Coral Reefs - Living Rocks or Jellyfish Cousins at Home?

When volcanic action pushes above the sea level in tropical areas and forms an island, when the new land cools, it is slowly populated. First, birds use it as a landing place after long periods of flying. They nest there and provide extra "fertilizer" to make the volcanic soil rich enough to support plant life. Some of the birds may have plant seeds stuck to their feathers or may have swallowed fruits only to defecate the seeds along with the fertilizer. These seeds germinate and take hold--eventually forming the ground cover plant life for the island. Floating fruits also arrive on the shores of the island. Some take hold and form the beach strand vegetation. In the shallows around the island, coral larvae finally settle on the substrate. Eventually, they form the fringing reef of the island. But this process takes thousands of years.

Sometimes the volcanic island starts to resubmerge after many thousands of years--or the sea level rises. When this happens, the corals at the reef front (at the edge of the open ocean) form a higher growing ridge and the corals on the island side of the reef start to die off as sand and silt deposit over them and as the water gets more stagnant with less oceanic flow. Over time, a lagoon surrounds the island, enclosed within the barrier reef. In the case of Australia, the barrier reef is far enough away from the Australian mainland such that it is growing on both sides.

Atoll pictured here: Majuro

When the volcanic island subsides enough (or the sea level rises enough) the coral at the reef front has grown higher such that it begins to approach the height of the peak of the volcano. Eventually after many years, the peak of the volcano subsides completely within the fringe of the reef, giving the resulting reef front and reef platform a ring shape--the classic appearance of an Atoll. Islands in Micronesia that are atolls are Kwajalein, Majuro, Eniwetok, and perhaps the most famous one, Bikini.

The island of Guam in the Western Pacific demonstrates two of the three reef types-- in the southern part of the island, there is a wide channel separating the island from a small island on what could be considered a barrier reef. The small island is Cocos Island--a popular tourist destination for day trips from the main island of Guam. The other reefs around the island are fringing reefs--usually wide platforms extending sometimes almost a mile away from the island shore to the reef front. On the Philippine Sea side (west) of the island, the reef flats are expansive on most of the indented bays. Except for Pago Bay on the East side of the island (which has a very large reef flat on both sides of the Pago River outlet, the reef flats are usually more narrow--they are exposed to the rougher seas of the open Pacific Ocean. This side of the island gets the brunt of incoming typhoons and the reefs sustain a lot of damage during the storms.

The islands of the Republic of Belau are mostly surrounded by fringing reefs--but they are also examples of karst topography--limestone outcroppings that form the famous "Rock Islands" of Palau. Babeldaob island, Koror, and Arakebesang form the three main islands of Belau... other islands in this group include Pelelu and Angaur and Ngermiaus. The island groups of Yap and Truk (Chuuk) are similar with their fringing reefs--but Chuuk is also an atoll--and is the site of the famous World War II battle between the Japanese fleet and American aircraft carrier planes. There are many sunken ships in the Chuuk lagoon. Pohnapei is a raised volcanic island with a large fringing reef. It is the site of mysterious basalt rock structures built by an ancient civilization to an aquatic city called Nan Madol. As one moves East across the Pacific toward Hawaii, you come to the true atolls of Majuro, Kwajelein, Eniwetok, and Bikini. Going north from there you reach Hawaii. Going south from there, across the equator, you will reach Tahiti and the Marquesas. Tahiti and the Marquesas are volcanic raised islands with fringing and barrier reefs.