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Cientistas descobrem falha junto à costa de Sagres


As obras do túnel do Marão

Cientistas descobriram fractura tectónica em formação ao largo da costa portuguesa

A descoberta de uma zona de subducção nas suas primeiríssimas fases de formação, ao largo da costa de Portugal, acaba de ser anunciada por um grupo internacional de cientistas liderados por João Duarte, geólogo português a trabalhar na Universidade de Monash, na Austrália. A confirmar-se que o fenómeno, em que uma placa tectónica da Terra mergulha debaixo de outra, está mesmo a começar a acontecer, como concluem estes cientistas num artigo publicado online pela revista Geology, isso significa que, daqui a uns 200 milhões de anos, o Oceano Atlântico poderá vir a desaparecer e as massas continentais de Europa e América a juntar-se num novo supercontinente.

Divisão da placa Indo-Australiana aumenta magnitude de sismos

   Há quase 50 milhões de anos, a placa Indo-Australiana começou a dividir-se em duas, ou mesmo em três, um processo muito lento que os sismólogos já conheciam. Associado a esta ruptura está o terramoto de 9.2 graus na escala de Richter, com epicentro em Banda Aceh, que ocorreu em Dezembro de 2004, e o posterior tsunami que provocou 228 mil vítimas no sudeste asiático, bem como os sismos que em Abril deste ano se sentiram em Sumatra (Indonésia), que alcançaram 8.7 e 8.2 graus na escala de Richter.

   A actividade sísmica entre a Índia e a Austrália era já significativa antes dos movimentos de Abril passado, mas aumentou consideravelmente desde o terramoto de Banda Aceh, explicam os investigadores. “A deformação da placa pode originar terramotos de magnitudes nunca antes registadas”, diz Delescluse, autor principal de um dos três estudos que analisam as causas e as consequências dos sismos.

   Não existe ainda uma fronteira clara que divida a placa, mas possivelmente surgirá entre o oeste de Sumatra e o sudeste da Índia. O primeiro terramoto, de 8.7 graus, foi desencadeado pela ocorrência de pelo menos quatro fissuras na placa, em apenas 160 segundos. O terramoto foi sentido desde a Índia até à Austrália, incluindo o sul e o sudeste asiático. Duas horas depois ocorreu um segundo sismo, de 8.2 graus na escala de Richter.

  “Nunca tínhamos visto um terramoto como este. Faz parte da cisão desordenada da placa. É um processo geológico que levará milhões de anos até que se forme uma nova fronteira e, provavelmente, será necessário haver milhares de terramotos de magnitudes semelhantes para que isso suceda”, diz Keith Koper, sismólogo e co-autor de um dos artigos.

   Ao contrário do sismo de Banda Aceh, os terramotos de Abril não deram lugar a tsunami apesar da sua elevada magnitude, já que foram provocados por movimentos horizontais de falhas oblíquas. Na região, nos dias seguintes a ambos os terramotos, o número de sismos com magnitudes superiores a 5.5 graus multiplicou-se cinco vezes, chegando a produzir-se até 1500 quilómetros do epicentro dos dois primeiros.

Artigos:

• April 2012 intra-oceanic seismicity off Sumatraboosted by the Banda-Aceh megathrust
• The 11 April 2012 east Indian Ocean earthquaketriggered large aftershocks worldwide
• En échelon and orthogonal fault ruptures of the 11April 2012 great intraplate earthquakes

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Adaptado de:

http://www.cienciahoje.pt/index.php?oid=55713&op=all

Supercontinente Amásia deverá formar-se no Pólo Norte

   

   Segundo um artigo publicado esta quinta-feira na revista Nature, os geólogos da Universidade de Yale estimamTerra terá um novo supercontinente dentro de 50 a 200 milhões de anos, a Amásia, que resultará da junção da América e da Ásia junto ao oceano Árctico. Os actuais continentes serão empurrados para uma massa de terra única, em redor do Pólo Norte.

