extends 868 km from Ihavandhippolhu in the north
(6
57
0 N) to Addu atoll (0
34
0 S) just south of the equator.
Comprising 2,041 reefs the archipelago is globally unique
in biological diversity, the diversity of reef structures it
possesses, and their mode of evolution. The reef system
is host to approximately 1,190 low-lying reef islands. As
one of only four atoll nations, the Maldives is commonly
regarded as extremely vulnerable to future sea-level and
climatic change.
Early exploration
Earliest scientific investigations of the Maldives involved
detailed hydrographic observations of the archipelago by
Commander James Moresby 1835–1838. This work produced detailed hydrographic charts of the reef system,
which were used by Charles Darwin in constructing his
global synthesis of reef evolution, and which remain the
most accurate records of bathymetry in the archipelago.
At the end of the nineteenth century and beginning of
the twentieth century a number of scientific expeditions
were undertaken to the Maldives to document the taxonomy and biogeography of reef biota and make observations of oceanographic conditions. Most notable were
the expeditions of James Stanley Gardiner in 1899–1900
who undertook the first detailed descriptions of marine
and terrestrial fauna and flora as well as reef structures
(Gardiner, 1903). Shortly thereafter, Alexander Agassiz
also visited the archipelago in 1901–1902 and undertook
sampling and description of biological aspects of the reefs
(Agassiz, 1903). Seymour Sewell led the “John Murray”
expedition to the Maldives (1933–1934) in which Sewell
undertook detailed geomorphic observations of a number
of atoll islands (Sewell, 1936a, b).
In 1957–1958 the “Xarifa” expedition undertook
detailed investigations of the sub-littoral ecology and
structure of reefs in many atolls in the archipelago (Hass,
1965). Introducing new systematic survey techniques, this
expedition was notable for its taxonomic investigation of
faunal and floral communities, identification of high species diversity, and establishment of baseline monitoring
locations for future reference of reef community dynamics
(Wallace and Zahir, 2007). British scientists also studied
the marine and terrestrial geomorphology and ecology of
Addu atoll (Stoddart, 1966).
Since these early expeditions the research effort in the
Maldives can be described as sparse and narrowly focused
on specific aspects of reef ecology, geology, and management. As a result large gaps exist in scientific knowledge
and many atolls have not been visited by scientists since
Gardiner and Sewell.
Geologic history
The Maldives constitute the central section of the
Laccadives – Maldives – Chagos submarine ridge, which
in turn is part of a larger feature that stretches from the early
Tertiary Deccan Traps of India to the volcanically active
zone of Reunion. The Deccan Traps were the first
manifestation of the stationary mantle hotspot that subsequently produced the volcanic ridge underlying the Maldives. Northward drift of the Indian Plate and
northeastward motion of the African plate has resulted in
northward age progression along this ridge (Duncan and
Pyle, 1988; Duncan and Hargreaves, 1990; Tiwari et al.,
2007).
The history of formation of the Maldivian atoll systems
differs from the subsidence model proposed by Darwin
(1842). Rather, carbonate accumulation through the
Tertiary was controlled by phases of aggradation and
progradation modulated by large oscillations in sea level
(Aubert and Droxler, 1992; Purdy and Bertram, 1993;
Belopolsky and Droxler, 2003). The basement rocks
underlying the Maldives are Eocene volcanics (55 million
years old). These basement volcanics are capped by up to
3,000 m of limestones. Carbonate banks were initiated in
the Eocene on topographic highs of the faulted volcanic
basin (Figure 1). A number of NNE-SSW oriented grabens
acted as channels that became filled with sediments shed
from surrounding carbonate banks. The Oligocene to early
Miocene were characterized by aggradation of carbonate
banks and backstepping in response to sea level rise. An
elevated outer rim developed and provided the antecedent
structure for the double chain of atolls separated by an
inner sea basin that characterizes the central section of
the contemporary archipelago (Figure 2). The midMiocene (15–10 million years ago) was dominated by lateral progradation from the outer margins of the twin atoll
chain toward the inner sea forming a shallow carbonate
platform. This period of lateral progradation was followed
in the late Miocene and Pliocene by flooding of the platforms and backstepping of the carbonate banks to the
periphery of the platform. These peripheral platforms
were exposed to karstification during the Plio–Pleistocene
sea level minima.
During the Quaternary, the dominant mode of reef
development has been vertical coral reef growth on top
of these karstified remnants. This vertical reef growth is
a consequence of high-amplitude sea-level fluctuations
during the Pliocene–Pleistocene, producing alternate
periods of exposure and submergence. The Holocene is
the last of these reef growth phases.
The contemporary surface morphology of the Maldivian reefs is the result of coral reef growth over the past
10,000 years upon Pleistocene foundations. The Holcoene
reef growth history of the Maldives has only recently been
resolved in detail based on drill cores from Ari and South
Maalhosmadulu atolls with the Holocene reef thickness
ranging from 12 to 14 m (Gischler et al., 2008; Kench
et al., 2009). These studies show the Holocene reef growth
initiated approximately 8,100 years ago as sea level
flooded the Pleistocene surface. Vertical reef growth
occurred at around 7 mm/year for the period 8,100–
6,500 years ago and subsequently slowed to 1 mm/year
thereafter. Evidence suggests the final stages of reef accretion in the late Holocene exhibited local variations. In situ
reef deposits located above sea level on some reefs show
MALDIVES
649
(6
57
0 N) to Addu atoll (0
34
0 S) just south of the equator.
Comprising 2,041 reefs the archipelago is globally unique
in biological diversity, the diversity of reef structures it
possesses, and their mode of evolution. The reef system
is host to approximately 1,190 low-lying reef islands. As
one of only four atoll nations, the Maldives is commonly
regarded as extremely vulnerable to future sea-level and
climatic change.
