Close you eyes and think of the ideal tropical island. What do you see? White sand beaches… palm trees… crystalline water lapping on the shore… verdant tropical vegetation… surrounded by bands of blue that blow your mind… You probably also think of it as being small – the better to have as your own. What this describes is a motu, and French Polynesia has more motu that anywhere on earth.
In fact motu is a Polynesian word that means “small island”, but it has also been adopted by geographers and given a scientific definition. In this case it represents a type of islet that forms on top of a coral reef in high-energy environments – i.e places that occasionally experience massive storm waves. It has a variety of sediment sizes and on the seaward edge may have a fringe of cemented coral rubble that is called a conglomerate. It can have lots of vegetation on it although it is usually only hardy native plants that can survive here. Another similar geographic term is a “sand cay” which is found in low energy environments and is just made up of sand sized sediment and usually doesn’t have any vegetation.
Motu are created in the first place my massive storm waves that tear chunks of coral off the outer reef slope and toss them up onto the reef flat. This rubble, with everything from car to gravel sized sediment in it, forms a bank parallel to the reef. This bank then creates a barrier to water flow and smaller sized sediment begins to settle out on the lagoon side. Eventually the rubble bank erodes down with wind and rain, and plants begin to arrive and stabilize the sediment, and voila you have a motu.
Once a motu forms it usually persists for a long time, although the shape and size of it can get re-arranged by subsequent storms. Infrequent storms often add large sediment input to the motu, while the normal day to day circulation of the lagoon often strips sand away, or spins it out into sand spits in the direction of the prevailing current.
Motu form on the reefs of high islands and once the volcanic interior erodes away (which takes about 5-6 million years) motu are all that are left forming a ring around a large lagoon. This island form is called an atoll and 74 of the 78 islands in the Tuamotu Archipelago are atolls.
Motu then are made up entirely of organic material – sediment made from coral and other reef organisms, and then the plants that grow on it. If you find a rock on a motu that is not from the reef then it is certain that it was either brought by a human or fell from the sky. In fact it is possible to find volcanic rock on motu because Polynesians used it in ancient times for tools, and fishing weights, and ballast for canoes. Meteorites are also found surprisingly often on motu since they stand out very well, a rusty red iron-laden rock, against a white background.
One of the great pleasures of Tahiti and her Islands is to get yourself to a remote atoll, and explore the pristine motu environment. Walk on wild beaches disturbed only by the tracks of hermit crabs, snorkel the clear waters along the shore, and wonder about the unique confluence of processes that created the little island under your feet.