on sand cays. During subsequent fair weather, storm rubble is redistributed on motu, while sand cays recover from
the erosional episode and buildup between storms.
Bayliss-Smith’s model has proved a robust one and has
been modified and expanded by several other researchers.
In contrast to the well-documented impacts of tropical
storms on atoll island construction and destruction, the
impact of tsunami is more ambiguous, even though atoll
islands throughout the Indo-Pacific region must have been
subject to multiple tsunami during their geological histories. Few investigations of the impact of tsunami on atoll
islands have been made, the Sumatran tsunami of
December 26, 2004 being a notable exception with several
studies of its effects on the Maldives. While the tsunami
had tragic consequences for many inhabited islands in
the Maldives, pre- and post-tsunami geomorphic surveys
on a number of uninhabited islands showed both erosional
and depositional changes, little major damage, and a net
long-term effect favoring island accretion rather than
reduction (Kench et al., 2008).
Stabilization processes
Once formed, atoll island deposits can be stabilized by
a number of different processes including colonization
by terrestrial and littoral vegetation and lithification of
island and beach sediments. Vegetation, through root binding and the accumulation of humus, stabilizes land surfaces and enables soil formation to proceed, while along
island margins root masses of coconut palms, pandanus,
and broadleaf trees often form conspicuous “phyto-revetments.” Island shores can also be protected by a fringe of
intertidal mangroves or Pemphis that serve to slow down
particle movement and entrap sediment.
Lithification of unconsolidated deposits can also take
place, both within islands and around their margins.
Examples of the former include the formation of phosphate rock and cay sandstone (Phosphatic Cay Sandstone), while beach rock and beach conglomerate
(Conglomerates) or breccia are common littoral rocks,
which give an island some resistance. Both form natural
sea walls and groynes and serve to protect an island from
erosion. They can also form a locus for island accretion,
and are included in MacNeil’s classification of atoll islets
(Types 1 and 3) and that of Richmond (Type 4).
Origin and development of atoll islands
Islands on sea-level atolls are geologically young, having
developed only in the last few millennia (mid-late
Holocene). Obviously, island accretion postdates the surfaces on which the islands are built, be that surface conglomerate platform, solid reef flat, or lagoonal sediment.
Many researchers have suggested that the formation of
islands on atolls was triggered by a slight fall in sea level
from a mid-Holocene high stand. Such an interpretation
has been advocated for a large number of atolls extending
from the southernmost atoll in French Polynesia (Temoe)
through the Tuamotu archipelago and Cook Islands in
the Pacific to Cocos (Keeling) Islands in the eastern Indian
Ocean, where the majority of islands are perched on
a conglomerate platform or emergent reef flat (Woodroffe
and McLean, 1994). On the other hand, there is evidence
to indicate that some atoll islands accumulated earlier,
when sea level was rising rather than falling, as demonstrated by Kench et al. (2005) for islands in South
Maalhosmadulu atoll in the central Maldives.
It is evident from this analysis that there are several
ways in which atoll islands can form, including through
storm wave action, and that they have formed at different
times over the last few millennia (McLean and Hosking,
1991). Moreover, some islands may have developed episodically and others incrementally. Woodroffe (2000)
has proposed a number of different scenarios to illustrate chrono-sequences of island accumulation such as
accretion away from a central core, progradation in a
lagoonward or oceanward direction, or through sediment
rollover or overwash. More recently, quantitative models
using morphodynamic principles have been used to test
field-based models and to suggest that there is a growthlimiting size to atoll islets (Barry et al., 2008).
Future status of atoll islands
Due to their small size, low elevation, and reliance on
locally generated reefal sediments, atoll islands are considered particularly vulnerable to the effects of climate
change and especially sea-level rise. The principal impacts
from sea-level rise are expected to include: shoreline erosion, inundation and flooding, and saline intrusion, the
consequences of which may be to reduce island size,
waterlog low-lying areas, and contaminate fresh groundwater supplies. Given these potential impacts, it is surprising so little substantive research has been done on these
issues in the atoll world. However, the research that has
been done suggests that atoll islands may be much less
vulnerable than portrayed in the media or by atoll governments. Geomorphic studies such as the recent comprehensive analysis by Woodroffe (2008) indicates that atoll
islands exhibit a degree of physical resilience, that sealevel rise is not the only climate change issue of relevance
to atoll islands, and that the key message is that islands
will differ in their susceptibility to sea-level rise.
Woodroffe’s conclusion reiterates an earlier comment by
Richmond (1993: 1193) that even within an atoll “different islet types could respond differently to such climate
changes as sea-level rise and increased (or decreased)
storminess.”
Finally, it is interesting to recall that nearly 50 years
ago Wiens (1962: 135) wrote about the fate of atoll land
with rising sea levels as follows: “In the next 5,000–
6,000 years it is possible that periods of rising sea levels
may inundate most present land on atolls and possibly
destroy most present reef islets.” Perhaps Wiens’ time
scale is an order of magnitude or two out as we may
already be in one of those “periods of rising sea levels.”
