of storm-deposited coarse material may form the island
core, but are surrounded by a sandy beach (e.g., Lady
Musgrave Island, southern GBR). Reef islands developed where wind direction seasonally reverses are also
less likely to show the classic bimodal distribution of sediments, especially on smaller reef platforms where
mixing is likely.
Criterion 2: Island location on the reef platform. Where
sediments are available and the reef platform size and
energy regime allow sorting to occur, shingle cays form
near the windward reef margin and sand cays to leeward. On some platforms, reef islands may occupy
a significant proportion of the reef flat; coral cays on
lagoonal reefs within Maldivian atolls can occupy as
much as 56% of the reef top (Kench et al., 2008). Where
cays are large relative to the reef platforms on which
they sit, it is difficult to define whether a cay is windward or leeward, as is also the case where cays are
located centrally on the reef due to either seasonal wave
climate reversals or a function of reef geometry.
Criterion 3: Island shape. Cay shape broadly falls
between elongate and compact (oval to round). This
trait is largely controlled by the interaction of reef shape
and wave refraction and transformation around and
across a reef. Compact islands form where sediment
transport to a single focal node is most efficient. Therefore, the directional consistency and energy of the
prevailing wave climate and the complicating effects
of obstructions such as adjacent reefs on the transfer
of this energy to the reef platform also influence island
shape. Generally, but not always, compact cays are
more stable than elongate cays; the ends of which can
be particularly sensitive and move with small changes
in wind and wave direction with a seasonal periodicity
(e.g., Flood, 1986).
Criterion 4: Vegetation cover. Coral cays are either
unvegetated or vegetated, with the extent of vegetation
often reflecting island size, age, and stability. Climate,
especially rainfall and frequency of storms, can also
be important, but larger, older, and more stable islands
generally possess better soils and groundwater aquifers
best able to sustain vegetation. Reef-island vegetation is
discussed further in Section Reef-island vegetation.
The four criteria above can be used to describe and classify
reef islands that range from small and unstable
unvegetated sandy cays to complex low wooded islands
(see Section Classification: island types). It is important
to note that two criteria – vegetation cover and island
shape – may change abruptly, especially on reefs exposed
to extreme events. For example, Pickersgill Cays on the
GBR were reported as slightly vegetated by the Australian
Pilot early in the twentieth century but unvegetated when
Spender (1930) and Steers (1929) visited in 1929 and were
again unvegetated in 1973 (Stoddart et al., 1978a). Rapid
changes in reef-island morphology can be established by
comparing aerial photographs or accurate GPS surveys at
event and seasonal time scales, and these methods
commonly document significant shifts in shoreline at
these short time scales (e.g., Frank and Jell, 2006). Historical maps available for some reef islands can be compared
with later versions to show significant change over the past
50–100 years (e.g., Stoddart et al., 1978b; Flood, 1986).
Relict beach rock outcrops indicate the position and shape
of cay shorelines in the longer term past that can be very
different to the present day morphology. Cay dynamics
are covered in Section Factors influencing cay stability.
Classification: island types
The main classes of coral cay are described in the following section. The defining features of major reef-island
classes are depicted schematically in Figure 2.
Unvegetated cays
Unvegetated cays clearly lack vegetation. They are generally small and unstable; both conditions constrain the
establishment and survival of plant cover. A mean area
of just 0.5 ha was determined for 18 unvegetated cays surveyed in the 1973 northern GBR expedition (Stoddart
et al., 1978a). They may be composed of sand and/or shingle and can be either elongate or compact. They are the
most common cay type (see Unvegetated Cays).
Unvegetated sand cays
Linear unvegetated sand cays form on medium to large
planar reefs (see Chapter Reef Classification by Hopley,
(1982)) of elongate shape where wave refraction generates
opposing wave trains that meet along a central axis and
where sediment accumulation takes place. Linear
unvegetated cays may also form where winds seasonally
reverse, producing an elongate, spit-like accumulation
toward the leeward reef margin. Hopley (1982) considered
that the migrating linear sand banks on the large planar
reefs of Princess Charlotte Bay, northern GBR resulted
from seasonal shifts between dominant south-easterly
trade winds and lighter, more northerly monsoonal winds.
Bidirectional monsoons have also been linked to the abundance of elongate unvegetated cays in Indonesia
(Tomascik et al., 1997). Compact unvegetated cays form
where strong centripetal sediment transport focuses deposition within a restricted locus (Figure 3a). Many are
highly mobile and unstable (Hopley, 1978; Aston, 1995),
especially the smaller (<0.1 ha) “ephemeral” cays that
may be submerged at high tide (Figure 3a). Stoddart
et al. (1982) concluded that many of these cays in the
Caribbean, such as Paunch and Curlew Cay, may be second generation features reformed following the destruction of larger cays during storms. Larger unvegetated
cays up to 400-m long and 120-m wide are often flanked
by beach rock, suggesting greater stability.
