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the Mid-Atlantic Ridge (Original)

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Carolyn Silver
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« Reply #75 on: July 30, 2008, 09:59:11 pm »

Carolyn Silver

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   posted 02-14-2006 10:05 PM                       
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Important naval battles were fought in the waters of the Azores in that period during the attacks by corsairs and pirates. The following centuries were calmer, but in 1829 the Azores returned to the pages of history with the role played by Terceira in the struggle against the absolutist forces, and as the base for the liberal forces that invaded the mainland Portugal.



The archipelago developed during the 19th and 20th centuries with the introductions of new crops, the developing of industries and progress made with stock-breeding and fisheries. The last few years have witnessed a slow but progressive improvement in the economic and social well-being of the population. Most of this improvement is due to the inflow of capital donations from emigrants (almost all from North America) and the annual leasing of the Lajes Air Field to the United States.





The nine islands of the Azores and the European Union's remotest outpost are spread over some 600km of ocean (the Economic Free Zone is about 940km2) and are located roughly 1,500km or two hours' flying time from Lisbon and about 3,900km or five hours from the east coast of North America . Running along a southeast to northwest axis they lie on either side of the line of latitude that links Lisbon with New York and are between latitudes 36°-39° north and longitudes 250-310 west. The total population is approximately 243,000.

The islands separate conveniently into three groups: the Eastern Group of São Miguel and Santa Maria ; the Central Group of Terceira , Graciosa, São Jorge, Pico and Faial ; the Western Group of Flores and Corvo. The closest islands are Pico and Faial at just 6km apart.

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ORIGIN AND INVASIONS

Geological origin
The nine islands are formed from the upper sections of old volcanoes. In mid- Atlantic on the ocean bed tectonic plates are pulling apart; the gap between them is filled by molten volcanic material that rises from the earth's mantle and continuously forms new oceanic crust. This extrusion wells up and forms a ridge, and the sea floor spreads. The ridge is frequently offset by cross- cutting fractures caused by spreading on a curved surface. These mid-ocean ridges occur beneath all our major oceans, and sometimes rise above sea level and form islands such as the Galapagos and Easter Island .
The Azores lie on the Mid-Atlantic Ridge, a fault line that runs for some 16,000km from beneath the northern icecap southwards, and turns east around the southern tip of Africa to meet with the Indian Ocean Ridge. Along its length lie also Iceland , the largest land made from oceanic crust, Ascension and Tristan da Cunha islands.
Beneath the Azores , three plates meet or rather diverge in a T-shaped triple junction. The North American, African and Eurasian plates meet at a point between the western and central group of islands between Flores and Faial . Flores and Corvo are on the American Plate. Graciosa,São Jorge, Terceira and São Miguel lie on the Eurasian Plate and FaiaI, Pico, Santa Maria and the Formigas Islets tend to move towards joining the African Plate. Thus the two western islands are distancing themselves from the rest of the archipelago by 2.5cm a year.
Southwest of Flores is the site of one of the deep water 'smoking' hydrothermal vents, 'Lucky Strike'.
The seismic tremors felt in the islands are mostly caused by magma flowing up the cracks left in the earth's crust as the plates separate. At times these tremors will occur with surprising frequency. Seismic activity in the Azores is measured on the McCawley scale which is twice the rating of the Richter scale used in North America . For example, six on a McCawley scale is rated as three on the Richter scale. Apart from Santa Maria , Flores and Corvo, the six remaining islands have all been subject to eruptions and earthquakes within historical times.
Santa Maria was the first island to rise above the sea some seven million years ago but changes in sea level and tectonic activity caused it to submerge again. Four million years ago the Formigas Islets and São Miguel rose above the sea and Santa Maria reappeared. It was during its long period of submergence that the island acquired its marine fossils. Sao Miguel is made from five volcanoes: the oldest is Pico da Vara, followed two million years ago by the Povoação caldera. Both are extinct, while the three other volcanoes are considered active but dormant. Água de Pau comprises an inner and outer caldera, the latter formed between 26,000 and 46,000 years ago. The inner encloses Lagoa do Fogo, and the caldera margin is about 15,000 years old. Sete Cidades dates from around 22,000 years ago, and the 500m-tall walls of the Furnas caldera date back 12,000 years. The remaining islands probably originated within the past two million years: Terceira two million years ago, Graciosa, São Jorge and Faial no more than one million years, and Pico just 300,000 years.
In 1811, about a mile off the coast of São Miguel opposite Ferreira, a new island appeared. One hundred metres tall and about 1.5krn long, it took about a month to create. The British frigate Sabrina was in the area and her captain, Captain Tillard, landed on the still steaming island, planted a Union Jack, named the island 'Sabrina' and claimed it as British territory! Unfortunately for his credibility, when the next surveyors arrived, there was no trace of the island; in just four months the sea had washed it away and all that remains now is a bank 40m below the sea. Most recently, in 1957, an eruption began just off the west coast of Faial that added a further 2km2 of land to that island. In this region much more frequent activity occurs under the ocean's surface on the seabed than is seen on the surface of the islands. Constantly, seamounts rise and fall, and new lava pressure ridges are formed. Currently there is a submarine eruption 10km west of Terceira on a small area known as the 'Serreta High'. It began in 1998, initially in three different areas, later spreading to six. Surface signs are plumes of smoke coming from floating lava debris as it cools. The basalt magma is rich in gases that are trapped inside, creating 'balloons' which float upwards towards the ocean's surface. As they rise the gases inside expand and cause the 'balloons' to explode. The debris floats on the surface for about 15 minutes and sinks when seawater enters as it cools. When great volumes of gas rise to the surface the sea becomes pale green, and it is very dangerous for boats, since they could easily sink. This surface evidence is not continuous and its absence reflects quieter periods of submarine activity. The earliest eruptions detected were at around 400m below sea-level, and the magma subsequently has risen to 180m.

http://www.azores.com/history/Background.asp
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