   Os investigadores propõem um modelo dos movimentos lentos dos continentes nas próximas dezenas de milhar de anos. “Primeiro deverão fundir-se as Américas e depois irão migrar para Norte, colidindo com a Europa e a Ásia, mais ou menos onde hoje existe o Pólo Norte”, disse Ross N. Mitchell, geólogo da Universidade de Yale e principal autor do estudo, na revistaNature. “A Austrália deverá continuar a mover-se para Norte e fixar-se perto da Índia”e o oceano Árctico e o mar das Caraíbas desaparecerão, dentro de 50 a 200 milhões de anos.

   A última vez que a Terra assistiu a um supercontinente foi há 300 milhões de anos, quando todas as massas terrestres se fundiram no equador dando origem à Pangeia, situada onde hoje está a África ocidental.

   Depois de estudarem a geologia das cadeias montanhosas em todo o mundo, os geólogos têm assumido que o próximo supercontinente se irá formar no mesmo local da Pangeia. Mas Ross N. Mitchell e os seus colegas têm uma ideia diferente: a Amásia deverá formar-se no Árctico, a 90 graus do centro geográfico do supercontinente anterior, a Pangeia.

   Os geólogos chegaram a esta conclusão depois de analisar o magnetismo das rochas mais antigas para determinar as suas localizações no globo terrestre ao longo do tempo. Além disso, mediram como o manto move os continentes que “flutuam” à sua superfície.

   “A forma como os continentes se movem tem implicações para a biologia – por exemplo, pode afectar os padrões da dispersão das espécies – e para as dinâmicas no interior da Terra”, disse Taylor M. Kilian, um dos autores do estudo, da Universidade de Yale.

   “Compreender a disposição das massas dos continentes é fundamental para compreendermos a história da Terra”, disse Peter Cawood, geólogo na universidade britânica de St Andrews, na revista Nature. “As rochas são a nossa janela para a história.”

   O geólogo David Rothery da Universidade Aberta, em Milton Keynes, no Sul da Inglaterra, disse à BBC que não está preocupado com o choque de continentes. “Podemos compreender melhor o Ambiente da Terra no passado se soubermos exactamente onde estavam os continentes”, disse. “Não me interessa se os continentes vão convergir no Pólo Norte ou se a Inglaterra vai colidir com a América num futuro longínquo. Prever o futuro tem muito menos importância do que saber o que aconteceu no passado.”

(Estudo em PDF)

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Adaptado de:

http://ecosfera.publico.pt/noticia.aspx?id=1532955

Pulsar tectónico no estreito de Gibraltar afecta o clima

  

  A cada milhão de anos, a actividade tectónica na região do estreito de Gibraltar parece intensificar-se, distanciando a Península Ibérica da África e aumentando repentinamente o fluxo de água que vem do Mediterrâneo para o Atlântico. Essa mudança, marcada nos sedimentos do chão marinho, foi uma das principais descobertas feitas pela expedição científica do navio JOIDES Resolution.

   “Provavelmente estamos a mover-nos para uma nova fase de actividade tectónica, que pode abrir mais ou fechar o estreito”, disse Dorrik Stow, numa conferência de imprensa em Lisboa. O cientista inglês é um dos líderes da equipa de 35 cientistas de 14 países, que passou os últimos dois meses no navio. 

    A viagem, que terminou com o navio atracado em Alcântara, fez parte do Programa Integrado de Perfuração dos Fundos Oceânicos (IODP), uma organização internacional que inclui Portugal. O navio tem 143 metros de comprimento e uma torre de sondagens com 62 metros de altura acima do nível do mar. Desde 1985, JOIDES está ao serviço do programa, depois de terminar os seus dias como navio de prospecção petrolífera. 

   Os sete núcleos de perfuração feitos nos últimos dois meses, cinco a Sul de Faro e dois a Oeste da costa alentejana, serviram para estudar a formação do mar Mediterrâneo e os ciclos climáticos da Terra. Há 5,6 milhões de anos, o Mediterrâneo ficou isolado do Atlântico, o que fez aumentar a salinidade. Passados 300 mil anos, o estreito voltou a abrir e surgiu uma nova corrente quente, salgada e muito mais densa, vinda do mar, que ao atravessar o estreito de Gibraltar se afunda no oceano. 

