Pohnpei-Between Time & Tide . How Pohnpei Formed

How the Island of Pohnpei Formed

ince 99.9% of the F.S.M.'s 1.5 million sq. mile area is ocean, the sea plays a paramount part in the lives of Micronesians. It is a resource, a livelihood, a highway, a creator, a destroyer, and the outer core of island life. Not yet ruined by development, Pohnpei's waters are vast and beautiful; clean, clear and seething with life and growth. Fed from the east by the Northern Equatorial Current, the ocean is like bathwater; nearly 85°F on the surface. Dubbed the Warm Pool by geographers, the area has the warmest ocean temperatures in the world. Pohnpei's barrier reef is a blossoming of coral and reef fish that swarm like multi-colored bees. The lagoon is a labyrinth of shallow reefs and azure channels, which flushes in and out through numerous passes in the reef. These passes are teaming with marine creatures; sharks, barracuda, manta rays, jacks, squid, and large pelagic fish.

Pohnpei Island is merely the tip of a five million year old extinct shield volcano. The entire island is made up of volcanic basalt rock, black in color. How did the island form? According to Darwin's theory⁽¹⁾ of hot-spot island formation concocted in the 1830's, occasionally magma from the Earth's mantle spurts up through openings in the oceanic crust. As the magma spills out, it cools and solidifies, building upon itself. A seamount begins to form and grows steadily toward the surface. (1) Eventually, the seamount, or underwater volcano, grows high enough that it breaks the surface of the ocean and can be called an island. If the volcanic activity continues to produce new rock, the island grows larger. This is currently happening on Hawaii's Big Island. (2) After the island's volcanism ceases, several things happen. First, the sheer weight of the island slowly depresses the oceanic crust on which it rests, and it begins to slowly sink (subsidence). Meanwhile, weathering processes erode the rocky peaks, soil forms and plant life begins to appear. Now provided with substrate on which to build, tiny marine animals called coral polyps⁽²⁾ establish themselves on the island's shallow fringe, creating a fringe reef. (3) As the island continues to subside, a space widens between the shore and the fringe reef, forming a semi-enclosed area called a lagoon. The original fringe reef now composes a barrier reef⁽³⁾. A new fringe reef may develop if the subsidence is slow enough and the water conditions allow it. (4) Ultimately, the island will submerge completely, leaving only a ring-like barrier reef around an empty lagoon. If sediment and coral debris build the barrier reef up above sea-level so that it can support plant growth, it becomes an atoll, or low island. All oceanic islands in the Pacific east of the Andesite Line are in various phases of this cycle.

POHNPEI'S INFLUENCE ON THE ART WORLD

Subsidence Phases (top view) (1) CHARLES DARWIN, the famed founder of evolutionary biology, made reference to Pohnpei Island in his book The Structure and Distribution of Coral Reefs. The Caroline Islands are perfect examples of the phases in his cycle. Because the oceanic plate was moving west over the stationary hot spot, islands get younger as one moves along the chain to the east. Kosrae, in the east, is the youngest, has the sharpest peaks and a fringing reef, but no lagoon or barrier reef. (A) Pohnpei's older mass still sits high above the surface, but has subsided enough that a sizable lagoon has developed. Its mountains are also more eroded than Kosrae's. (B) Chuuk is the oldest of these three. Its island has subsided considerably, leaving only a few peaks still poking up into the large mostly empty lagoon. (C) The atolls of western Chuuk and Yap represent the final stage, where the original island has long since submerged.

Crinoid (2) CORAL POLYPS are small animals that live in colonies, consisting of millions of individual organisms. They are generally active at night. Polyps fasten onto existing rock and create exoskeletal armor around themselves. The reef that is produced by these skeletal structures is the foundation of an intricate web of other organisms, such as crinoids. Crinoids look like strange marine plants with their waving, feathery extremities, but they are actually animals.

(3) A BARRIER REEF is a thick ring of coral surrounding an island, but not contiguous with its shores. Pohnpei's barrier reef is about 135 miles in circumference and is 1-2 miles off-shore. It is broken by half a dozen passes that lead to the open ocean.

Text by Alex Zuccarelli 2003.
Image/Photo credits: Crinoid photo courtesy of Jeffrey Jeffords ©1997-2000. Reef photo source unknown. All other photos, images and maps property of Alex Zuccarelli.
All photographs on this page were used with the written permission of their perspective owners, except where otherwise indicated.

Primary Sources

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On-Line Resources