Popular Science Monthly/Volume 79/September 1911/Floating Islands

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IF one should see in a story the statement that a floating island 100 feet square, upon which were growing trees thirty feet in height, was used by the hero as a place of refuge, and that the island traveled 1,000 miles out to sea, he would probably accuse the author of an abuse of the imagination. But such an island—without the hero—was seen off the coast of North America and is known to have traveled at least 1,000 miles. Floating islands also occasionally occur in the lakes of some of our northern states. It is the purpose of this article to point out the location of some of these islands and to explain their origin.

In order to understand the formation of a floating island let us imagine a pond on the edges of which rushes and grasses are growing. These gradually push their way out into the deeper water, leaving a mass of decaying vegetable matter upon which, in time, mosses such as sphagnum may secure a foothold, and start a shelf which will extend out into the water. As soon as the sphagnum has become well established, water-loving plants and shrubs such as alders, sheep laurel and sweet gale will grow with the moss. Cranberries and pitcher plants may also aid in the formation of the mat, forming the familiar cranberry bog. This shelf will be attached to the shore for several feet, the distance depending on the depth of the water, but the peat will seldom attain a thickness of more than three feet. After the mat has become firm, black spruces and larches may grow upon it, often anchoring it and always making it more compact by means of their roots. Such a mat is illustrated by the drawing on page 304.

After the shelf has extended itself some distance into the pond, if the water level is raised unusually high by excessive rainfall or the construction of a dam across the outlet, the mat may break off and form a floating island. This island will either become attached near its former position by roots extending underneath it, or it will float around the lake.

In case the pond is small and nearly circular the mat may surround it. If the water in the pond is raised, at the time when all but the center of the pond is overgrown, an atoll or ring-shaped island may be formed. For the persistence of this atoll the water must remain at its high level and the atoll must become anchored.

Finally, let us suppose that the vegetation completely covered the lake before the water rose. There is sufficient elasticity in such a mat to permit it to rise slightly with a rise in the water level. The mat, however, is not sufficiently elastic to permit its center to rise more than
Cross-section of a Lake, showing a mat of vegetation growing out from both sides. A rise in water level might break off this shelf, and thus form a floating island.

a few feet, if the sides remain attached, and consequently it may disappear with unusually high water. In order for it to reappear either the water must go down or some agency must push the center up higher. This agency is marsh gas. This gas with its light specific gravity could exert such a force on the center of the mat that it would he buoyed up. In order for the gas to act in such a manner it must be present in large quantities and must not escape until that mat has reached the surface. When the gas escapes the mat will again disappear. This alternate appearance and disappearance of the mat makes it a periodic island. A true periodic island would not be attached to the sides. Such islands are rare, and conclusive evidence is lacking to show that they are not attached.

The various ways in which floating islands, floating atolls and periodic islands may originate are as follows:

I. Floating Islands are divided into—(A) natural islands and (B) artificial islands.

(A) Natural islands may be formed by:

1. The coming together of floating vegetable masses. This hypothesis demands sufficient floating material upon which there is plant life of a suitable kind, or upon which plant life may start. There must also be some favorable agency to collect this material. In a large lake where high waves could break off pieces of sphagnum from the shore, the waves might collect the pieces so as to form a floating island. In small lakes, cat-tails or other rushes form a favorable place for such material to lodge.

2. The action of waves beating against a mat of vegetation may by their force break off large islands. This could happen only in the case of a large body of water and would probably account for the origin of the floating island mentioned in the introduction which was seen in the Atlantic Ocean in 1892. When first noticed in July in latitude

Cross-section of Sadawga Lake, Whitingham, Vt., showing two floating islands. The large island grew out from the shore and was broken off by high water. It is now attached at the east side.

39.5° N., longitude 65° W. the island was about 9,000 square feet in extent, with trees thirty feet in height upon it, which made it visible for seven miles. It had apparently become detached from the coast of this country and been carried out to sea by the Gulf Stream. It was again seen in September in latitude 45° 29' N, longitude 42° 39' W., after it had passed through a severe storm. By this time it must have traveled over 1,000 nautical miles, and it may have eventually arrived at the coast of Europe. A Section of an Outline Map of North America, showing the course of a floating island which was seen floating in the Atlantic Ocean in 1892.

3. The raising of the water level separates mats from the shore, as previously explained. An interesting case under this heading is that in which there are two kinds of peat in an overflowed bog. If the peat is arranged in layers, the bottom layer being heavy, since it is formed by the decompositon of woody material, may separate from the top layer which is light on account of its sphagnum content. In this circumstance the bottom layer remains in its former position, the upper layer breaking away to form a floating island.

4. The action of ice in the northern climates sometimes separates masses of vegetation from the shore. This expansion and contraction of ice is often an important factor in forming shore topography and an overgrowing mat could offer but feeble resistance to this force.

