The Galápagos Islands, an archipelago located about 600 miles west of Ecuador in the Pacific Ocean, are renowned for their unique biodiversity. This collection of islands famously inspired Charles Darwin’s theory of evolution and serves as one of the world’s finest natural laboratories. The islands are a classic example of a volcanic oceanic archipelago formed by a deep-seated mantle hotspot. This relentless volcanic activity, which began millions of years ago, continues to shape the westernmost islands today.
Formation via a Stationary Hotspot
The creation of the Galápagos Islands is driven by a geological mechanism involving the stationary Galápagos Hotspot and the movement of the Nazca Tectonic Plate. The hotspot is a relatively fixed mantle plume—an area of unusually hot material rising toward the crust. The islands sit on the Nazca Plate, which slowly drifts in an east-southeasterly direction. This plate movement over the stationary hotspot is the fundamental engine of the archipelago’s formation.
As the Nazca Plate moves, the fixed hotspot provides a continuous source of magma that forces its way through the crust. This molten rock initiates the formation of a seafloor volcano, which eventually emerges above sea level to become an island. The process is analogous to a conveyor belt, carrying the newly formed island away from its magmatic source. This ensures the hotspot continually creates new volcanoes on the crust passing over it, resulting in a chain of islands of varying ages.
The Nazca Plate moves slowly, estimated at about 2.5 inches (6.4 centimeters) per year. This consistent drift is responsible for the distinct age progression observed across the archipelago. The hotspot is currently located beneath the western islands, such as Isabela and Fernandina, which are the most volcanically active. The older islands to the east were created first and carried away from the plume by the plate’s motion.
Defining Characteristics of Oceanic Shield Volcanoes
The volcanic structures emerging from this hotspot mechanism are known as oceanic shield volcanoes. This classification is due to the composition and viscosity of the lava that forms them. The Galápagos volcanoes are primarily constructed from basaltic lava, a mantle-derived molten rock characterized by low silica content. This low-silica composition results in lava that is highly fluid and low in viscosity.
When this fluid lava erupts, it flows easily and spreads out over vast distances before solidifying. This creates a volcano with a broad base and a gently sloping profile, resembling a shield. Eruptions are typically effusive and non-explosive, involving gentle outpourings of lava rather than the violent eruptions associated with stratovolcanoes. Common features on the western shield volcanoes include large, nearly circular depressions at their summits called calderas. These calderas form when the underlying magma chamber is partially emptied, causing the overlying rock to collapse inward.
The sheer size of these calderas is a notable characteristic, often large in proportion to the overall volcano. For example, the Sierra Negra volcano on Isabela Island has a caldera measuring 10 kilometers in diameter. Isabela, the largest island, is a coalescence of six distinct shield volcanoes that have merged above sea level. The accumulated layers of basaltic rock have built the islands up from the seafloor, a process that continues on the western edge of the chain.
The Age Gradient and Island Evolution
The movement of the Nazca Plate over the stationary hotspot results in a distinct age gradient across the archipelago, influencing the islands’ landscapes and geological activity. Islands become progressively older and more eroded moving from the active western region toward the quiescent eastern islands. The youngest islands, such as Fernandina, are located closest to the hotspot and are still undergoing active formation. Fernandina is estimated to be less than 700,000 years old and features a rugged, barren landscape dominated by recent lava flows from La Cumbre, one of the world’s most active volcanoes.
In contrast, the eastern islands, carried away from the magma source, are significantly older and geologically inactive. San Cristóbal and Española islands are among the oldest, estimated between 2.4 and 4 million years. Since volcanism ceased, erosion has broken down the basaltic rock, leading to gentler slopes and mature soils that support greater biodiversity. As these older islands move away from the hotspot’s thermal support, their underlying oceanic crust cools and contracts, causing the landmass to slowly subside beneath the ocean surface.
This subsidence means the oldest islands, once much larger, are gradually shrinking and will eventually become submerged seamounts, demonstrating the life cycle of this oceanic island type. The age difference between the youngest and oldest emergent islands is approximately four million years. This provides a clear chronosequence for studying how geological processes lead to ecological evolution. The western islands are characterized by frequent eruptions, while the eastern islands are lower, heavily eroded, and lack active volcanic features.