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Great Barrier Reef Islands, Geology

Around 1000 islands occur on Australia’s Great Barrier Reef (GBR), with the exact number varying depending on where the boundaries of the GBR are drawn and how islands are defined (e.g., when is a sand bar an unvegetated cay?). The numbers cited below are for the Great Barrier Reef Marine Park (GBRMP). Two main island types occur: continental high islands and low reef islands. Continental high islands are continental outcrops that were separated from the mainland as rising seas flooded the continental shelf after the last ice age. The low reef islands are accumulations of reef-derived carbonate sediments deposited on reef flats formed after the mid-Holocene.