The island of Santiago is located near the center of the Galapagos
Islands, off
the coast of Ecuador. The Galapagos Islands are situated near the equator, and
were formed from volcanism related to a large mantle plume (also known as a hot spot).
This hot spot is very close to the tectonic boundary between the Galapagos
Ridge—a plate boundary that is also an oceanic spreading center—and the Nazca
and Cocos plates. This combination of mantle plumes and tectonic plate
movements produces a unique geological environment, including underwater
ridges of volcanoes that
influence the water circulation around the Galapagos. All of these aspects
contribute to the geology and biology of the Galapagos.
Isla Santiago itself was formed from a shield
volcano (also
called Santiago). This type of volcanic structure is recognized by low, flat
summits surrounded by extensive flow fields of lava; the lava is not very
viscous, so it can flow for great distances from the source vents. Several dark
lava flow fields are visible in this astronaut photograph, the largest along
the eastern, western, and southern coastlines. The small Isla Rábida to the
south is the peak of another, mostly submerged shield volcano. (Note that the
image is rotated so that south is to the top left.)
In addition to the lava flows, other volcanic features known as
tuff cones are visible on the eastern and western sides of the island. These
cones are formed by the rapid interaction of hot flowing lava and water. The
water underneath the lava flow flashes to steam explosively, and this both
fragments the lava and rapidly cools it, leading to the formation of cones of
glassy, relatively fine-grained volcanic material. The most recent volcanic
activity on Isla Santiago occurred during 1904–1906.
The summit ridge of the Santiago shield volcano is located in
the northwestern part of the island. Also at image center is a large but
isolated region of green vegetation on the south-facing slope, below the summit
ridge. This image was taken during the dry, or garúa, season that lasts from
June to November. The season is dominated by cooler air transported by
southeast trade winds and cooler waters from the Humboldt and Cromwell
currents. The combination of cool air and water results in rain falling only in
the island highlands, with south- and east-facing slopes receiving the most
precipitation. Despite the favorable location, the yellow-green color of the
vegetation may indicate water (or other) stress.
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