Unvegetated shingle cays
Linear unvegetated shingle cays are very unstable reef
islands, vulnerable to reworking and erosion near the reef
front (Figures 2b and 3b). They are typically the product of
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CORAL CAY CLASSIFICATION AND EVOLUTION
core, but are surrounded by a sandy beach (e.g., Lady
Musgrave Island, southern GBR). Reef islands developed where wind direction seasonally reverses are also
less likely to show the classic bimodal distribution of sediments, especially on smaller reef platforms where
mixing is likely.
Criterion 2: Island location on the reef platform. Where
sediments are available and the reef platform size and
energy regime allow sorting to occur, shingle cays form
near the windward reef margin and sand cays to leeward. On some platforms, reef islands may occupy
a significant proportion of the reef flat; coral cays on
lagoonal reefs within Maldivian atolls can occupy as
much as 56% of the reef top (Kench et al., 2008). Where
cays are large relative to the reef platforms on which
they sit, it is difficult to define whether a cay is windward or leeward, as is also the case where cays are
located centrally on the reef due to either seasonal wave
climate reversals or a function of reef geometry.
Criterion 3: Island shape. Cay shape broadly falls
between elongate and compact (oval to round). This
trait is largely controlled by the interaction of reef shape
and wave refraction and transformation around and
across a reef. Compact islands form where sediment
transport to a single focal node is most efficient. Therefore, the directional consistency and energy of the
prevailing wave climate and the complicating effects
of obstructions such as adjacent reefs on the transfer
of this energy to the reef platform also influence island
shape. Generally, but not always, compact cays are
more stable than elongate cays; the ends of which can
be particularly sensitive and move with small changes
in wind and wave direction with a seasonal periodicity
(e.g., Flood, 1986).
Criterion 4: Vegetation cover. Coral cays are either
unvegetated or vegetated, with the extent of vegetation
often reflecting island size, age, and stability. Climate,
especially rainfall and frequency of storms, can also
be important, but larger, older, and more stable islands
generally possess better soils and groundwater aquifers
best able to sustain vegetation. Reef-island vegetation is
discussed further in Section Reef-island vegetation.
The four criteria above can be used to describe and classify
reef islands that range from small and unstable
unvegetated sandy cays to complex low wooded islands
(see Section Classification: island types). It is important
to note that two criteria – vegetation cover and island
shape – may change abruptly, especially on reefs exposed
to extreme events. For example, Pickersgill Cays on the
GBR were reported as slightly vegetated by the Australian
Pilot early in the twentieth century but unvegetated when
Spender (1930) and Steers (1929) visited in 1929 and were
again unvegetated in 1973 (Stoddart et al., 1978a). Rapid
changes in reef-island morphology can be established by
comparing aerial photographs or accurate GPS surveys at
event and seasonal time scales, and these methods
commonly document significant shifts in shoreline at
these short time scales (e.g., Frank and Jell, 2006). Historical maps available for some reef islands can be compared
with later versions to show significant change over the past
50–100 years (e.g., Stoddart et al., 1978b; Flood, 1986).
Relict beach rock outcrops indicate the position and shape
of cay shorelines in the longer term past that can be very
different to the present day morphology. Cay dynamics
are covered in Section Factors influencing cay stability.
Classification: island types
The main classes of coral cay are described in the following section. The defining features of major reef-island
classes are depicted schematically in Figure 2.
Unvegetated cays
Unvegetated cays clearly lack vegetation. They are generally small and unstable; both conditions constrain the
establishment and survival of plant cover. A mean area
of just 0.5 ha was determined for 18 unvegetated cays surveyed in the 1973 northern GBR expedition (Stoddart
et al., 1978a). They may be composed of sand and/or shingle and can be either elongate or compact. They are the
most common cay type (see Unvegetated Cays).
Unvegetated sand cays
Linear unvegetated sand cays form on medium to large
planar reefs (see Chapter Reef Classification by Hopley,
(1982)) of elongate shape where wave refraction generates
opposing wave trains that meet along a central axis and
where sediment accumulation takes place. Linear
unvegetated cays may also form where winds seasonally
reverse, producing an elongate, spit-like accumulation
toward the leeward reef margin. Hopley (1982) considered
that the migrating linear sand banks on the large planar
reefs of Princess Charlotte Bay, northern GBR resulted
from seasonal shifts between dominant south-easterly
trade winds and lighter, more northerly monsoonal winds.
Bidirectional monsoons have also been linked to the abundance of elongate unvegetated cays in Indonesia
(Tomascik et al., 1997). Compact unvegetated cays form
where strong centripetal sediment transport focuses deposition within a restricted locus (Figure 3a). Many are
highly mobile and unstable (Hopley, 1978; Aston, 1995),
especially the smaller (<0.1 ha) “ephemeral” cays that
may be submerged at high tide (Figure 3a). Stoddart
et al. (1982) concluded that many of these cays in the
Caribbean, such as Paunch and Curlew Cay, may be second generation features reformed following the destruction of larger cays during storms. Larger unvegetated
cays up to 400-m long and 120-m wide are often flanked
by beach rock, suggesting greater stability.
Unvegetated shingle cays
Linear unvegetated shingle cays are very unstable reef
islands, vulnerable to reworking and erosion near the reef
front (Figures 2b and 3b). They are typically the product of
240
CORAL CAY CLASSIFICATION AND EVOLUTION