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 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.
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 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!?
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