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Ants In The Sea! EarthCache

Hidden : 03/07/2010
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Geocache Description:

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Formigas Islet!


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.


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.


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.


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.


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.

Additional Hints (No hints available.)