Bibliography
Barry, S. J., Cowell, P. J., and Woodroffe, C. D., 2008. Growthlimiting size of atoll islets: morphodynamics in nature. Marine
Geology, 247, 159–177.
Bayliss-Smith, T. P., 1988. The role of hurricanes in the development of reef islands, Ontong Java atoll, Solomon Islands. The
Geographical Journal, 154, 377–391.
Kench, P. S., McLean, R. F., and Nichol, S. N., 2005. New model of
reef-island evolution: Maldives, Indian Ocean. Geology, 33,
145–148.
Kench, P. S., Brander, R. W., Parnell, K. P., and McLean, R. F.,
2006. Wave energy gradients across a Maldivian atoll:
implications for island geomorphology. Geomorphology, 81,
1–17.
Kench, P. S., Nichol, S. N., Smithers, S. G., McLean, R. F., and
Brander, R. W., 2008. Tsunami as agents of geomorphic change
in mid-ocean reef islands. Geomorphology, 95, 361–383.
MacNeil, F. S., 1972. Physical and biological aspects of atolls in the
northern Marshalls. In Mukundan, C., and Pillai, C. S. G. (eds.),
Proceedings First International Symposium of Corals and Coral
Reefs. Mandapan Camp, India: Marine Biological Association of
India, pp. 507–567.
McLean, R. F., and Hosking, P. L., 1991. Geomorphology of reef
islands and atoll motu in Tuvalu. South Pacific Journal of Natural History, 11, 167–189.
Richmond, B. M., 1993. Development of atoll islets in the central
Pacific. In Proceedings of the Seventh International Coral Reef
Symposium, 1992. Guam: University of Guam Press, Vol. 2,
pp. 1185–1194.
Stoddart, D. R. and Steers, J. A., 1977. The nature and origin of
coral reef islands. In Jones, O. A, and Endean, R. (eds.), Biology
and Geology of Coral Reefs. New York: Academic, Vol. 1V,
Geology 2, pp. 59–105.
Wiens, H. J., 1962. Atoll Environment and Ecology. New Haven and
London: Yale University Press, 532 pp.
Wood-Jones, F., 1912. Coral and Atolls: A history and description
of the Keeling-Cocos Islands, with an account of their fauna
and flora, and a discussion of the method of development and
transformation of coral structure in general. London: Lovell
Reeve, 302 pp.
Woodroffe, C. D., 2000. Reef-island sedimentation on Indo-Pacific
atolls and platform reefs. In Proceedings of the Ninth International Coral Reef Symposium, Bali, Indonesia, Vol. 2,
pp. 1187–1192.
Woodroffe, C. D., 2008. Reef-island topography and the vulnerability of atolls to sea-level rise. Global and Planetary Change, 62,
77–96.
Woodroffe, C. D., and McLean, R. F., 1994. Reef islands of Cocos
(Keeling) Island. Atoll Research Bulletin, 403, 36.
Cross-references
Atolls
Cay Formation
Cocos (Keeling) Islands
Coral Cay Classification and Evolution
Coral Cays-Geohydrology
Coral Cays, Vegetational Succession
Enewetak Atoll, Marshall Islands
Funafuti Atoll
Maldives
Mururoa Atoll
Shingle Ridges
Soils of Low Elevation Coral Structures
Vegetated Cays
Wave Shoaling and Refraction
ATOLLS
Colin D. Woodroffe, Naomi Biribo
University of Wollongong, Wollongong, NSW, Australia
Definition
Atolls are annular mid-ocean reefs; the reef rim supports
isolated, or near-continuous, reef islands composed of
unlithified or poorly consolidated sand or gravel, and
encloses a central lagoon.
