flat close to the crest, are a major contributor to the sands
from which the islands are composed in Kiribati, Tuvalu,
and the Marshall Islands (Woodroffe and Morrison,
2001; Collen and Garton, 2004; Fujita et al., 2009). Elsewhere islands may be composed of coarser material,
including coral rubble and shingle. Islands composed of
coarser material are often referred to by the Polynesian
term motu (Atoll Islands (Motu)) and differentiated from
sandy islands that are called cays (Stoddart and Steers,
1977).
Prominent on many atolls is a conglomerate of
disoriented coral blocks, often forming a platform that
represents highly resistant substrate where it occurs.
Many reef islands appear to be anchored on such outcrops
of conglomerate (Montaggioni and Pirazzoli, 1984).
Although disoriented coral boulders and cobbles give the
conglomerate an irregular appearance, the upper surface
is often relatively horizontal, and internal structure may
contain distinct beds of various thicknesses. The conglomerates are cemented by coralline algae and marine cements
such as isopachous rims of fibrous aragonite or highmagnesian calcite.
Less well-lithified outcrops around the margins of
islands include beachrock, which is cemented beach sand,
preserving the dip of the original beach. The sands appear
to have been cemented beneath the water table, and
beachrock may be preserved where sand has been eroded
away, indicating former shoreline positions. Cay sandstone is a less well-cemented limestone; this poorly lithified carbonate is horizontally bedded and seems to form
associated with the water table within the island interior.
On a few islands, the droppings of nesting seabirds have
infiltrated into sands in the island interior and they are
cemented with a phosphatic cement (Rodgers, 1989).
Island surfaces can also be colonized by vegetation, which
contributes both directly through roots that stabilize sand
and humus that gives the soil greater structure.
Morphological differences have been identified between
atolls in storm-prone areas, and those closer to the equator
where tropical cyclones (typhoons, hurricanes) are not
experienced. Where storms are frequent, reef flats contain
abundant coral rubble and large blocks of reef limestone
called reef blocks (Bayliss-Smith, 1988). For example,
megablocks (Megablocks) up to 4 m high and 10 m long
were reported to have been deposited on the reef flat of
Nukutipipi Atoll in the Duke of Gloucester group in the
Tuamotu Archipelago, by the cyclones Veena and Orama
that occurred in 1983 (Salvat and Salvat, 1992). Significant impacts were observed after a storm on Jaluit
(Blumenstock, 1961). Under some circumstances it appears
that tsunami may detach and emplace large blocks of similar dimensions on coral reefs (Bourrouilh-Le Jan, 1998),
but as a tsunami has only a small amplitude in mid ocean,
the prevailing view is that these megablocks are emplaced
by cyclones. Rubble ramparts are often formed by individual storms (Scoffin, 1993); for example the “Bebe” bank on
Funafuti (Maragos et al., 1973; Baines et al., 1974). This
rampart has been gradually reworked shoreward across
the reef flat, since it was thrown up during Cyclone Bebe,
and in places it has accreted onto similar, but earlier
storm-derived deposits on the reef islands (Baines and
McLean, 1976). Where storms are less frequent, boulders
may accumulate and contribute to a conglomerate that
underpins the island. In those atolls closest to the equator
and outside the storm belt, such as the Maldives, the islands
are predominantly built of sand. Boulder ramparts (Boulder
Zone/Ramparts) and shingle ridges (Shingle Ridges) are
indicators of former storm events, and may be eroded and
only partially preserved (Pirazzoli, 1987). Rubble storm
ridges, deposited over the past 3000 years, are prominent
features on islands in Lakshadweep (formerly the
Laccadives) to the north of the Maldives (Siddiquie, 1980).
Processes of island formation
Almost all material which comprises reefs islands is amenable to radiocarbon dating, and a series of models of reefisland formation on atolls was proposed by Woodroffe
et al. (1999). Radiocarbon ages yield estimates of the time
of death of skeletal organisms, and deposition may occur
some time later, after an undefined period of transport,
breakdown, erosion, and redeposition. However, a pattern
of gradual oceanward accretion of reef islands appears to
have occurred on an elongate island (West Island) on
Cocos (Woodroffe et al., 1999), and on Makin at the northern end of the Gilbert chain in Kiribati (Woodroffe and
Morrison, 2001). These, and results from comparison of
multitemporal aerial photography or satellite imagery
(Webb, 2006), indicate ongoing accretion, where sediment
production and transport are sustained, on many of the
oceanward shores of atoll islands. In other settings there
may be more complex trends in shoreline erosion and
deposition (Richmond, 1992). For example, detailed
resurveys of beaches on the islands of Betio and Buariki
in South Tarawa indicate fluctuations of island outline that
correspond with wind changes associated with the El
Niño-Southern Oscillation phenomenon (Gillie, 1993;
Solomon and Forbes, 1999), and in the Maldives seasonal
adjustments follow reversal of the monsoon (Kench et al.,
2006). An alternative model of island build-up has been
proposed for the small sandy islands that occur on patch
reefs in the center of the lagoons on atolls in the northern
Maldives (Kench et al., 2005). A further approach to determining island formation has been recently developed with
the use of computer modeling (Barry et al., 2007, 2008).
