On our Galapagos cruise we sail between the spectacular young volcanoes of Isabela and Fernandina Islands. To understand their existence, one has to understand plate tectonics. The Galapagos Archipelago lies on an oceanic platform formed by the presence of a volcanic hotspot under the Nazca tectonic plate. This tectonic plate is the fastest-moving plate in the world, moving at an estimated 15 cm a year. Its western margin is formed by a north-south spreading ridge which brings deep mantle material to the surface which then flows away from the ridge. To the east of the ridge, therefore, the sea floor is slowly moving east towards the South American continent to the north, and about 100 miles to the north of the Galapagos Archipelago is the Galapagos Spreading Ridge, a much-studied area of ocean floor in the last decades, complicated by transverse faulting which has affected the distribution of the northernmost islands in the Galapagos group. The material emerging from this spreading ridge flows in a southerly direction, however not at an equivalent rate as from the west. The result of material coming from the north and west, is a plate movement of an east-southeasterly orientation. The eastern margin of the Nazca plate is being subducted under the continental plate of South America, which starting 30+ million years ago began the formation of the Andes mountain range.
The volcanoes of the Galapagos Archipelago are generally believed to have been formed by what is termed a “hotspot,” where mantle material rises from depth in one or more plumes of varying diameters until the material breaks through the crust of the moving plate, forming active volcanoes. As the volcanoes ride on the surface of the tectonic plate, over tens of thousands of years they are moved away from the source, which stays in one location for millions of years. As volcanoes are distanced from the “hotspot,” their activity levels start to diminish, material brought to the surface declines, the weight of the deposited material causes subsidence and erosion plays a role in diminishing their size and height. In Galapagos it is obvious that the largest islands and volcanoes are found in the west and the smaller in the southeast. This pattern is often a good key to the direction of plate movement, which in Galapagos is distinctly moving in an east-southeasterly direction at about 15 cm per year.
It is thought that some of the northern islands of the Galapagos Archipelago were formed at a time when the Galapagos Spreading Ridge lay over the hotspot several million years ago. This complicates the understanding of their formation as the Galapagos Spreading Ridge contains many transverse faults which could have influenced the depth from which the material was brought to the surface, and the direction of island/volcano movement. Therefore the age of the northern islands seem anomalous relative to the main grouping of islands.
The volcanoes of Galapagos are called “shield volcanoes,” or “over-turned soup-plates.” Basaltic magma is very fluid when compared to continental/Andesitic material, and is responsible for the gentle lower slopes of our volcanoes. The presence of a high number of circumferential fissures around the rims, something not common in other volcanic oceanic islands, has built up steep slopes around the calderas.
The tallest of all the Galapagos volcanoes is Wolf Volcano standing at approximately 600 feet above sea-level. As Wolf Volcano sits on the western edge of the Galapagos platform which descends another 5,000 feet below sea level, one could say it is among the tallest of all Pacific Ocean volcanoes. Isabela Island and Fernandina Island in the western region of the archipelago have the youngest volcanoes and are currently sitting over the “hotspot.” These islands are estimated to have volcanoes possibly less than half a million years old. Eruptions occur on an average of every three years, but are gentle emissions of fluid material, similar to what can be seen on the islands of Hawaii.
To the east lie islands that moved off the hotspot millions of years ago and have subsided and been exposed to the forces of erosion. The furthest southeast in the archipelago, Española Island, is around 4 to 5 million years old and just a remnant of the original volcano. Researchers studying the ocean floor further south and east of the archipelago have found material indicating the presence of past islands, now eroded well beneath the ocean’s surface. This means the archipelago could possibly have formed as far back as 10 million years ago, if not more, a significance not lost on those studying the ancestry of present Galapagos species.
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