Surely chunks of solid earth on which we stand cannot drift easily about the surface of a water body. But floating islands do indeed exist on six of the seven continents and sometimes in the oceans between them. These islands are kept buoyant by the light spongy tissues of certain aquatic plants, by gases released into their soil by decomposing vegetation, or by both these forces.
Floating islands have attracted the attention of ancient authors, and the Roman writer Pliny the Younger has left us an evocative description of the floating islands in Lacus Vadimonis -- now a marshy pond known as Lago di Bassano, Laghetto di Bassano, or Laghetto di Basanello, on the bank of the Tiber about 60 km north of Rome. He wrote: "No boats are allowed on the lake, as its waters are sacred; but several floating islands swim about it, covered with reeds, rushes, and whatever other plants the fertile marshy ground nearby and the edge of the lake produce.Each island has its peculiar shape and size, but the edges of all of them are worn away by their frequent collisions with the shore and one another. They are all of the same thickness and buoyancy, for their shallow bases are shaped like the hull of a boat. This may be clearly observed from all sides: the islands lie half above and half below the water's surface. Sometimes they cluster together and seem to form a little continent; sometimes they are dispersed by the shifting winds; at other times, when the wind falls dead, they float in isolation. Often a large island sails along with a small island joined to it, like a ship with its tender, or as if one were striving to out-sail the other; then again they are all driven to one spot on the shore, whose limits they thus advance; and now here, and now there, they diminish or restore the area of the lake, until at last they occupy the center again and so restore it to its usual size. Sheep, seeking grass, proceed not only to the shores of the lake, but also upon these islands, unaware that the ground is mobile, until, far from the shore, they are alarmed to find themselves surrounded by water, as though they have been suddenly placed there. When the wind drives them back again, they little perceive their return as their departure."
Jagged edged wonder:
Pliny's observation about the edges of the islands being worn away by collision is a common feature of floating islands in lakes. These include the great floating islands of papyrus in the lakes of the Upemba Basin, upper Lualaba River, Zaire; Orange Lake, Florida, usa, which are being studied by Mark Clark of the University of Florida; the Iber Wetlands near Corrientes, Argentina; the Lago di Posta Fibreno southeast of Rome; and on the surface of the Zacatn sinkhole in Tamaulipas, Mexico. In India they can be found in Loktak lake, Manipur and in Khajjiar Lake, Himachal Pradesh.
Floating islands commonly rise in newly-flooded reservoirs.This happens if the flooded area has plenty of peaty soil (such soil contains decomposing vegetation). Once the bottom of the reservoir is filled with certain types of peat, it becomes buoyant. If the peat is covered by deep water, the weight of the water over it holds the peat to the bottom, but in shallow parts of the reservoir, less than two metres deep or so, this buoyancy can tear away sections of peaty soil up from the bottom of the reservoir, and they rise to the surface as floating islands.
The island can be colonised by various plants, including trees. In hydroelectric reservoirs, these islands can cause serious problems if they are drawn into the intake for power generation equipment.
It is difficult and expensive to remove floating islands from reservoirs. It is, however, possible to prevent or mitigate this problem before a reservoir is flooded. Soils in flood-prone areas can be studied, and their rising can be forestalled by loading gravel.
Floating islands also form during floods of the great tropical rivers when large masses of aquatic vegetation or chunks of their banks are torn away and carried downriver. The Congo in Africa is one such river, and floating islands that came down the Congo were reported 240 km out to sea from the river's mouth. Floating islands are also common in the Sepik river in Papua New Guinea following the monsoon rains. The islands are called "Lik Lik Aislans" in pidgin English, and can be up to 100 metres across with living trees on them. The Ro Paran and Ro de la Plata in South America also generate floating islands -- when they flood they are filled with floating islands called camalotes , which are matted masses of water hyacinth. A famous episode at Convento de San Francisco in Santa Fe, Argentina, which is located on the Ro Paran, involved the killing of two friars at the Convento by a jaguar that arrived on a camalote during a flood of the Paran on April 18, 1825.
Loktak lake, Manipur
In the flood of 1905, the Ro de la Plata at Buenos Aires was covered with camalotes as far as the eye could see, some half a mile long and 100 feet wide, others just a few feet in diameter. As they came down the river, these islands hit moored ships and tore them from their anchor And the passengers that accompanied them included species of tropical snakes, deer, a puma, parrots, and monkeys. An Indian baby was found on a floating island that came ashore near Rosario, and although he was weak from hunger and exposure, the kid was brought back to health.
Of course, floating islands that come down rivers end up at sea; many are quickly destroyed by the waves, but others survive for quite some time. An article in the November 8, 1908, edition of the Washington Post reports that a us cruiser in the Caribbean north of Honduras encountered an island which they soon discovered was floating (this is certainly one of the largest floating islands ever seen at sea).
The paper reported: "It proved to be a little island about three quarters of a mile around and a quarter wide. In shape it was long and narrow, with a thick growth of vines and bushes reaching down to the water's very edge. Three tall cocoanut palms grew in the middle of it. No life of any kind was on the island, nor was there any water, though instead of being sandy or rocky as such islands usually are, the soil was rich, dark and very moist. After gathering the cocoanuts the sailors returned to the cruiser, which, oddly enough, seemed much further off, and considerably more to the southwest than when they left her. Then it just dawned on them that they had been visiting one of the floating islands so often heard about but seldom seen in the South Atlantic. Further observation confirmed the suspicion, as the cruiser remained near it long enough to see the island change its position."
A story published in several newspapers in June and July of 1902, gives a remarkable account of two floating islands spotted at sea in the Caribbean. The Norwegian ship Donald , steaming from Banes, Cuba, on its way to Philadelphia, encountered a floating island about 48 km from the island of San Salvador.
One us correspondent reported, "On passing Watlins island, which lay off about 48 km," said Skipper Warnecke, "we steamed close to a floating island. Upon it were what appeared to be a large number of stately palm trees. I had never encountered anything like this before. The floating island was moving, and that, too, at a slow rate. Curious for a thorough investigation, I steamed still closer to the object, and was amazed to find what I took to be palm trees were full-grown cocoanut trees, and laden with fruit of the largest kind. Then I ordered a boat lowered and, together with the first mate, made a landing on the still moving island. Then another surprise awaited us. High up in the trees was a small colony of mischievous monkeys, and as we got nearer they shied a number of cocoanuts at us. After a lot of trouble we secured two of the attacking simians and at least a dozen cocoanuts. Then we took to our boats, boarded the steamer, ordered full steam ahead, and soon the strange floating island was lost in haze . But another surprise was in store for us the following day, when we passed within glass sight of another singular floating object just off the port bow. The lookout sung out 'Land ahead.' This amazed me, for I knew according to the chart land was not miles near. Still, curious from the previous day's experience, I determined to solve this further mystery of the sea, so I gave orders for the ship to steam close to what I now made out to be another floating island. Again I had a boat lowered, and with the same crew we landed on the island. We found it to be an exact duplicate of the day before, with an exception --instead of monkeys we found a big covey of parrots of most brilliant plumage. Among them was one who was evidently the patriarch of the tribe, and I do not exaggerate when I say that the aged fellow could cuss in two languages. He was evidently a lost pet. We took him and a couple of his fellows aboard the steamer, and soon left the floating island in the distance."
These accounts are of particular interest to biologists. They support the theory that floating islands have been important in the dispersal of plant and animal species across the oceans, and are important in explaining evolution.
Chet Van Duzer has recently published Floating Islands: A Global Bibliography. (Cantor Press, www.cantorpress.com ). The addenda to the book is available as a free download in PDF format