“Temos uma grande fonte de água fria no mar da Noruega que arrefece, afunda-se e move-se para Sul. A água quente (vinda de Sul) corre à superfície”; “Mas é preciso um input de águas mais quentes e mais salgadas a latitudes mais altas, o Mediterrâneo providencia este equilibrio.”, disse Stow.

   O fluxo deixa um rasto de sedimentos pelo Atlântico, que se acumulou ao longo das Eras e que o JOIDES Resolution perfurou.  O navio recolheu um cilindro de 5,5 quilómetros de comprimento de solo marinho. As amostras acabaram divididas em cilindros de 1,5 metros. A equipa descobriu através das amostras que nos últimos 4 M.a. existiram três grandes episódios de mudança na água vinda do Mediterrâneo, devido a “um pulso tectónico que tem afectado o fluxo mediterrânico e por isso o clima”. As amostras confirmaram os ciclos climáticos de 20.000 anos causados pela órbita terrestre, que origina temporadas quentes e eras glaciares. “Todas as amostras que recolhemos através das perfurações, foram similares entre si por terem esse ciclo climático forte”.

   Os sedimentos recolhidos vão até os 1,4 M.a., até agora, o período de tempo que se tinha deste tipo de registos não ultrapassava os 800.000 anos, o que pode ajudar a perceber melhor o futuro do clima tendo em conta as alterações climáticas originadas pelo Homem. 

   Outra descoberta mais inesperada foi o surgimento de uma camada de areia na região do golfo de Cádis, que é óptima para acumular crude ou gás natural. Contudo, “a areia que perfurámos é provavelmente demasiado recente para conter crude ou gás natural”, afirmou Stow.

  “A única forma de confirmar é fazendo sondagens”, disse Fernando Barriga, do departamento de geologia da Faculdade de Ciências da Universidade de Lisboa. O professor faz parte do conselho português da ECORD, que é o grupo que reúne 17 países europeus e o Canadá e faz parte da IODP. “Para se compreender a Terra abaixo do mar é preciso ter acesso à terceira dimensão”, disse na conferência.

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Adaptado de:

http://www.publico.pt/Ci%C3%AAncias/ha-uma-pulsar-tectonico-no-estreito-de-gibraltar-que-afecta-o-clima-1529695

New Force Driving Earth's Tectonic Plates

Steve Cande and Dave Stegman have now discovered a new force that drives plate tectonics: plumes of hot magma pushing up from Earth's deep interior. Their research is published in the July 7 issue of the journal Nature.

Using analytical methods to track plate motions through Earth's history, Cande and Stegman's research provides evidence that such mantle plume "hot spots," which can last for tens of millions of years and are active today at locations such as Hawaii, Iceland and the Galapagos, may work as an additional tectonic driver, along with push-pull forces.

Their new results describe a clear connection between the arrival of a powerful mantle plume head around 70 million years ago and the rapid motion of the Indian plate that was pushed as a consequence of overlying the plume's location. The arrival of the plume also created immense formations of volcanic rock now called the "Deccan flood basalts" in western India, which erupted just prior to the mass extinction of dinosaurs. The Indian continent has since drifted north and collided with Asia, but the original location of the plume's arrival has remained volcanically active to this day, most recently having formed Réunion island near Madagascar.

The team also recognized that this "plume-push" force acted on other tectonic plates, and pushed on Africa as well but in the opposite direction.

"Prior to the plume's arrival, the African plate was slowly drifting but then stops altogether, at the same time the Indian speeds up," explains Stegman, an assistant professor of geophysics in Scripps' Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics. "It became clear the motion of the Indian and African plates were synchronized and the Réunion hotspot was the common link."

After the force of the plume had waned, the African plate's motion gradually returned to its previous speed while India slowed down.

