Darwin had part of the answer on the Galapagos islands.
The answer to the origin of the GalÃ¡pagosÂ was only available after 1958, when continental drift, or plate tectonics, was discovered.Â We now understand that the surface of the earth is divided into massive tectonic plates which slowly drift across the globe.Â The formation of the GalÃ¡pagos is intimately tied to the history of the Nazca plate, on which they lie.
The GalÃ¡pagos are located on the very northern edge of the Nazca plate, which is bounded by the Cocos (north) and the Pacific (west) plates (see map). The Nazca plate itself is currently drifting southeast, away from the Cocos plate and from the Pacific plate.Â This movement of the Nazca plate relative to the Cocos plate is responsible for producing the cluster of volcanic islands we call GalÃ¡pagos.
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 GalÃ¡pagos formed.
In regions of extensive and repeated fissure eruptions, ridges are formed. Often these underwater ridges have substantial height (as much as 2,000 to 3,000 meters) and are considered to include the longest mountain chains in the world. As new oceanic crust forms at the ridges, older crust is progressively moved farther and farther from the ridge, creeping along at a rate of a few centimeters per year. This process is referred to as seafloor spreading. For this reason, we often refer to divergent boundaries as spreading boundaries. As the new oceanic crustal rock moves away from the heated ridge, it cools and contracts, decreasing the ridge height (i.e., increasing the water depth) of the ridge flanks.
Recently, the use of undersea submersibles has provided a window to view the mid-ocean ridges. Scientists have actually observed new ocean floor being produced as red-hot lava extrudes from active fissures, instantly œfreezing, or cooling, in the 2Â°C bottom water. Associated with the ridges are hydrothermal vents, where super-heated water, gases, and minerals escape from deep within the Earth.
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 CristÃ³bal, 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 GalÃ¡pagos islands, now submerged, have been discovered between Isla San CristÃ³bal 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 GalÃ¡pagos Islands. Who can imagine what course further evolution will take!?
Although the Galapagos area is mainly a Hot Spot where the Nazca plate and the Cocos plate are pushed away from each other (divergent boundaries) , which allows molten lava to reach the earth's surface, a small portion of the fault line is a transform fault where the 2 plates are striking along each other with regular earthquakes as a result (see graphic)
Mid-ocean islands like the Galapagos are formed from basalt, the most basic of all types of lava.
Basalt has a very different chemical composition from the lavas that erupt from continental volcanoes, and is much more fluid. Consequently, as the lava flows build up to produce a volcanic cone, the island cones have a much shallower slope than those on the mainland.
These shallow-sloped volcanoes are called shield volcanoes and in the Galapagos, they are often compared to over-turned soup bowls.
Such shield volcanoes can clearly be seen in the younger western islands of Isabela and Fernandina. To the east, the volcanoes are lower and more eroded.
(some text Courtesy - Dr. Robert Rothman, Professor Biological Sciences)
(Pictures and graphics : Dr. Robert Rothman, Armand Vervaeck and NOAA oceanexplorer)