Island Formation Through Underwater Volcanism

Many of the world's islands and island chains originated as underwater volcanoes that eventually grew large enough to emerge above sea level. This process of island formation through submarine volcanism demonstrates the enormous scale and long-term persistence of underwater volcanic activity, with some volcanic islands representing millions of years of continuous volcanic growth from the deep ocean floor.

The Process of Volcanic Island Building

Volcanic island formation typically begins with eruptions on the deep ocean floor, often at depths of 3,000-5,000 meters below sea level. At these depths, volcanic eruptions operate under extreme pressure conditions that favor the formation of pillow lavas and other volcanic structures adapted to the underwater environment.

The initial stages of island building involve the accumulation of volcanic rocks on the ocean floor, typically forming broad, low shield volcanoes similar to those seen in Hawaii. However, unlike terrestrial shield volcanoes, these underwater volcanic edifices must grow through several kilometers of water depth before their eruptions can interact with the atmosphere.

As underwater volcanoes grow upward through the water column, they eventually reach depths where water pressure decreases enough to allow more explosive eruptions. This transition typically occurs at depths of 500-1,000 meters below sea level, where reduced pressure allows volcanic gases to expand and create more explosive eruption styles.

The emergence of volcanic islands above sea level represents a critical transition in volcanic behavior, as eruptions can now interact with air rather than water and can produce the full range of terrestrial volcanic phenomena. However, the underwater foundation of these volcanic islands continues to influence their structure and eruption characteristics.

The final stages of island building often involve the construction of subaerial volcanic edifices that may bear little resemblance to their underwater foundations. However, the enormous volume of underwater volcanic rocks typically far exceeds the volume of subaerial volcanic deposits, with underwater portions of volcanic islands commonly representing 80-90% of the total volcanic structure.

Hawaiian Islands: A Classic Example

The Hawaiian Island chain represents one of the best-studied examples of island formation through underwater volcanism, providing detailed insights into how volcanic islands grow from the deep ocean floor over millions of years. The Hawaiian hotspot has been active for at least 85 million years, creating a chain of volcanic islands and seamounts that extends across the entire North Pacific.

Kilauea and Mauna Loa, the currently active Hawaiian volcanoes, sit atop massive underwater volcanic foundations that extend over 5,000 meters below sea level. The total height of these volcanic structures from their base on the ocean floor to their summit exceeds 9,000 meters, making them among the tallest mountains on Earth when measured from their true base.

The underwater structure of Hawaiian volcanoes is dominated by pillow lavas and other submarine volcanic rocks that formed during the early stages of volcanic growth. Drilling and submersible studies have revealed that these underwater foundations contain complex sequences of volcanic rocks that record millions of years of submarine volcanic activity.

The transition from submarine to subaerial volcanism in Hawaiian volcanoes is preserved in their geological structure, with distinctive rock units that formed during the emergence process. These transitional volcanic rocks show evidence of explosive interactions between magma and seawater as the volcanoes approached and broke through sea level.

Loihi Seamount, located southeast of the Big Island, represents the next Hawaiian volcano in the process of formation and provides a modern example of underwater volcanic island building. Loihi currently rises to within about 1,000 meters of sea level and shows active volcanic growth that may eventually create a new Hawaiian island.

Galápagos Islands: Hotspot Volcanism on a Spreading Ridge

The Galápagos Islands provide a unique example of volcanic island formation where hotspot volcanism interacts with a mid-ocean spreading ridge. This setting creates complex volcanic processes that differ from typical hotspot island chains like Hawaii and provides insights into how different tectonic environments affect underwater volcanism.

The underwater volcanic foundations of the Galápagos Islands are built on relatively young oceanic crust created at the Galápagos Spreading Center, creating a different thermal and structural environment compared to Hawaiian volcanoes built on old, thick oceanic lithosphere.

The interaction between hotspot magmatism and spreading ridge volcanism creates complex patterns of volcanic activity both underwater and on the volcanic islands themselves. This interaction can enhance volcanic productivity and create distinctive volcanic rock compositions that reflect the mixing of different magma types.

Fernandina and Isabela, the westernmost and most volcanically active Galápagos islands, show evidence of ongoing underwater volcanic activity around their shorelines. Submarine volcanic vents and recent lava flows have been documented in the shallow waters surrounding these islands, demonstrating the continuation of island-building processes.

The Galápagos platform, the underwater volcanic foundation that supports the island chain, extends far beyond the subaerial islands and includes numerous seamounts and underwater volcanic features. This platform represents a massive underwater volcanic construction that has been built through millions of years of combined hotspot and spreading ridge volcanism.

Caribbean and Lesser Antilles Arc Islands

The Lesser Antilles island arc provides examples of volcanic island formation in a subduction zone setting, where underwater volcanism is driven by the subduction of oceanic crust beneath the Caribbean Plate. This setting creates more explosive volcanic activity than hotspot systems and produces volcanic islands with different characteristics and hazards.

Many Lesser Antilles islands have complex underwater foundations that include both volcanic rocks and sedimentary deposits related to the subduction process. The underwater portions of these islands often show evidence of explosive volcanism, landslides, and other processes related to the more volatile nature of arc volcanism.

Kick-'em-Jenny, an active underwater volcano near Grenada, exemplifies ongoing underwater volcanism in the Lesser Antilles arc. This seamount has shown repeated cycles of explosive growth and collapse, with eruptions that have been documented since the 1930s and that continue to modify the underwater volcanic structure.

The hazards associated with underwater volcanism in arc settings include the potential for explosive eruptions that can affect shipping and aviation, landslides that can generate tsunamis, and the gradual emergence of new volcanic islands that can alter regional geography.

Soufrière Hills volcano on Montserrat demonstrates how subduction-related volcanism can create ongoing hazards for established volcanic islands, with eruptions that have destroyed much of the island's infrastructure and forced the evacuation of much of the population since the 1990s.