"There is a dramatic slow down in the northwards motion of the Indian plate around 50 million years ago that has long been attributed to the initial collision of India with the Eurasian plate," said Cande, a professor of marine geophysics in the Geosciences Research Division at Scripps. "An implication of our study is that the slow down might just reflect the waning of the mantle plume-the actual collision might have occurred a little later."

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Source: sciencedaily.com

East Africa's Great Rift Valley: A Complex Rift System Part II

If the rifting process described occurs in a continental setting, then we have a situation similar to what is now occurring in Kenya where the East African/Gregory Rift is forming. In this case it is referred to as "continental rifting" (for obvious reasons) and provides a glimpse into what may have been the early development of the Ethiopian Rift. As mentioned in Part I, the rifting of East Africa is complicated by the fact that two branches have developed, one to the west which hosts the African Great Lakes (where the rift filled with water) and another nearly parallel rift about 600 kilometers to the east which nearly bisects Kenya north-to-south before entering Tanzania where it seems to die out .Lake Victoria sits between these two branches. It is thought that these rifts are generally following old sutures between ancient continental masses that collided billions of years ago to form the African craton and that the split around the Lake Victoria region occurred due to the presence of a small core of ancient metamorphic rock, the Tanzania craton, that was too hard for the rift to tear through. Because the rift could not go straight through this area, it instead diverged around it leading to the two branches that can be seen today. As is the case in Ethiopia, a hot spot seems to be situated under central Kenya, as evidenced by the elevated topographic dome there .This is almost exactly analogous to the rift Ethiopia, and in fact, some geologists have suggested that the Kenya dome is the same hotspot or plume that gave rise to the initial Ethiopian rifting. Whatever the cause, it is clear that we have two rifts that are separated enough to justify giving them different names, but near enough to suggest that they are genetically related.

Conclusions

The East African Rift System is a complicated system of rift segments which provide a modern analog to help us understand how continents break apart. It is also a great example of how many natural systems can be intertwined - this unique geological setting may have altered the local climate which may have in turn caused our ancestors to develop the skills necessary to walk upright, develop culture and ponder how such a rift came to be. Just like the Grand Canyon, the East African Rift System should be high on any geologist's list of geologic marvels to visit.

This was taken at the Njorowa Gorge in Hell's Gate National Park (Kenya). The gorge was carved by water, and is quite spectacular in many regards, but here we have an igneous dike cutting through the wall of the canyon, with Dr. Wood and one of our guides for scale. To enlarge (http://geology.com/articles/east-africa-rift/hells-gate-dike-750.jpg)

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Fonte:: Geology.com

East Africa's Great Rift Valley: A Complex Rift System Part I

The East African Rift System

The East African Rift System (EARS) is one the geologic wonders of the world, a place where the earth's tectonic forces are presently trying to create new plates by splitting apart old ones. In simple terms, a rift can be thought of as a fracture in the earth's surface that widens over time, or more technically, as an elongate basin bounded by opposed steeply dipping normal faults. Geologists are still debating exactly how rifting comes about, but the process is so well displayed in East Africa (Ethiopia-Kenya-Uganda-Tanzania) that geologists have attached a name to the new plate-to-be; the Nubian Plate makes up most of Africa, while the smaller plate that is pulling away has been named the Somalian Plate (Figure 1). These two plates are moving away form each other and also away from the Arabian plate to the north. The point where these three plates meet in the Afar region of Ethiopia forms what is called a triple-junction. However, all the rifting in East Africa is not confined to the Horn of Africa; there is a lot of rifting activity further south as well, extending into Kenya and Tanzania and Great Lakes region of Africa. The purpose of this paper is to discuss the general geology of these rifts are and highlight the geologic processes involved in their formation.

Figure 1: Colored Digital Elevation Model showing tectonic plate boundaries, outlines of the elevation highs demonstrating the thermal bulges and large lakes of East Africa. To enarlge (http://geology.com/articles/east-africa-rift/figure1.jpg)

Figure 2: Rift segment names for the East African Rift System. Smaller segments are sometimes given their own names, and the names given to the main rift segments change depending on the source. To enalrge (http://geology.com/articles/east-africa-rift/figure2.jpg)

What is the East Africa Rift System?

