A low island, virtually synonymous with an atoll, is a distinctive geological feature in tropical oceans that stands in stark contrast to the mountainous, volcanic “high islands.” Low islands are entirely biological and sedimentary, constructed completely from the skeletal remains of marine organisms. This unique origin means the entire island structure is composed of calcium carbonate, or limestone.
The Primary Building Material
The most significant contributor is the coral polyp, a tiny invertebrate that secretes a hard, cup-shaped exoskeleton of calcium carbonate. Over millennia, the accumulation of these skeletons forms the massive, underlying structure of the coral reef.
This fundamental framework is reinforced by other organisms, including calcareous algae, which deposit a hard, cement-like layer of calcium carbonate, helping to bind the reef together. As waves and storms break apart the living reef structure, they create vast quantities of coral fragments, sand, and rubble. These sediments, along with the shells of mollusks and foraminifera, are continually deposited and compacted. This process of lithification, or natural cementation, transforms the loose debris into solid limestone rock, forming the low island’s foundation.
The Geological Process of Formation
The creation of a low island, or atoll, is a geological process that typically begins with a volcanic high island. The accepted model, first proposed by Charles Darwin, describes a three-stage sequence tied to the subsidence of a volcanic peak. The process starts when a volcanic island emerges from the seafloor, providing a shallow-water platform for coral polyps to colonize and grow, which forms a fringing reef directly against the shoreline.
As the underlying tectonic plate moves away from the volcanic hotspot, the extinct volcano begins a slow, continuous process of sinking, or subsidence, and erosion. The coral reef, however, is a living entity and continues to grow upward, maintaining its position in the shallow, sunlit waters it needs to survive. This upward growth, while the island sinks, causes the reef to separate from the shore by a developing body of water, transitioning it into a barrier reef.
The geological transformation is complete when the volcanic cone has entirely sunk beneath the ocean surface, sometimes thousands of feet below. All that remains visible is the continuous, ring-shaped coral reef, with a central body of water where the volcano once stood. The final atoll is thus a thick cap of limestone resting on the peak of a submerged, ancient volcano.
Anatomy of an Atoll
The resulting low island structure is characterized by three distinct physical features. The most prominent feature is the central lagoon, a shallow, protected basin of water that sits above the submerged volcano. These lagoons are often between 100 to 300 feet deep and connect to the open ocean through channels in the reef rim, allowing for water exchange.
Encircling the lagoon is the reef flat, which is the broad, shallow platform of living and dead coral that is often exposed during low tide. Perched atop this reef flat are the motus, which are the small, habitable islets that constitute the actual landmass of the low island.
Motus are not part of the living reef itself but are formed when storms and wave action deposit broken coral, shells, and sand onto the reef flat, accumulating enough material to rise permanently above sea level. These small islets are typically low in elevation, often less than 15 feet above the high-tide line, with a geology consisting of unconsolidated, biologically derived carbonate sand and gravel. The motus are constantly being reshaped by the constructive and destructive forces of ocean waves and currents.