Fernandina Island, located at the western edge of the Galápagos Archipelago, is a land mass born of intense volcanic activity. It is home to La Cumbre, one of the world’s most active volcanoes, which constantly reshapes the island’s landscape. Its frequent eruptions and unique geological structure make it a focal point for understanding the processes that build oceanic islands. The island’s dynamic environment has allowed its ecosystems to remain largely pristine, providing a natural laboratory.
Defining the Shield Volcano
Fernandina is classified as a shield volcano, named for its low, broad profile that resembles a warrior’s shield lying on the ground. This characteristic shape results from the properties of the magma it erupts. The volcano produces highly fluid, low-viscosity basaltic lava, which flows easily and spreads out over vast distances before solidifying.
Effusive eruptions slowly build up the volcano’s massive, dome-like structure. The slopes are typically gentle, often only a few degrees near the base, gradually steepening to about ten degrees at higher elevations. This contrasts sharply with the steep, conical form of stratovolcanoes, which erupt thicker, more viscous lava. The constant layering of thin, wide sheets of basaltic lava has given Fernandina its characteristic expansive form.
The Galapagos Hotspot and Fernandina’s Formation
Fernandina’s existence is directly linked to the underlying Galapagos hotspot, a stationary plume of heat rising from deep within the Earth’s mantle. This plume provides a sustained source of high-volume, low-silica magma that feeds the volcano.
The Nazca tectonic plate, on which the island rests, is slowly moving east-southeast over the hotspot. Fernandina is the youngest and westernmost island in the chain, positioning it almost directly above the hotspot’s center. This location explains its high level of activity compared to the older, more easterly islands that have drifted away from the primary magma source.
The low-silica chemistry of the magma inherently produces the runny basalt required to form a shield volcano. This process of plate movement over a stationary plume is responsible for the entire linear chain of islands in the archipelago.
Activity and Unique Features of Fernandina
Fernandina is the most active volcano in the entire Galápagos chain, with nearly thirty recorded eruptions since the early 1800s. Its activity is characterized by flank eruptions where lava emerges from fissures on the outer slopes. These lava flows are typically effusive, producing both smooth pāhoehoe and rough, blocky ʻaʻā lava. The flows often extend to the ocean, creating new land in the form of lava deltas.
A distinctive feature of Fernandina is its large, steep-walled summit caldera, measuring approximately 6.5 kilometers across. This central depression often experiences collapse events, which occur when the volume of magma beneath the volcano withdraws.
The most significant collapse occurred in 1968, when a section of the caldera floor dropped by as much as 350 meters. While most eruptions are effusive, the 1968 event was preceded by an unusual, more explosive phase known as a hydromagmatic eruption.
This explosive phase was caused by the interaction of rising basaltic magma with water that had collected on the caldera floor. The island remains the most pristine in the archipelago, having never suffered from the introduction of invasive species. This allows scientists to study volcanic processes and ecological recovery in a virtually untouched environment.