Much of the western Pacific is shaken by the clash of tectonic plates (a phenomenon once referred to as continental drift), when one section of earth's drifting surface dives beneath another. The northern and central Pacific rest on the Pacific Plate, while New Guinea, Australia, Fiji, New Caledonia, and part of New Zealand sit on the Indo-Australian Plate. The western edge of the Pacific Plate runs northeast from New Zealand up the eastern side of Tonga to Samoa, where it swings west and continues up the southwestern side of Vanuatu and the Solomons to New Britain. North of New Guinea the Pacific Plate faces the Eurasian Plate, with a series of ocean trenches defining the boundary. The greatest depths in any ocean are encountered in the western Pacific, reaching 10,924 meters in the Marianas Trench, the deepest point on earth.
The circum-Pacific "Ring of Fire" marks the boundary between the Pacific Plate and the plates to the west. West of this divide, much of New Caledonia remains from the submerged Australasian continent of Gondwana of 100 million years ago. To the east, only volcanic and coralline islands exist.
Three-quarters of the world's active volcanoes occur around the edge of the Pacific Plate and 85 percent of the world's annual release of seismic energy occurs in this area.
As the thinner Pacific Plate pushes under the thicker Indo-Australian Plate at the Tonga Trench it melts; under tremendous pressure, some of the molten material escapes upward through fissures, causing volcanoes to erupt and atolls to rise. Farther west the Indo-Australian Plate dives below the Pacific Plate, causing New Caledonia to slowly sink as parts of Vanuatu belch, quake, and heave. Fiji, between these two active areas, is relatively stable.
A circular or horseshoe-shaped coral reef bearing a necklace of sandy, slender islets (motu) of debris thrown up by storms, surf, and wind is known as an atoll. Atolls can be up to 100 km across, but the width of dry land is usually only 200-400 meters from inner to outer beach. The central lagoon can measure anywhere from one km to 50 km in diameter; huge Rangiroa Atoll is 77 km long. Entirely landlocked lagoons are rare; passages through the barrier reef are usually found on the leeward side. Most atolls are no higher than four to six meters.
A raised or elevated atoll is one that has been pushed up by some trauma of nature to become a platform of coral rock rising as much as 70 meters above sea level. Raised atolls are often known for their huge sea caves and steep oceanside cliffs. The largest uplifted coral platform of this kind in the world is 1,146-square-km Lifou in New Caledonia. If you include lagoon area, at 2,174 square km Kwajalein in the Marshall Islands is the largest true atoll in the world.
Where the volcanic island remains there's often a deep passage between the barrier reef and shore; the reef forms a natural breakwater, which shelters good anchorages. Soil derived from coral is extremely poor in nutrients, while volcanic soil is known for its fertility. Dark-colored beaches are formed from volcanic material; the white beaches of travel brochures are entirely calcareous. The black beaches are cooler and easier on the eyes, enabling plantlife to grow closer and providing patches of shade; the white beaches are generally safer for swimming, as visibility is better.
To understand how a basalt volcano becomes a limestone atoll, it's necessary to know a little about the growth of coral. Coral reefs are the world's oldest ecological system covering some 200,000 square km worldwide, between 25 degrees north and 25 degrees south latitude. A reef is created by the accumulation of millions of calcareous skeletons left by myriad generations of tiny coral polyps, some no bigger than a pinhead. Though the skeleton is usually white, the living polyps are of many different colors. The individual polyps on the surface often live a long time, continuously secreting layers to the skeletal mass beneath the tiny layer of flesh.
Coral polyps thrive in clear salty water where the temperature never drops below 18°C or goes over 30°C. They must also have a base not more than 50 meters below the water's surface on which to form. The coral colony grows slowly upward on the consolidated skeletons of its ancestors until it reaches the low-tide mark, after which development extends outward on the edges of the reef. Sunlight is critical for coral growth. Colonies grow quickly on the windward sides of reefs due to clearer water and a greater abundance of food. A strong, healthy reef can grow four to five centimeters a year. Fresh or cloudy water inhibits coral growth, which is why villages and ports all across the Pacific are located at the reef-free mouths of rivers. Hurricanes can kill coral by covering the reef with sand, preventing light and nutrients from getting through. Erosion caused by logging or urban development can have the same effect.
Polyps extract calcium carbonate from the water and deposit it in their skeletons. All limy reef-building corals also contain microscopic algae within their cells. The algae, like all green plants, obtain energy from the sun and contribute this energy to the growth of the reef's skeleton. As a result, corals behave (and look) more like plants than animals, competing for sunlight just as terrestrial plants do. Many polyps are also carnivorous; with minute stinging tentacles they supplement their energy by capturing tiny planktonic animals and organic particles at night. A small piece of coral is a colony composed of large numbers of polyps.
Corals belong to a broad group of stinging creatures, which includes polyps, soft corals, stony corals, sea anemones, sea fans, and jellyfish. Only those types with hard skeletons and a single hollow cavity within the body are considered true corals. Stony corals such as brain, table, staghorn, and mushroom corals have external skeletons and are important reef builders. Soft corals, black corals, and sea fans have internal skeletons.
The fire corals are recognized by their smooth, velvety surface and yellowish brown color. The stinging toxins of this last group can easily penetrate human skin and cause swelling and painful burning that can last up to an hour. The many varieties of soft, colorful anemones gently waving in the current might seem inviting to touch, but beware: many are also poisonous.
The corals, like most other forms of life in the Pacific, colonized the ocean from the fertile seas of Southeast Asia. Thus the number of species declines as you move east. More than 800 species of reef-building coral make their home in the Pacific, compared to only 48 in the Caribbean. The diversity of coral colors and forms is endlessly amazing. This is our most unspoiled environment, a world of almost indescribable beauty.