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GEOLOGY
The
Galapagos Archipelago comprises 13 large islands, 6 small islands, 42
islets and a number of small rocks and pinnacles, which make up a
total land surface of 8,000km2.
The
Galapagos Islands, located on the equator about 1000km (600m) west of
Ecuador, were never part of mainland South America. They are a group
of submarine volcanoes that grew progressively from the ocean floor,
until they finally emerged above sea level about 4.5 million years ago
and formed a group of islands. The islands have been added to and new
islands have been forming ever since. Each island is formed from a
single volcano, with the exception of Isabela, which comprises 6
volcanoes strung together.
The
Galapagos Islands are not formed at the junction of two or more
tectonic plates, as are many of the world’s volcanoes. They occur
within the Nazca Plate, and are interpreted to be the result of a
‘hot spot’. A "hot spot" is region of high thermic flux
due to the presence of a magmatic plume ascending from the earths’
mantle. The rising magma pierces the oceanic crust in a weak part of
the plate (e.g. where the plate is fractures) and magma is extruded
onto the sea floor. Another classic hot spot is responsible for the
formation of the Hawaiian Islands.
The
Galapagos Archipelago is
a chain of islands. This is not the result of movement of the hot
spot, rather, the hot spot remains stationary and the Nazca plate
drifts over it to the southeast (at a rate of about 3 inches, or about
6.5cm, per year), taking the older islands with it, while new islands
form the to the North west. Thus the oldest island is Isla Espanola in
the South west, while Fernandina and Isabela in the northwest are the
youngest and most volcanically active.
Like
the Hawaiian Islands, the Galapagos are basaltic in composition.
Basalt has a relatively low viscosity and typically forms volcanoes
with gently sloping flanks (<10 degrees), known as shield
volcanoes. In plan, shield volcanoes are
roughly circular or elliptical in shape. They are built up by
frequent eruptions of fluidal basaltic lava issuing from a central
vent or the flanks. Two main types of subaerial basaltic lava have
been distinguished; Pahoehoe (which is the Hawaiian word
meaning ropey) which is characterised by smooth, billowy, ropey and
toe surfaces; and Aa (the Hawaiian name for hurt) which has a
spinose and fragmented surface.
The
submarine, or seamount stage of growth is represented by basaltic
pillow lavas, hyaloclastites (quenched fragmented lava), and, as the
seamount approaches the surface, by coherent submarine lavas. Above
sea level, shield volcanoes are composed of lava flows, with limited
scoria fall and spatter deposits.
The
Galapagos shields have gentle lower slopes that rise to steeper
central slopes (34 degrees) and ultimately flatten off to form
spectacular summit calderas between 3 and 9km in diameter, the largest
being on Sierra Negra. Calderas are large, broadly circular volcanic
depressions that are usually formed by the collapse of the roof of a
subsurface magma chamber. Collapse often occurs during or after the
evacuation of the magma chamber by an eruption. An event of this type
occurred for example, on Volcan Fernandina in 1968, when the caldera
floor subsided by 300m.
The
dome-like shape of the Galapagos shields has been likened to an
overturned soup plate, in comparison to the gently sloping overturned
saucer-shape of the Hawaiian shields. Scientists have suggested that
the presence of intrusive
rocks (e.g. basalt dykes and sills injected into the lava pile)at a
high level may account for their characteristic shape.
The
Galapagos Islands are among the world’s most active volcanic areas
today. There have been over 50 eruptions in the last 200 years, and
many are recent for example; Fernandina has erupted on a regular
basis, every 4-5 years since 1968, with the last eruption being in
1995 when lava flowed into the sea, also Volcan Cerro Azul on Isabella
has erupted regularly over periods of approximately tens years since
the 1950’s (intervals were closer together before that), with the
last eruption being only last year, when lava flowed down the south
flank of the volcano.
MARINE CURRENTS
The
Galapagos Islands are not only a marine biogeographic province on
their own (due to the high proportion of unique marine species), but
are also an area subdivided into different ecoregions. Warm an cool
oceanic currents meet in the islands and produce this unique setting,
where fur seals and penguins meet tropicbirds and tropical flamingos.
The Galapagos Marine Reserve is one of the largest marine protected
areas in the world and is becoming a top destination for international
diving tourism. Of course, climate and weather throughout the
Galapagos depend directly upon oceanic conditions.
Despite their tropical location, two moving currents affect the
islands. The cold Humboldt Current (or Peru Current)
produces the cool and
dry garúa season from June to November and the warm marine Panama
Current which produces the warm and wet season from December
to May. During the garúa season, cooler waters from the Humboldt
Current are driven to the Galapagos by the southeast trade winds, with
an average sea temperature of 71°F. As a result, there is warm
tropical air passing over cool water. The moisture evaporating from
the sea is concentrated in an inversion layer (300 to 600 m above sea
level) and the higher parts of the islands, which intercept this
layer, receive precipitation in the form of garúa (mist rain). While
lowland areas remain dry though cool. During the warm season the
southeast trade winds diminish in strength and warmer waters from the
Panama Basin flow through the islands. The average sea temperature
rises to 77°F. Warmer waters cause the cool season inversion layer to
break up, and Galapagos experience a more typical tropical climate
with blue skies and occasionally heavy showers. In some years, the
flow of warm water is much greater than normal, and an "El Niño"
year results. Surface water temperatures are higher and rainfall can
increase greatly. Life on land blossoms but seabirds and sea life,
which depend on the more productive, cooler waters, may experience
dramatic breeding failures. |
