Page 550 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

514 part III The earth–atmosphere interface
into calcium carbonate that is lithified into rock. Coral reefs form through many generations, with live corals near the ocean’s surface building on the foundation of older coral skeletons, which, in turn, may rest upon a volcanic seamount or some other submarine feature built up from the ocean floor. Thus, a coral reef is a bio- logically derived sedimentary rock that can assume one of several distinctive shapes.
In 1842, Charles Darwin proposed a hypothesis for reef formation. He suggested that, as reefs develop around a volcanic island and the island itself gradually subsides, equilibrium is maintained between the subsid- ence of the island and the upward growth of the corals (to keep the living corals at their optimum depth, not too far below the surface). This idea, generally accepted today, is
portrayed in Figure 16.20. Note the specific examples of each reef stage: fringing reefs (platforms of surrounding coral rock), barrier reefs (reefs that enclose lagoons), and atolls (circular, ring-shaped reefs).
Earth’s most extensive fringing reef is the Bahama Platform in the western Atlantic (Figure 16.21), covering some 96000 km2. The largest barrier reef, the Great Barrier Reef along the shore of the state of Queensland, Australia, exceeds 2025 km in length, is 16–145 km wide, and includes at least 700 coral-formed islands and keys (coral islets or barrier islands).
Coral Bleaching As mentioned previously, coral reefs may experience a phenomenon known as bleaching, in which normally colourful corals turn stark white by
Fringing
149° W)
Barrier
Atoll
Bikini
(12° N 165° E)
Eniwetok
(12° N 162° E)
Tahiti (18° S
O‘ahu
(22° N 158° W)
Rarotonga
(21° S 160° W)
Mayotte (13° S
Truk
(7° N 152° E)
Clipperton
(10° N 109° W)
Hawai‘i (20° N
45° E) Santa Cruz
156° W) Grand Comoro (12° S 44° E)
(11° S 166° E) Bora Bora
Aitutaki
(19° S 160° W)
Kwajalein
(9° N 167° E)
(16° S 151° W)
(a) Common coral formations in a sequence of reef growth around a subsiding volcanic island: fringing reefs, barrier reefs, and an atoll.
Tikehau atoll
(d) Barrier reefs surround Bora Bora, Tahiti.
(b) Tikehau Atoll, French Polynesia.
Pacific Ocean
1000 m
(c) Tuherahera village and Tikehau airport.
▲Figure 16.20 Coral formations. [(a) after D. r. Stoddart, The Geographical Magazine 63 (1971): 610. (b), (c) EO-1 image, naSa. (d) wilar/Shutterstock.]
Lagoon
(c) Bahamian coral platform