and the final stage is the formation of an atoll,
where the original island has disappeared, leaving
only the coral ring surrounding a shallow water
lagoon (Fig. 2.5). As Ridley (1994) noted, Darwin
thought out the outline of his theory on the west
coast of South America before he had seen a true
coral reef! Although the theory may not be globally
applicable and requires modification, for instance
in the light of contemporary understanding of sea-
level change, his basic model remains viable for
most oceanic atolls and can account for most of the
massive coral reefs (Steers and Stoddart 1977).
Coral reefs are built by small coelenterate ani-
mals (corals) that secrete a calcareous skeleton.
Within the tissues and calcareous skeleton, numer-
ous algae and small plants are lodged. The algae
are symbionts critical to reef formation, providing
the food and oxygen supplement necessary to
account for the energetics of coral colonies, while
obtaining both growth sites and nutrients from the
coral (Mielke 1989). Reef-building corals generally
grow in waters less than 100 m deep (exceptionally
they can be found as deep as 300 m); they require
water temperatures between 23C and 29C, and are
thus found principally in tropical and subtropical
areas, notably in the Indo-Pacific Ocean and in the
Caribbean Sea (Fig. 2.6).
Coral growth has been found to vary between 0.5
and 2.8 cm/year, with the greatest growth rates
occurring in water of less than 45 m depth (Mielke
1989). Historically, these growth rates have been
great enough to maintain reefs in shallow water as
either the underlying sea floor subsides or the sea
level has risen (or both). Drilling evidence from
islands with barrier reefs demonstrates this ability,
just as predicted by Darwin’s (1842) theory. For
example, the thicknesses of coral that have accumu-
lated on the relatively young islands of Moorea
(1.5–1.6 Ma), Raiatea (2.4–2.6 Ma), and Kosrae
(4.0 Ma) vary between 160 and 340 m, whereas the
rather older islands of Mangareva (5.2–7.2 Ma),
Ponape (8.0 Ma), and Truk (12.0–14.0 Ma) have accu-
mulated depths of 600–1100 m of coral (Menard
1986). However, it should be noted that massive reef
accretion also occurs in the western Atlantic on conti-
nental shelves that have not experienced long-term
subsidence (Mielke 1989). This lends support to
another theory: that reefs established on wave-cut
platforms during periods of lowered sea level associ-
ated with glaciation, and that reef development then
kept pace with rising sea levels in the interglacial
period. This illustrates that several processes—subsi-
dence, eustatic sea-level changes, temperature
changes, and wave action—must each be considered
with respect to the influence that they have had on
the growth of coral reefs (cf. Nunn 1997, 2000).
Given the vagaries of tectonic processes and sea-
level changes, coral-capped islands can becomeENVIRONMENTAL CHANGES OVER LONG TIMESCALES 23IslandHigh water
Low water(a) Fringing reefIslandHigh water
Low waterLagoon(b) Barrier reef(c) AtollSubmerged islandHigh water
Low waterLagoon Figure 2.5The developmental sequence of coral
reefs as a result of subsidence, from (a) to (b) to (c), as
hypothesized by Darwin (1842). (Redrawn from Mielke
1989, Fig. 7.10.)