VOLCANIC ISLANDS ON THE MOVE AND STILL IN THE MAKING
Though Simon & Garfunkel might have expressed their separation and independence from others by proclaiming, “I am a rock/I am an island,” this axiom doesn’t stand true for the Galápagos Islands.
True: the Galápagos islands are volcanic rock, and they are islands. But their current—and continued—existence is fully reliant on perpetually moving tectonic plates and repeated volcanic action at the earth’s mantle. The Galápagos Islands may have never been part part of the South American continent, but they are by no means untouched by the same geologic forces.
TECTONIC PLATES AND THE GALÁPAGOS CONVEYOR BELT
THE NAZCA PLATE
The Nazca plate’s eastward movement has shaped much of the world as we know it. As the Nazca plate shifts away from both the northern Cocos plate and eastern Pacific plate, it collides with the southeastern South American plate. The lighter South American plate, comprised of continental crust, rides up over the dense Nazca plate. In the process of subduction, the Nazca plate’s oceanic crust is forced into the mantle, where it melts and eventually rises up as volcanoes. This same process results in crumpled up crust and raised land.
The Nazca plate moves at a rate of 6-8 cm per year—a rather significant shift over the course of time. In effect, the Nazca plate’s tectonic shift played a significant role in the swiftly (in geologic terms) growing Andes mountain chain as well as the volcanic islands that make up the Galápagos archipelago.
VOLCANIC HOTSPOTS AND THE CONVEYOR BELT ISLANDS
Volcanic hotspots—exceptionally hot, stationary places within the earth’s mantle—occur throughout the world and produce island volcanoes within a plate’s interior rather than near plate boundaries. The Hawaiian Islands are an example of hotspot-produced island volcanoes, and the Galápagos Islands are another.
Hotspots exist below plats and produce thermal plumes—sustained, localized sources of heat—which then produce magma, which rises through cracks in the plate to form volcanoes. These underwater volcanic cones continue to grow until it reaches the surface of the ocean.
Of course, as the tectonic plate shifts over the stationary hotspot, the volcano begins to shift away from its sustaining hotspot. Then, the process begins again, and a new volcano begins to take form. Over time, a chain of volcanic islands (what is: archipelago) appears. Thus, the conveyor belt theory.
Islands formed by hotspots are basically like volcanic siblings, and in the case of the Galápagos, the oldest siblings can be found toward the east and the youngest found toward the west. The Galápagos hostpot lies at the convergence of the Carnegie Ridge and the Cocos ridge, currently to the east of Isla Fernandina—the youngest addition to the Galápagos Islands.
Some volcanic islands are completely submerged, such as the Carnegie Ridge. The submerged mountain range extends east of the archipelago toward mainland Ecuador. Carnegie Ridge is comprised of volcanic islands estimated to be nine million years old, now eroded, and lying 2,000 feet beneath the surface of the ocean.
The oldest above-water island in the archipelago, Española, has been so eroded by time and weather that no evidence of the island’s original volcano exists anymore. That being said, evidence supporting Española Island’s role in the hot spot theory has been discovered as some of the rocks on Isla Española are dated around 3.25 million years old, whereas Isabela and Fernandina Islands have rocks dating back to a mere 750,000 years ago.
GALÁPAGOS VOLCANOES AND RECENT ERUPTIONS
GALÁPAGOS SHIELD VOLCANOES
Most of the volcanoes found throughout the Galápagos are rounded shield volcanoes rather than the more iconic cone-shaped volcanoes. Younger, western islands such as Isabela and Fernandina retain the shallowly sloping shape of shield volcanoes (making them look like overturned soup bowls), though the older volcanoes to the east have eroded more over time and much diminished.
Many of the Galápagos volcanoes are capped with the large cavity of a caldera. The largest caldera throughout the islands can be found on Volcan Sierra Negra on Isabela, and the caldera on Fernandina collapsed in 1968—resulting in a 300 m drop in the caldera floor.
GALÁPAGOS PARASITIC CONES
Lava can erupt from fissures and vents throughout the main volcano’s structure as well as from the main crater. The outflow from these fissure eruptions mount and create small cones, known as parasitic cones. There are two types of parasitic cones: tuff cones and spatter cones.
Tuff cones are generally found on the coast, just off the shorelines. They are composed of cemented volcanic ash that explodes from vents when sea water contacts magma. Charles Darwin found Galápagos tuff cones the most interesting, and Darwin was actually the first person who came up with the explanation of how they are formed.
Tuff cones can be found around Sullivan Bay, James Bay, Buccaneer Cove, and Pinnacle Rock. The tuff cones at Pinnacle Rock and Buccaneer Cove have eroded over time, and a cross-section of their interior can now be seen.
Spatter cones occur when gobbets of lava (de-gassed magma) are flung into the air, where they are cooled and begin to solidify. They fall back down in “splats” and form a small cone. Isla Bartolomé has excellent examples of splatter cones.