Geology
Posted September 21st, 2008 by Bob the Sea LionThe oldest of the islands visible today were
formed roughly four to five million years
ago by underwater volcanoes erupting and
rising above the ocean’s surface (the islands
were never connected to the mainland).
The Galápagos region is volcanically very
active – more than 50 eruptions have been
recorded since their discovery in 1535. The
most recent eruption occurred in October,
2005 when Volcán Sierra Negra on Isabela
spewed ash and lava for several days. Thus,
the formation of the islands is an ongoing
process; the archipelago is relatively young
compared with the age of the earth (which
is about 1000 times older).
Geologists generally agree that two relatively
new geological theories explain the
islands’ formation. The theory of plate tectonics
holds that the earth’s crust consists
of several rigid plates that, over geological
time, move relative to one another over the
surface of the earth. The Galápagos lie on
the northern edge of the Nazca Plate, close
to its junction with the Cocos Plate. These
two plates are spreading apart at a rate of
about 1km every 14,000 years, pretty fast
by plate-tectonic standards.
The hotspot theory states that deep within
the earth (below the moving tectonic plates)
are certain superheated areas that remain
stationary. At frequent intervals (measured
in geological time), the heat from these
hotspots increases enough to melt the earth’s
crust and produce a volcanic eruption of sufficient
magnitude to cause molten lava to
rise above the ocean floor and, eventually,
above the ocean’s surface.
The Galápagos are moving slowly to the
southeast over a stationary hotspot, so it
makes sense that the southeastern islands
were formed first and the northwestern islands
formed most recently. The most ancient
rocks yet discovered on the islands are about
3.25 million years old and come from Isla Española
in the southeast. In comparison, the
oldest rocks on the islands of Isla Fernandina
and Isla Isabela are less than 750,000 years
old. The northwestern islands are still in the
process of formation and contain active volcanoes,
particularly Isabela and Fernandina.
In addition to the gradual southeastern drift
of the Nazca Plate, the northern drift of the
Cocos Plate complicates the matter, so that
the islands do not get uniformly older from
northwest to southeast.
Most of the Galápagos are surrounded
by very deep ocean. Less than 20km off
the coasts of the western islands, the ocean
is over 3000m deep. When visitors cruise
around the islands, they can see only about
the top third of the volcanoes – the rest is
underwater. Some of the oldest volcanoes
in the area are, in fact, completely underwater.
The Carnegie Ridge, a submerged
mountain range stretching to the east of
the Galápagos, has the remnants of previous
volcanic islands, some of which were as
much as nine million years old. These have
been completely eroded away; they now
lie 2000m beneath the ocean surface and
stretch about half the distance between the
Galápagos and the mainland.
Most of the volcanic rock forming the
Galápagos Islands is basalt. Molten basalt
has the property of being more fluid than
other types of volcanic rock, so when an
eruption occurs, basalt tends to erupt in
the form of lava flows rather than in the
form of explosions. Hence the Galápagos
Islands have gently rounded shield volcanoes
rather than the cone-shaped variety
most people associate with the formations.
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