Indian Ocean Islands

Abstract

The Indian Ocean contains ~2000 islands (sensu lato) of diverse origins, sizes, topographies, and geologies. Ranging from the equator to subpolar regions, some have a continental origin, some are oceanic, and the origin of many is enigmatic. Sizes span a range from <1 to >7000km2, and elevations vary from sea level to 3070m. The vast majority of islands are low-lying, with little topography; numerically, coralline atolls, islands, and islets are the overwhelmingly dominant type of Indian Ocean Island. In terms of land area and varying topography, however, volcanic islands prevail. In a global context, Indian Ocean islands encompass several geologic “type” examples. Seychelles constitutes the quintessential microcontinent. Heard, Marion, and Réunion are among the most active mantle hotspots globally, and for which both short-lived “plume head” and long-lived “plume tail” magmatic products are preserved. Great Chagos Bank is the world's largest atoll, Diego Garcia has the longest continuous dryland rim of any atoll worldwide, and Aldabra is the largest raised coral reef globally. Charles Darwin developed a well-accepted model of atoll formation from observations at Cocos (Keeling), and for which Europa, Mayotte, and Rodrigues are also exemplars. Nevertheless, many mysteries remain. Mantle plumes may or may not be responsible for creating the foundations of Amsterdam/Saint Paul, Comoro, Crozet, and Rodrigues islands. Similarly, the origins of the foundations of multiple islands and island groups—Agaléga, Aldabra, Amirante (including Alphonse), Bassas da India, Christmas, Europa, Farquhar, Glorioso, Juan de Nova, and Tromelin—are enigmatic. A sea level rise of several meters would drown a great majority of Indian Ocean islands: Agaléga, Aldabra, Amirante, Bassas da India, Chagos, Cocos (Keeling), Europa, Farquhar, Glorioso, Juan de Nova, Lakshadweep (Laccadive), Maldives, Saint Brandon, some Seychelles, and Tromelin.