" Matupás, she and her colleagues report in a paper recently published in PLOS ONE, begin when aquatic and semi-aquatic plants begin to crowd together and transform, over time, into a buoyant soil disc where shrubs and trees feel at home. The islands can be almost 10 feet thick and anywhere from a few square yards to a few acres in size. Some are strong enough for people to walk on—strong enough even to grow food on."
http://www.atlasobscura.com/articles/the-science-and-legend-of-the-amazon-s-floating-forest-islands
http://www.atlasobscura.com/articles/the-science-and-legend-of-the-amazon-s-floating-forest-islands