Introduction
The term “atoll” is derived from a Maldivian (divehi) word,
atolu. Atolls are ring-shaped reefs that occur in mid-ocean,
often in linear island chains or archipelagoes. The most
extensive groups of atolls occur in the Pacific Ocean but
there are also numerous atolls in the central Indian Ocean
(Figure 1). The reef platforms that form atolls are generally
characterized by reefs that reach sea level, especially on the
windward margin of the reef platform, although there is
considerable variation in the extent to which the reef crest
is continuous around the entire perimeter of the central
lagoon. There are several hundred atolls which occur across
a wide range of climatic and oceanographic conditions
(Bryan, 1953), and show a wide variety of shapes (Stoddart,
1965; Shimazaki et al., 2006).
Although there is consensus that many mid ocean reefs
are atolls, it is more difficult to settle on an unambiguous
definition of what constitutes an atoll. The scientific study
of atolls owes a considerable debt to Charles Darwin, first
because of his insight into reef development and the evolution of atolls, but also because he produced a map of
the distribution of known atolls, as well as other reef types,
which focused on the geological factors related to where
atolls have formed.
Definitions of an atoll vary, but emphasize several factors in common. Shepard considered an atoll: “an ovalshaped coral reef surrounding a lagoon in which there
are no islands other than slightly emerged reefs or small
sand cays” (Shepard, 1948, p. 251). Wiens, in his book
on atolls, suggested that atoll shape is too irregular to be
captured by this definition, and believed that the definition
by Kuenen is preferable: “all more or less continuous reefs
surrounding a distinctly deeper lagoon with or without
lagoon reefs. . . which rise from a sea bottom which is
too deep for the growth of coral reefs” (Newell and Rigby,
1957, p. 21, following Kuenen). Wiens proposed his
own definition: “an atoll is a more or less continuous
emerged or slightly submerged calcareous reef surrounding a distinctly deeper lagoon or several such lagoons
without emerged volcanic islands, which stand apart from
other islands, and whose upper seaward slopes rise steeper
than the repose angle of loose sediments from a generally
volcanic foundation too deep for the growth of reef corals”
(Wiens, 1962, p. 8).
ATOLLS
51
Barry, S. J., Cowell, P. J., and Woodroffe, C. D., 2008. Growthlimiting size of atoll islets: morphodynamics in nature. Marine
Geology, 247, 159–177.
Bayliss-Smith, T. P., 1988. The role of hurricanes in the development of reef islands, Ontong Java atoll, Solomon Islands. The
Geographical Journal, 154, 377–391.
Kench, P. S., McLean, R. F., and Nichol, S. N., 2005. New model of
reef-island evolution: Maldives, Indian Ocean. Geology, 33,
145–148.
Kench, P. S., Brander, R. W., Parnell, K. P., and McLean, R. F.,
2006. Wave energy gradients across a Maldivian atoll:
implications for island geomorphology. Geomorphology, 81,
1–17.
Kench, P. S., Nichol, S. N., Smithers, S. G., McLean, R. F., and
Brander, R. W., 2008. Tsunami as agents of geomorphic change
in mid-ocean reef islands. Geomorphology, 95, 361–383.
MacNeil, F. S., 1972. Physical and biological aspects of atolls in the
northern Marshalls. In Mukundan, C., and Pillai, C. S. G. (eds.),
Proceedings First International Symposium of Corals and Coral
Reefs. Mandapan Camp, India: Marine Biological Association of
India, pp. 507–567.
McLean, R. F., and Hosking, P. L., 1991. Geomorphology of reef
islands and atoll motu in Tuvalu. South Pacific Journal of Natural History, 11, 167–189.
Richmond, B. M., 1993. Development of atoll islets in the central
Pacific. In Proceedings of the Seventh International Coral Reef
Symposium, 1992. Guam: University of Guam Press, Vol. 2,
pp. 1185–1194.
Stoddart, D. R. and Steers, J. A., 1977. The nature and origin of
coral reef islands. In Jones, O. A, and Endean, R. (eds.), Biology
and Geology of Coral Reefs. New York: Academic, Vol. 1V,
Geology 2, pp. 59–105.
