Formigas
The Formigas Islet and
Dollabarat Reef, sometimes referred to as the Formigas Bank, are a
remote group of rock outcroppings in the eastern group of the
Azores archipelago, pertaining to Portugal. The bank is located 43
km northeast of Santa Maria Island and southeast of São Miguel
Island covering a surface area of approximately 9,000 m². The bank
is only disturbed by a lighthouse located on the largest mound.
Due to strong currents and frequent large swells the linear rock
outcroppings lack any terrestrial flora or fauna. Along their
perimeter, the seabed drops steeply to a depth of 50–70 m on
either side, and gently at the northern and southern tips. The area
of the Bank referred to as the Dollabarat reef, is an area of
shallower gradient situated 5 km along the southwest of the
Formigas Islets.
The name Formigas is Portuguese for ants, since these geological
features are small and dispersed rocks in the ocean, like
disorganized "ants". The largest islet, Formigão (or literally
"large/enormous ant"), reaches 11 m. Generally, the islets are
useful for navigation and visible up to 19 km away on clear days,
but during intemperate weather a ship can run aground without even
seeing the lighthouse.
Geology
The mid-atlantic ridge
The opening of the Atlantic
ocean was initiated about 180 million years ago in the Jurassic
period during the fragmentation of the super-continent known as
Pangaea, and the separation of the American plates from the African
and Eurasian plates. The early phase of this plate separation was
similar to the situation in the East African Rift Valley today. The
gradual enlargement of the ocean basin since Jurassic times can be
traced in the pattern of ages of ocean crust preserved in the ocean
basins. New crust is continually being created at the spreading
ridge, and gradually moves away from the ridge as spreading
continues as if on a conveyor belt. This creates a pattern of
symmetrical age bands on either side of the spreading ridge, as
shown in the diagram below. A similar pattern is seen in the
Pacific.
In the Atlantic ocean, the oldest ocean crust rocks are
preserved at the continental margins adjacent to the continental
shelves. However, in the Pacific, the oldest continental crust is
being destroyed at the subduction zones at the margins of the
ocean:
The rate of seafloor spreading and the formation of new oceanic
crust varies over time, giving rise to different widths of age
bands and magnetic stripes. At times, an excess of magma is
injected into the spreading line, and this overflows onto the
seafloor to build up a large submarine ridge with a series of
smaller, sub-parallel ridges. An example of this is the Mid
Atlantic Ridge. The ridge forms a very prominent feature near the
spreading line, where it develops on the crust domed up by the
rising convection currents in the underlying mantle. As the
newly-formed crust is carried away from the spreading line, the
doming is reduced as the underlying mantle is cooler. In places,
the magma outpouring and doming can be so extensive that volcanic
islands can be built up, such as Iceland which is currently located
directly on the Mid Atlantic Ridge. As these volcanic islands are
carried away from the spreading line by plate movement they
subside, become volcanically extinct and sometimes become submerged
to form guyots. Numerous examples of these extinct volcanic islands
and guyots can be seen in the Atlantic basin - e.g., the Azores,
Madeira, Cape Verde and the Canary Islands. These islands were
formed along the spreading line and were carried away by plate
motion so that the age of the islands increases with distance from
the spreading zone.
Pillow Lavas
The rocks making up the ocean
floor, mid ocean ridge and volcanic islands are mainly basaltic in
composition. The magma is produced by partial melting of the mantle
peridotite through mantle decompression below the spreading ridge.
The magma collects in a large chamber below the central spreading
line, and is gradually injected as dykes into the spreading zone to
form sheets of a rock known as dolerite (medium grained gabbro).
Above the layer of sheeted dolerite dykes, the lava cools very
rapidly on contact with seawater and a layer of fine grained pillow
lavas form.
Pillow lavas are lavas that contain characteristic pillow-shaped
structures that are attributed to the extrusion of the lava under
water, or subaqueous extrusion. Pillow lavas in volcanic rock are
characterized by thick sequences of discontinuous pillow-shaped
masses, commonly up to one metre in diameter. They form the upper
part of 'Layer 2' of normal oceanic crust.
Composition
Pillow lavas are commonly of basaltic composition, although
pillows formed of komatiite, picrite, boninite, basaltic andesite,
andesite or even dacite are known. In general the more intermediate
the composition, the larger the pillows, due to the increase in
viscosity of the erupting lava.
Ocurrence
They occur wherever mafic to intermediate lavas are extruded under
water, such as along marine hotspot volcano chains and the
constructive plate boundaries of mid-ocean ridges. As new oceanic
crust is formed, thick sequences of pillow lavas are erupted at the
spreading center fed by dykes from the underlying magma chamber.
Pillow lavas and the related sheeted dyke complexes form part of a
classic ophiolite sequence when a segment of oceanic crust is
obducted onto continental crust.
The presence of pillow lavas in the oldest preserved volcanic
sequences on earth, the Isua and Barberton greenstone belts,
confirms the presence of large bodies of water on the Earth's
surface early in the Archean. Pillow lavas are used generally to
confirm subaqueous volcanism in metamorphic belts.
Pillow lavas are also found associated with some subglacial
volcanoes at an early stage of an eruption which was descovered by
Gabriel A. Doudine and his companions.
Formation
They are created when magma reaches the surface but, as there is
a large difference in temperature between the lava and the water,
the surface of the emergent tongue cools very quickly, forming a
skin. The tongue continues to lengthen and inflate with more lava,
forming a lobe, until the pressure of the magma becomes sufficient
to rupture the skin and start the formation of a new eruption point
nearer the vent. This process produces a series of interconnecting
lobate shapes that are pillow-like in cross-section. The skin cools
a lot faster than the inside of the pillow, so it is very fine
grained, with a glassy texture. The magma inside the pillow cools
more slowly, so is slightly coarser grained than the skin, but
still classified as fine grained.
The Earthcache
To claim this Earthcache you must go near the published coordenates
and send to me the answers for the following questions after
visiting the published coordinates:
1. In the given coordenates what can you see?
2. Witch kind of composition is this pillow lava islet made
of?
3. How many islet can you see?
You may take a picture of your favorite "ant" (Islet), be creative!
After you receive my mail validating your answers you may put it in
your log.