Formation of the


Plate Tectonics and the Formation of the Galapagos Islands

But Darwin only had part of the answer. A more complete answer to the origin of the Galápagos could not be had until after 1958, when continental drift, or plate tectonics, was discovered. We now understand that the surface of the earth is divided into massive tectonic plates which slowly drift across the globe. The formation of the Galápagos is intimately tied to the history of the Nazca plate, on which they lie.

The Galápagos are located on the very northern edge of the Nazca plate, which is bounded by the Cocos (north), the Pacific (west), the South American (east), and the Antarctic (south) plates (see map). The Nazca plate itself is currently drifting south, away from the Cocos plate, and east, away from the Pacific plate. Since the net direction of drift is southeast, the Nazca plate is colliding with the South American plate. At the point of collision, the South American plate, which is made of light continental crust, is riding up over the Nazca plate, which is made of dense oceanic crust. This type of plate interaction is called subduction.

As the Nazca plate is forced into the mantle, it melts and its melt products work their way up to the surface to form volcanoes. The land is further raised by the crumpling effect as the western edge of the continent rides up over the descending plate. The result of all of this is the Andes, a young, highly volcanic, rapidly growing mountain chain. This same movement of the Nazca plate is responsible for producing the cluster of volcanic islands we call Galápagos.

There is a large body of geophysical evidence for the existence of enormous plumes of hot mantle material that originate near the earth's core and rise all the way to the crust. These plumes seem to be stable over many millions of years. and with time, they burn through the crust to form an underwater volcano which may eventually grow big enough to become an island.. But, because the crustal plate is in constant motion, the island will eventually move off of the hot spot. thereby making room for a second volcanic island. And a third, and a fourth.... Thus are archipelagos like the Galápagos formed.

Islands farthest from the hot spot are older and more eroded while islands near or on the hot spot are younger and steeper. Thus Isla San Cristóbal, the nearest to the mainland, is approximately four million years old and composed of eroded, rounded cones, while Isla Fernandina dates at less than 7000 years and is considered to be one of the most active volcanoes in the world. Recently former Galápagos islands, now submerged, have been discovered between Isla San Cristóbal and the mainland. This discovery may double the age of the islands. Indeed, several million years from now the present islands may likewise sink beneath the waves only to be replaced by a new set of Galápagos Islands. Who can imagine what course further evolution will take!?

Next: Galapagos Volcanoes

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