Wiens, H. J., 1962. Atoll Environment and Ecology. New Haven and
London: Yale University Press, 532 pp.
Wood-Jones, F., 1912. Coral and Atolls: A history and description
of the Keeling-Cocos Islands, with an account of their fauna
and flora, and a discussion of the method of development and
transformation of coral structure in general. London: Lovell
Reeve, 302 pp.
Woodroffe, C. D., 2000. Reef-island sedimentation on Indo-Pacific
atolls and platform reefs. In Proceedings of the Ninth International Coral Reef Symposium, Bali, Indonesia, Vol. 2,
pp. 1187–1192.
Woodroffe, C. D., 2008. Reef-island topography and the vulnerability of atolls to sea-level rise. Global and Planetary Change, 62,
77–96.
Woodroffe, C. D., and McLean, R. F., 1994. Reef islands of Cocos
(Keeling) Island. Atoll Research Bulletin, 403, 36.
Cross-references
Atolls
Cay Formation
Cocos (Keeling) Islands
Coral Cay Classification and Evolution
Coral Cays-Geohydrology
Coral Cays, Vegetational Succession
Enewetak Atoll, Marshall Islands
Funafuti Atoll
Maldives
Mururoa Atoll
Shingle Ridges
Soils of Low Elevation Coral Structures
Vegetated Cays
Wave Shoaling and Refraction
ATOLLS
Colin D. Woodroffe, Naomi Biribo
University of Wollongong, Wollongong, NSW, Australia
Definition
Atolls are annular mid-ocean reefs; the reef rim supports
isolated, or near-continuous, reef islands composed of
unlithified or poorly consolidated sand or gravel, and
encloses a central lagoon.
Introduction
The term “atoll” is derived from a Maldivian (divehi) word,
atolu. Atolls are ring-shaped reefs that occur in mid-ocean,
often in linear island chains or archipelagoes. The most
extensive groups of atolls occur in the Pacific Ocean but
there are also numerous atolls in the central Indian Ocean
(Figure 1). The reef platforms that form atolls are generally
characterized by reefs that reach sea level, especially on the
windward margin of the reef platform, although there is
considerable variation in the extent to which the reef crest
is continuous around the entire perimeter of the central
lagoon. There are several hundred atolls which occur across
a wide range of climatic and oceanographic conditions
(Bryan, 1953), and show a wide variety of shapes (Stoddart,
1965; Shimazaki et al., 2006).
Although there is consensus that many mid ocean reefs
are atolls, it is more difficult to settle on an unambiguous
definition of what constitutes an atoll. The scientific study
of atolls owes a considerable debt to Charles Darwin, first
because of his insight into reef development and the evolution of atolls, but also because he produced a map of
the distribution of known atolls, as well as other reef types,
which focused on the geological factors related to where
atolls have formed.
Definitions of an atoll vary, but emphasize several factors in common. Shepard considered an atoll: “an ovalshaped coral reef surrounding a lagoon in which there
are no islands other than slightly emerged reefs or small
sand cays” (Shepard, 1948, p. 251). Wiens, in his book
on atolls, suggested that atoll shape is too irregular to be
captured by this definition, and believed that the definition
by Kuenen is preferable: “all more or less continuous reefs
surrounding a distinctly deeper lagoon with or without
lagoon reefs. . . which rise from a sea bottom which is
too deep for the growth of coral reefs” (Newell and Rigby,
1957, p. 21, following Kuenen). Wiens proposed his
own definition: “an atoll is a more or less continuous
emerged or slightly submerged calcareous reef surrounding a distinctly deeper lagoon or several such lagoons
without emerged volcanic islands, which stand apart from
other islands, and whose upper seaward slopes rise steeper
than the repose angle of loose sediments from a generally
volcanic foundation too deep for the growth of reef corals”
(Wiens, 1962, p. 8).
ATOLLS
51