(B) Artificial islands may be formed by the damming of ponds or lakes, which is a very common mode of origin of floating islands in this country. An example of this type of island is that in Sadawga Lake shown in cross-section in the second sketch.

II. Atolls

Two atolls in central Minnesota, which are characteristic of all such atolls, have been described by Conway MacMillan in the Minnesota Botanical Studies,[1] and the following description of them is taken from that article.

The [larger] pond is about 150 yards across and almost circular in shape. It is surrounded, except for a short distance on the west, by rather precipitous morainic hills 50-75 feet in height. . . . [The atoll] is about 75 feet in diameter and of uniform width of about ten feet. On the west its continuity is broken by a channel, twelve feet across, which furnishes communication between the waters of the ultra-insular lagoon and those of the pond outside of the atoll. The depth of the water is nowhere great. The greatest depth is about twelve feet and this maximum of depth is in the middle, within the lagoon. . . . The general texture of the atoll was loose, so that one standing anywhere upon it soon sank into the soft and spongy mass up to the knees. . . .

[The smaller] pond is barely fifty yards across, with high banks, and the atoll ring is within a foot or two of twenty yards in diameter. Its breadth, however, is greater than that of [the other] atoll, being twelve feet on the average from the outer to the inner aspect. The lagoon, then, is slightly less than fifty feet across. . . . The water of the pond was shallow, averaging four feet, just outside the atoll ring. . . . In general the texture of the atoll was much firmer than that of [the other] atoll. One could stand anywhere upon it without sinking in above the insteps.

The explanation of the presence of sphagnum atolls may be derived from the assumed changes in level of the pond water, and indeed their presence may, conversely, be held to indicate, or to demonstrate, fluctuations in the pond level. If it be possible to conceive that in these two atoll-producing ponds there has been, during the course of years, a gradual diminution in size followed by a rather rapid increase in diameter and depth, I believe the formation of the atolls would become a phenomenon readily comprehensible. . . . Concomitantly with the diminution in size, doubtless extending over a term of years, vegetation of the shoreward area would have established itself in characteristic zones. The littoral flora and the submerged plants just outside the shores would have formed a loose turf lining the edges of the pond. This turf would have gradually become more solid as it extended landward and would therefore at a little distance from the water's edge have become modified in character, giving a foothold for plants of larger growth. . . . When, subsequently to this epoch of gradual diminution, the ponds began to increase again, the effect of the rise in level of the water was to detach from the shore a ring of the loose littoral turf, and this mass of vegetation with its attendant soil, buoyed up at first as a circular floating bog, appears to-day as the characteristic sphagnum atoll. . . . The increase in size [of the pond] left the annular ring far out in the waters of the pond. . . and as the mass of vegetation and soil became thoroughly saturated with the water below, its character may gradually have changed until
A View of Sadawga Lake, Whitingham, Vt. Two floating islands are shown in the central part of the picture, and behind them is the main floating island from which they have become detached.

Another View of Sadawga Lake, showing the main floating island (1) and three smaller floating islands which have been broken off from it and have drifted ashore.
the sphagnum plants retained vitality. Generations of these succeeding each other contributed to the weight of the ring and finally pressed it down upon the bottom of the pond, forming the anchored atoll of the present. [The essential requirements in the formation of atolls are] a small parent pond, height and regularity of banks, regular and gentle slope of the bottom, suitable original littoral vegetation, small lateral pressure and tension of winter ice, and comparatively prompt anchoring of the bottom.

III. Periodic Islands

As previously explained, periodic islands may rise without the formation of gas, but as gas usually escapes from them after they have risen, irrespective of the force causing them to rise, it is a factor which demands attention. Under the average floating island there is little gas, owing to the loose texture of the peat. Consequently, in order to have gas support islands it is necessary that an upper layer of the bog be made of dense material, thus allowing the gas to escape but slowly. The most favorable place for it to collect is between the layers of peat. This is illustrated by experiments[2] in digging peat from a bog filled with water. Holes were dug in the evening of one day and the next morning they were found to be filled with peat masses in the form of domes, cleft in the middle. These domes were formed of one of the lower layers of the peat. The gas under this layer combined with lateral pressure forced it up.

Periodic islands usually rise in spring and sink in fall, owing to the activity of gas-producing organisms in warm water. Some periodic islands have been reported which rose for only a few days. Because of the short time which most of these islands have been known, it has been impossible to study them. Attention is now being given to one in this country and it is to be hoped that through its study valuable information will be obtained regarding this type of island.

  1. 1894, Bul. 9, Vol. 1.
  2. Früh und Schröeter, "Die Moore der Schweiz," Beitr. geol. Schweiz Geotech., ser. 3, Bern, 1904.