The oldest and best defined rift occurs in the Afar region of Ethiopia and this rift is usually referred to as the Ethiopian Rift. Further to the South a series of rifts occur which include a Western branch, the "Lake Albert Rift" or "Albertine Rift" which contains the East African Great Lakes, and an Eastern branch that roughly bisects Kenya north-to-south on a line slightly west of Nairobi (Figure 2). These two branches together have been termed the East African Rift (EAR), while parts of the Eastern branch have been variously termed the Kenya Rift or the Gregory Rift (after the geologist who first mapped it in the early 1900's). The two EAR branches are often grouped with the Ethiopian Rift to form the East Africa Rift System (EARS). The complete rift system therefore extends 1000's of kilometers in Africa alone and several 1000 more if we include the Red Sea and Gulf of Aden as extensions. In addition there are several well-defined but definitely smaller structures, called grabens, that have rift-like character and are clearly associated geologically with the major rifts. Some of these have been given names reflecting this such as the Nyanza Rift in Western Kenya near Lake Victoria. Thus, what people might assume to be a single rift somewhere in East Africa is really a series of distinct rift basins which are all related and produce the distinctive geology and topography of East Africa.

Figure 3: "Textbook" horst and graben formation (left) compared with actual rift terrain (upper right) and topography (lower right). Notice how the width taken up by the trapezoidal areas undergoing normal faulting and horst and graben formation increases from top to bottom in the left panel. Rifts are considered extensional features (continental plates are pulling apart) and so often display this type of structure. To enlarge (http://geology.com/articles/east-africa-rift/figure3.jpg)

How did these Rifts form?

The exact mechanism of rift formation is an on-going debate among geologists and geophysicists. One popular model for the EARS assumes that elevated heat flow from the mantle (strictly the asthenosphere) is causing a pair of thermal "bulges" in central Kenya and the Afar region of north-central Ethiopia. These bulges can be easily seen as elevated highlands on any topographic map of the area (Figure 1). As these bulges form, they stretch and fracture the outer brittle crust into a series of normal faults forming the classic horst and graben structure of rift valleys (Figure 3). Most current geological thinking holds that bulges are initiated by mantle plumes under the continent heating the overlying crust and causing it to expand and fracture. Ideally the dominant fractures created occur in a pattern consisting of three fractures or fracture zones radiating from a point with an angular separation of 120 degrees. The point from which the three branches radiate is called a "triple junction" and is well illustrated in the Afar region of Ethiopia (Figure 4), where two branches are occupied by the Red Sea and Gulf of Aden, and the third rift branch runs to the south through Ethiopia.

Figure 4: Triple Junction in the Afar region of Ethiopia. Image shows areas of stretched and oceanic crust as well as areas of exposed flood basalts that preceded rifting. Areas unshaded or covered by flood basalts represent normal continental crust. As the crust is pulled apart you end up with thinned crust with a complex mixture of continental and volcanic rock. Eventually the crust thins to the point where oceanic-type basalts are erupted which is the signal that new ocean crust is being formed. This can be seen in the Gulf of Aden as well as a small sliver within the Red Sea. The original extent of the flood basalts would have been greater, but large areas have been buried within the rift valley by other volcanic eruptions and sediments. To enlarge (http://geology.com/articles/east-africa-rift/figure4.jpg)

The stretching process associated with rift formation is often preceded by huge volcanic eruptions which flow over large areas and are usually preserved/exposed on the flanks of the rift. These eruptions are considered by some geologists to be "flood basalts" - the lava is erupted along fractures (rather than at individual volcanoes) and runs over the land in sheets like water during a flood. Such eruptions can cover massive areas of land and develop enormous thicknesses (the Deccan Traps of India and the Siberian Traps are examples). If the stretching of the crust continues, it forms a "stretched zone" of thinned crust consisting of a mix of basaltic and continental rocks which eventually drops below sea level, as has happened in the Red Sea and Gulf of Aden. Further stretching leads to the formation of oceanic crust and the birth of a new ocean basin.