Island soils, vegetation, and ecosystems
The calcareous sediments that form reef islands on the
margin of atolls produce poorly developed and immature
soils (Morrison, 1990). Soil characteristics depend primarily on the incorporation of plant matter and the development of a humus layer. An exception is the occurrence
of phosphate-rich areas, first identified by Fosberg
(1957), but subsequently described in several situations
where the vegetation is, or was previously, dominated
by the tree Pisonia grandis. This tree attracts seabirds
62
ATOLLS
from which the islands are composed in Kiribati, Tuvalu,
and the Marshall Islands (Woodroffe and Morrison,
2001; Collen and Garton, 2004; Fujita et al., 2009). Elsewhere islands may be composed of coarser material,
including coral rubble and shingle. Islands composed of
coarser material are often referred to by the Polynesian
term motu (Atoll Islands (Motu)) and differentiated from
sandy islands that are called cays (Stoddart and Steers,
1977).
Prominent on many atolls is a conglomerate of
disoriented coral blocks, often forming a platform that
represents highly resistant substrate where it occurs.
Many reef islands appear to be anchored on such outcrops
of conglomerate (Montaggioni and Pirazzoli, 1984).
Although disoriented coral boulders and cobbles give the
conglomerate an irregular appearance, the upper surface
is often relatively horizontal, and internal structure may
contain distinct beds of various thicknesses. The conglomerates are cemented by coralline algae and marine cements
such as isopachous rims of fibrous aragonite or highmagnesian calcite.
Less well-lithified outcrops around the margins of
islands include beachrock, which is cemented beach sand,
preserving the dip of the original beach. The sands appear
to have been cemented beneath the water table, and
beachrock may be preserved where sand has been eroded
away, indicating former shoreline positions. Cay sandstone is a less well-cemented limestone; this poorly lithified carbonate is horizontally bedded and seems to form
associated with the water table within the island interior.
On a few islands, the droppings of nesting seabirds have
infiltrated into sands in the island interior and they are
cemented with a phosphatic cement (Rodgers, 1989).
Island surfaces can also be colonized by vegetation, which
contributes both directly through roots that stabilize sand
and humus that gives the soil greater structure.
Morphological differences have been identified between
atolls in storm-prone areas, and those closer to the equator
where tropical cyclones (typhoons, hurricanes) are not
experienced. Where storms are frequent, reef flats contain
abundant coral rubble and large blocks of reef limestone
called reef blocks (Bayliss-Smith, 1988). For example,
megablocks (Megablocks) up to 4 m high and 10 m long
were reported to have been deposited on the reef flat of
Nukutipipi Atoll in the Duke of Gloucester group in the
Tuamotu Archipelago, by the cyclones Veena and Orama
that occurred in 1983 (Salvat and Salvat, 1992). Significant impacts were observed after a storm on Jaluit
(Blumenstock, 1961). Under some circumstances it appears
that tsunami may detach and emplace large blocks of similar dimensions on coral reefs (Bourrouilh-Le Jan, 1998),
but as a tsunami has only a small amplitude in mid ocean,
the prevailing view is that these megablocks are emplaced
by cyclones. Rubble ramparts are often formed by individual storms (Scoffin, 1993); for example the “Bebe” bank on
Funafuti (Maragos et al., 1973; Baines et al., 1974). This
rampart has been gradually reworked shoreward across
the reef flat, since it was thrown up during Cyclone Bebe,
and in places it has accreted onto similar, but earlier
storm-derived deposits on the reef islands (Baines and
McLean, 1976). Where storms are less frequent, boulders
may accumulate and contribute to a conglomerate that
underpins the island. In those atolls closest to the equator
and outside the storm belt, such as the Maldives, the islands
are predominantly built of sand. Boulder ramparts (Boulder
Zone/Ramparts) and shingle ridges (Shingle Ridges) are
indicators of former storm events, and may be eroded and
only partially preserved (Pirazzoli, 1987). Rubble storm
ridges, deposited over the past 3000 years, are prominent
features on islands in Lakshadweep (formerly the
Laccadives) to the north of the Maldives (Siddiquie, 1980).
Processes of island formation
Almost all material which comprises reefs islands is amenable to radiocarbon dating, and a series of models of reefisland formation on atolls was proposed by Woodroffe
et al. (1999). Radiocarbon ages yield estimates of the time
of death of skeletal organisms, and deposition may occur
some time later, after an undefined period of transport,
breakdown, erosion, and redeposition. However, a pattern
of gradual oceanward accretion of reef islands appears to
have occurred on an elongate island (West Island) on
Cocos (Woodroffe et al., 1999), and on Makin at the northern end of the Gilbert chain in Kiribati (Woodroffe and
Morrison, 2001). These, and results from comparison of
multitemporal aerial photography or satellite imagery
(Webb, 2006), indicate ongoing accretion, where sediment
production and transport are sustained, on many of the
oceanward shores of atoll islands. In other settings there
may be more complex trends in shoreline erosion and
deposition (Richmond, 1992). For example, detailed
resurveys of beaches on the islands of Betio and Buariki
in South Tarawa indicate fluctuations of island outline that
correspond with wind changes associated with the El
Niño-Southern Oscillation phenomenon (Gillie, 1993;
Solomon and Forbes, 1999), and in the Maldives seasonal
adjustments follow reversal of the monsoon (Kench et al.,
2006). An alternative model of island build-up has been
proposed for the small sandy islands that occur on patch
reefs in the center of the lagoons on atolls in the northern
Maldives (Kench et al., 2005). A further approach to determining island formation has been recently developed with
the use of computer modeling (Barry et al., 2007, 2008).
Island soils, vegetation, and ecosystems
The calcareous sediments that form reef islands on the
margin of atolls produce poorly developed and immature
soils (Morrison, 1990). Soil characteristics depend primarily on the incorporation of plant matter and the development of a humus layer. An exception is the occurrence
of phosphate-rich areas, first identified by Fosberg
(1957), but subsequently described in several situations
where the vegetation is, or was previously, dominated
by the tree Pisonia grandis. This tree attracts seabirds
62
ATOLLS