Early exploration
Earliest scientific investigations of the Maldives involved
detailed hydrographic observations of the archipelago by
Commander James Moresby 1835–1838. This work produced detailed hydrographic charts of the reef system,
which were used by Charles Darwin in constructing his
global synthesis of reef evolution, and which remain the
most accurate records of bathymetry in the archipelago.
At the end of the nineteenth century and beginning of
the twentieth century a number of scientific expeditions
were undertaken to the Maldives to document the taxonomy and biogeography of reef biota and make observations of oceanographic conditions. Most notable were
the expeditions of James Stanley Gardiner in 1899–1900
who undertook the first detailed descriptions of marine
and terrestrial fauna and flora as well as reef structures
(Gardiner, 1903). Shortly thereafter, Alexander Agassiz
also visited the archipelago in 1901–1902 and undertook
sampling and description of biological aspects of the reefs
(Agassiz, 1903). Seymour Sewell led the “John Murray”
expedition to the Maldives (1933–1934) in which Sewell
undertook detailed geomorphic observations of a number
of atoll islands (Sewell, 1936a, b).
In 1957–1958 the “Xarifa” expedition undertook
detailed investigations of the sub-littoral ecology and
structure of reefs in many atolls in the archipelago (Hass,
1965). Introducing new systematic survey techniques, this
expedition was notable for its taxonomic investigation of
faunal and floral communities, identification of high species diversity, and establishment of baseline monitoring
locations for future reference of reef community dynamics
(Wallace and Zahir, 2007). British scientists also studied
the marine and terrestrial geomorphology and ecology of
Addu atoll (Stoddart, 1966).
Since these early expeditions the research effort in the
Maldives can be described as sparse and narrowly focused
on specific aspects of reef ecology, geology, and management. As a result large gaps exist in scientific knowledge
and many atolls have not been visited by scientists since
Gardiner and Sewell.
Geologic history
The Maldives constitute the central section of the
Laccadives – Maldives – Chagos submarine ridge, which
in turn is part of a larger feature that stretches from the early
Tertiary Deccan Traps of India to the volcanically active
zone of Reunion. The Deccan Traps were the first
manifestation of the stationary mantle hotspot that subsequently produced the volcanic ridge underlying the Maldives. Northward drift of the Indian Plate and
northeastward motion of the African plate has resulted in
northward age progression along this ridge (Duncan and
Pyle, 1988; Duncan and Hargreaves, 1990; Tiwari et al.,
2007).
The history of formation of the Maldivian atoll systems
differs from the subsidence model proposed by Darwin
(1842). Rather, carbonate accumulation through the
Tertiary was controlled by phases of aggradation and
progradation modulated by large oscillations in sea level
(Aubert and Droxler, 1992; Purdy and Bertram, 1993;
Belopolsky and Droxler, 2003). The basement rocks
underlying the Maldives are Eocene volcanics (55 million
years old). These basement volcanics are capped by up to
3,000 m of limestones. Carbonate banks were initiated in
the Eocene on topographic highs of the faulted volcanic
basin (Figure 1). A number of NNE-SSW oriented grabens
acted as channels that became filled with sediments shed
from surrounding carbonate banks. The Oligocene to early
Miocene were characterized by aggradation of carbonate
banks and backstepping in response to sea level rise. An
elevated outer rim developed and provided the antecedent
structure for the double chain of atolls separated by an
inner sea basin that characterizes the central section of
the contemporary archipelago (Figure 2). The midMiocene (15–10 million years ago) was dominated by lateral progradation from the outer margins of the twin atoll
chain toward the inner sea forming a shallow carbonate
platform. This period of lateral progradation was followed
in the late Miocene and Pliocene by flooding of the platforms and backstepping of the carbonate banks to the
periphery of the platform. These peripheral platforms
were exposed to karstification during the Plio–Pleistocene
sea level minima.
During the Quaternary, the dominant mode of reef
development has been vertical coral reef growth on top
of these karstified remnants. This vertical reef growth is
a consequence of high-amplitude sea-level fluctuations
during the Pliocene–Pleistocene, producing alternate
periods of exposure and submergence. The Holocene is
the last of these reef growth phases.
The contemporary surface morphology of the Maldivian reefs is the result of coral reef growth over the past
10,000 years upon Pleistocene foundations. The Holcoene
reef growth history of the Maldives has only recently been
resolved in detail based on drill cores from Ari and South
Maalhosmadulu atolls with the Holocene reef thickness
ranging from 12 to 14 m (Gischler et al., 2008; Kench
et al., 2009). These studies show the Holocene reef growth
initiated approximately 8,100 years ago as sea level
flooded the Pleistocene surface. Vertical reef growth
occurred at around 7 mm/year for the period 8,100–
6,500 years ago and subsequently slowed to 1 mm/year
thereafter. Evidence suggests the final stages of reef accretion in the late Holocene exhibited local variations. In situ
reef deposits located above sea level on some reefs show
MALDIVES
649