50
ATOLL ISLANDS (MOTU)
the erosional episode and buildup between storms.
Bayliss-Smith’s model has proved a robust one and has
been modified and expanded by several other researchers.
In contrast to the well-documented impacts of tropical
storms on atoll island construction and destruction, the
impact of tsunami is more ambiguous, even though atoll
islands throughout the Indo-Pacific region must have been
subject to multiple tsunami during their geological histories. Few investigations of the impact of tsunami on atoll
islands have been made, the Sumatran tsunami of
December 26, 2004 being a notable exception with several
studies of its effects on the Maldives. While the tsunami
had tragic consequences for many inhabited islands in
the Maldives, pre- and post-tsunami geomorphic surveys
on a number of uninhabited islands showed both erosional
and depositional changes, little major damage, and a net
long-term effect favoring island accretion rather than
reduction (Kench et al., 2008).
Stabilization processes
Once formed, atoll island deposits can be stabilized by
a number of different processes including colonization
by terrestrial and littoral vegetation and lithification of
island and beach sediments. Vegetation, through root binding and the accumulation of humus, stabilizes land surfaces and enables soil formation to proceed, while along
island margins root masses of coconut palms, pandanus,
and broadleaf trees often form conspicuous “phyto-revetments.” Island shores can also be protected by a fringe of
intertidal mangroves or Pemphis that serve to slow down
particle movement and entrap sediment.
Lithification of unconsolidated deposits can also take
place, both within islands and around their margins.
Examples of the former include the formation of phosphate rock and cay sandstone (Phosphatic Cay Sandstone), while beach rock and beach conglomerate
(Conglomerates) or breccia are common littoral rocks,
which give an island some resistance. Both form natural
sea walls and groynes and serve to protect an island from
erosion. They can also form a locus for island accretion,
and are included in MacNeil’s classification of atoll islets
(Types 1 and 3) and that of Richmond (Type 4).
Origin and development of atoll islands
Islands on sea-level atolls are geologically young, having
developed only in the last few millennia (mid-late
Holocene). Obviously, island accretion postdates the surfaces on which the islands are built, be that surface conglomerate platform, solid reef flat, or lagoonal sediment.
Many researchers have suggested that the formation of
islands on atolls was triggered by a slight fall in sea level
from a mid-Holocene high stand. Such an interpretation
has been advocated for a large number of atolls extending
from the southernmost atoll in French Polynesia (Temoe)
through the Tuamotu archipelago and Cook Islands in
the Pacific to Cocos (Keeling) Islands in the eastern Indian
Ocean, where the majority of islands are perched on
a conglomerate platform or emergent reef flat (Woodroffe
and McLean, 1994). On the other hand, there is evidence
to indicate that some atoll islands accumulated earlier,
when sea level was rising rather than falling, as demonstrated by Kench et al. (2005) for islands in South
Maalhosmadulu atoll in the central Maldives.
It is evident from this analysis that there are several
ways in which atoll islands can form, including through
storm wave action, and that they have formed at different
times over the last few millennia (McLean and Hosking,
1991). Moreover, some islands may have developed episodically and others incrementally. Woodroffe (2000)
has proposed a number of different scenarios to illustrate chrono-sequences of island accumulation such as
accretion away from a central core, progradation in a
lagoonward or oceanward direction, or through sediment
rollover or overwash. More recently, quantitative models
using morphodynamic principles have been used to test
field-based models and to suggest that there is a growthlimiting size to atoll islets (Barry et al., 2008).
Future status of atoll islands
Due to their small size, low elevation, and reliance on
locally generated reefal sediments, atoll islands are considered particularly vulnerable to the effects of climate
change and especially sea-level rise. The principal impacts
from sea-level rise are expected to include: shoreline erosion, inundation and flooding, and saline intrusion, the
consequences of which may be to reduce island size,
waterlog low-lying areas, and contaminate fresh groundwater supplies. Given these potential impacts, it is surprising so little substantive research has been done on these
issues in the atoll world. However, the research that has
been done suggests that atoll islands may be much less
vulnerable than portrayed in the media or by atoll governments. Geomorphic studies such as the recent comprehensive analysis by Woodroffe (2008) indicates that atoll
islands exhibit a degree of physical resilience, that sealevel rise is not the only climate change issue of relevance
to atoll islands, and that the key message is that islands
will differ in their susceptibility to sea-level rise.
Woodroffe’s conclusion reiterates an earlier comment by
Richmond (1993: 1193) that even within an atoll “different islet types could respond differently to such climate
changes as sea-level rise and increased (or decreased)
storminess.”
Finally, it is interesting to recall that nearly 50 years
ago Wiens (1962: 135) wrote about the fate of atoll land
with rising sea levels as follows: “In the next 5,000–
6,000 years it is possible that periods of rising sea levels
may inundate most present land on atolls and possibly
destroy most present reef islets.” Perhaps Wiens’ time
scale is an order of magnitude or two out as we may
already be in one of those “periods of rising sea levels.”
50
ATOLL ISLANDS (MOTU)