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Fonte: Geology.com

Ethiopia Will Become an Ocean

Confirming a Suspected Rift Valley

Geologists Show that Seafloor Dynamics Are at Work in Splitting African Continent.

In 2005, a gigantic, 35-mile-long rift broke open the desert ground in Ethiopia. At the time, some geologists believed the rift was the beginning of a new ocean as two parts of the Africa

n continent pulled apart, but the claim was controversial.

Now, scientists from several countries have confirmed that the volcanic processes at work beneath the Ethiopian rift are nearly identical to those at the bottom of the world's oceans, and the rift is indeed likely the beginning of a new sea.

Aerial view of the rift near Afar, Ethiopia.

"Rifting In Concert"

"For the first time they demonstrate that activity on one rift segment can trigger a major episode of magma injection and associated deformation on a neighboring segment. Careful study of the 2005 mega-dike intrusion and its aftermath will continue to provide extraordinary opportunities for learning about continental rifts and mid-ocean ridges."

Is Ethiopia Analogous to Ocean-Ridge Rifting?

"The whole point of this study is to learn whether what is happening in Ethiopia is like what is happening at the bottom of the ocean where it's almost impossible f

or us to go," says Ebinger. "We knew that if we could establish that, then Ethiopia would essentially be a unique and superb ocean-ridge laboratory for us. Because of the unprecedented cross-border collaboration behind this research, we now know that the answer is yes, it is analogous."

Aerial view looking down the rift valley bounded by tall, near-vertical fault scarps

Twenty Feet of Rifting in a Few Days

Atalay Ayele, professor at the Addis Ababa University in Ethiopia, led the investigation, painstakingly gathering seismic data surrounding the 2005 event that led to the giant rift opening more than 20 feet in width in just days. Along with the seismic information from Ethiopia, Ayele combined data from neighboring Eritrea with the help of Ghebrebrhan Ogubazghi, professor at the Eritrea Institute of Technology, and from Yemen with the help of Jamal Sholan of the National Yemen Seismological Observatory Center. The map he drew of when and where earthquakes happened in the region fit tremendously well with the more detailed analyses Ebinger has conducted in more recent years.

Ayele's reconstruction of events showed that the rift did not open in a series of small earthquakes over an extended period of time, but tore open along its entire 35-mile length in just days. A volcano called Dabbahu at the northern end of the rift erupted first, then magma pushed up through the middle of the rift area and began "unzipping" the rift in both directions, says Ebinger.

Since the 2005 event, Ebinger and her colleagues have installed seismometers and measured 12 similar—though dramatically less intense—events.

"We know that seafloor ridges are created by a similar intrusion of magma into a rift, but we never knew that a huge length of the ridge could break open at once like this," says Ebinger. She explains that since the areas where the seafloor is spreading are almost always situated under miles of ocean, it's nearly impossible to monitor more than a small section of the ridge at once so there's no way for geologists to know how much of the ridge may break open and spread at any one time. "Seafloor ridges are made up of sections, each of which can be hundreds of miles long. Because of this study, we now know that each one of those segments can tear open in a just a few days."

Ebinger and her colleagues are continuing to monitor the area in Ethiopia to learn more about how the magma system beneath the rift evolves as the rift continues to grow.

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Fonte: geology.com

Novo ciclo geológico da Terra pode estar a começar junto à Península Ibérica

Actualmente não há vulcões activos em Portugal continental, no entanto o planeta poderá estar a entrar num novo ciclo geológico e é provável que se forme uma nova zona de subducção a sudoeste da Península Ibérica, reactivando essa actividade ígnea. "Com base na distribuição dos sismos, há quem diga que podemos estar a entrar num novo ciclo geológico, que poderá ter como consequência o vulcanismo", afirmou o geólogo José Francisco à agência Lusa.

Na origem da nova actividade vulcânica estará um fenómeno de subducção, ou seja o mergulho de uma placa sob outra - no caso concreto, da placa oceânica (limite Norte Africano) sob a placa continental Europeia, em cujo extremo está Portugal.