An island is simply a landmass, smaller than a continent, that is completely surrounded by water. While this definition is straightforward, the geology of islands is far more complex, as their origins determine their composition and fate. The question of whether islands are underwater mountains has a nuanced answer: many are the exposed peaks of massive submarine volcanoes, but a significant number are not. Islands are broadly categorized by their formation process, which can involve deep-ocean volcanism or the fragmentation of continental crust.
Defining Seamounts and the Direct Answer
A seamount is the geological term for an underwater mountain that rises from the seafloor but does not break the ocean surface. These structures are typically extinct or dormant volcanoes built up by successive lava flows over millions of years. Oceanographers define a seamount as a feature that rises at least 1,000 meters (3,281 feet) above the surrounding seabed.
When a volcanic structure continues to grow and pierces the water’s surface, it becomes an oceanic island. The visible island is only the tip of an enormous underwater peak, as the vast majority of the mountain remains submerged. For example, the Big Island of Hawaii is the exposed portion of Mauna Kea, which rises over 10,000 meters from its base on the ocean floor.
Islands Born from Volcanic Eruptions
The formation of true oceanic islands is driven by volcanic activity far from continental margins, creating landmasses built entirely of igneous rock. These islands form through two primary geological mechanisms: hotspot volcanism and plate boundary volcanism.
Hotspot Volcanism
Hotspot volcanism occurs when a stationary plume of hot magma, called a mantle plume, rises through a tectonic plate. As the oceanic plate moves slowly over this fixed hotspot, a chain of volcanoes is created. The Hawaiian Islands exemplify this process, where the Pacific Plate moves over the Hawaiian Hotspot, resulting in older, extinct volcanoes further down the chain.
Plate Boundary Volcanism
The other mechanism is volcanism at convergent plate boundaries, where one plate is forced beneath another in a process called subduction. This action causes melting rock to rise to the surface, creating an island arc. Examples of these curved chains of volcanoes include the Aleutian Islands and the islands of Japan.
Islands That Are Fragments of Continents
Not all islands are volcanic peaks; a large number are continental islands, which are geologically distinct from oceanic islands. These islands sit on the continental shelf and are extensions of the nearest landmasses. Continental islands share the same rock composition and structure as the adjacent mainland.
Their formation is attributed to two processes: rising sea levels or tectonic rifting. Many continental islands were once connected to the mainland by land bridges that were submerged after the last ice age. Larger continental islands, such as Greenland and Madagascar, separated from their continents through tectonic movements that broke off a fragment of the crust.
The Evolution of Oceanic Islands and Atoll Formation
Once the volcanic activity that formed an oceanic island ceases, the mountain begins a process of degradation and transformation. The exposed landmass is constantly eroded by wind, rain, and waves, which gradually reduce its size and elevation. Simultaneously, the volcano causes the underlying oceanic crust to sink, a process known as subsidence.
As the island subsides, coral reefs growing around its perimeter continue to build upward, maintaining their position near the ocean surface. This progression moves from a fringing reef to a barrier reef separated from the land by a lagoon. The final stage is the formation of an atoll, a ring-shaped coral island that encircles a central lagoon after the original volcanic peak has subsided entirely beneath the waves.