The idea that islands might drift away suggests they are temporary objects floating on the ocean’s surface. In reality, an island is a piece of the Earth’s solid crust surrounded by water. The ocean water simply occupies the lowest elevations of the planetary surface. Therefore, the island is not floating on the water but is rooted firmly in the seafloor beneath it.
Islands Are Part of the Earth’s Crust
Islands are physically connected to the rigid outer layer of the Earth, known as the lithosphere, which includes the crust and the uppermost part of the mantle. This lithosphere is composed of solid rock, and the ocean water rests directly on top of it.
Islands are essentially the peaks of underwater mountains, hills, or plateaus that rise high enough to pierce the ocean surface. Since islands are made of rock, they are significantly denser than the water surrounding them. They are stationary because they are fundamentally anchored to the planet’s solid foundation.
Continental Islands: Anchored to the Shelf
Continental islands are those that sit on the continental shelf, which is the submerged, gently sloping extension of a continent. These islands are geologically similar to the nearby mainland, sharing the same type of rock and formation history. Examples include Great Britain and Madagascar.
Their stability comes from being firmly rooted in the continental shelf, which is itself a stable part of a tectonic plate. These islands became separated from the main continent either through continental rifting or, more commonly, when global sea levels rose. The rise in water simply submerged the lower-lying land that connected the island to the mainland, leaving the higher ground isolated.
Volcanic Islands: Built from the Seafloor
Volcanic islands, often found far from continental margins, are the exposed peaks of massive underwater volcanoes. These structures are built up over thousands to millions of years by successive eruptions of molten rock from the Earth’s mantle.
The formation process occurs in two primary ways. The first is along tectonic plate boundaries, such as subduction zones, where melting creates magma that rises to form volcanic island arcs. Another element is stationary hotspots, which are plumes of hot material rising from deep within the mantle, creating volcanoes as the tectonic plate moves over them. The Hawaiian Islands are a classic example of this, with the newest islands forming over the active plume.
The islands are anchored because their base is a huge volcanic mountain whose foundation extends deep into the seafloor. For instance, the Hawaiian volcanoes rise from the deep ocean floor and have roots extending at least 1,500 kilometers down into the mantle. This massive, deep-seated structure ensures they remain fixed to the underlying oceanic crust.
Coral Islands and Atolls
Coral islands and atolls represent a different formation, where the visible land is built primarily through biological processes. These islands are created by the accumulation of calcium carbonate skeletons secreted by tiny coral polyps. Even these biologically-formed landmasses require a solid, stationary foundation to begin their growth.
The process typically begins when coral larvae attach to the submerged, hard surfaces of a volcanic island or an underwater seamount. As the volcanic peak slowly begins to subside, the coral continues to grow upward toward the sunlight near the water’s surface, maintaining its position.
Over vast periods, the central volcanic island may completely disappear beneath the waves, leaving a ring of coral reef and small islands encircling a central lagoon. This final ring structure is known as an atoll. The coral structure is firmly cemented to the hard rock base beneath it, which remains part of the Earth’s crust, ensuring the island complex does not drift away.