beneath which
is the denser saline water originating from the surrounding ocean and lagoon. Traditionally, the fresh water lens
has been used as the primary source of potable water by
atoll communities.
Processes of formation and change on atoll islands
Islands accumulate on atoll reefs where sediment is
available and where wave processes are focused, often
as a result of wave refraction around the reef. Frequently, island shape mimics that of the reef, several
examples from Tuvalu being cited by Richmond
(1993). Whether or not islands will form on atoll reefs
is also dependent on the strength of wave energy.
A west to east traverse across South Maalhosmadulu
atoll in the Maldives shows that continual high wave
energy on the western reef rim sweeps sediment right
across the reef platform and islands only occur on the
broadest reef flats. In contrast, on the eastern atoll rim
where wave energy levels are lower, islands occur on
most of the available reef platforms. The more equal
and lowest magnitude wave energy of the central atoll
ensures mobile sediments are trapped on reef tops, the
resulting islands occupying a large proportion of the
available reef surface (Kench et al., 2006).
Catastrophic storms have also been regarded as
a primary mechanism in the initiation of atoll islands and
especially motu as recognized in the classifications of
MacNeil (1972) and Richmond (1993) outlined earlier.
The hurricane/typhoon hypothesis of island formation
has a long history with strong support from some of the
greatest late nineteenth- to early twentieth century reef scientists based on observations particularly in the Cocos
(Keeling) Islands and Funafuti Atoll. Modern analogues
of the delivery of large masses of coral debris to reef surfaces by tropical storms abound; the massive rubble banks
formed during Typhoon Ophelia at Jaluit atoll in January
1958 and Hurricane Bebe at Funafuti in October 1972 provide two excellent examples. Indeed, as MacNeil (1972)
and many others have noted, without storms most islets
would not be built, and because of storms some islets
become larger, while others are reduced or disappear.
One implication of this is that sediment delivery is episodic, a point stressed by Wood-Jones (1912: 261) who
based on his experience on Cocos (Keeling) proposed
a “law of atoll growth” stating that “the processes of formation take place in fits and starts.”
A more critical assessment of the role of episodic
storms and fair weather periods was developed by
Bayliss-Smith (1988) who modeled the difference in
response between gravel motu and sand cays on Ontong
Java atoll, Solomon Islands. Storms result in a net input
of coarse rubble on motu, but they cause shore erosion
ATOLL ISLANDS (MOTU)
49