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As duas caras metamórficas

A cache by joom Send Message to Owner Message this owner
Hidden : 11/18/2017
Difficulty:
4.5 out of 5
Terrain:
4.5 out of 5

Size: Size:   other (other)

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Perguntas para responder - questions to answer

Para reclamar esta earthcache deverá enviar-me, através do meu perfil e antes de qualquer registo, as respostas às seguintes questões. Se algo estiver incorrecto será contactado. Não é necessário esperar por qualquer autorização. Por favor não envie fotografias. Essas são mais úteis no registo.

Nas coordenadas da earthcache a jusante da marmita de gigante com uma forma fora do normal. Como aqui poderá a recepção do sinal tente encontrar as duas caras da imagem. Aí tem um exemplo de uma rocha metamórfica e de uma ígnea. Ou seja uma cara ígnea e outra metamórfica.

1 - Qual é a rocha metamórfica e a ígnea? Ou seja comparando com a foto é a rocha da esquerda ou da direita?

2 - Classifique a rocha metamórfica segundo o texto. Há foliação? É uma rocha xistosa, gnássica ou granular?

3 - E dos cinco exemplos de rocha apresentados qual será o mais aproximado para a rocha metamórfica no ponto zero? Porquê?

4 - Qual é a cor predominante da rocha ígnea? Ou seja qual é a cor do protólito?

Opcional: uma foto sua no local, e no registo, será apreciada, e prova que esteve lá. Note que não deverá ser possível responder às perguntas através da fotografia. Obrigado pela visita.

To claim this earthcache one should send me, through my profile, the answers to the following questions identifying this earthcache. If something is incorrect I'll contact you. Please do not send photos. These are more useful on the log.

At the earthcache coordinates slightly downstream the pothole with an unusual shape. As the signal reception is poor try do find the two faces from the image. There you have an example of a metamorphic rock and one igneous one.

1 - Regarding the photo which side is the metamorphic rock?

2 - Classify the metamorphic rock according to the text. Is foliation present? Is it a schistose rock, a gneissose,  or granoblastic?

3 - And regarding the existing five rock examples, which one is the most similar to the metamorphic rock at ground zero? Why?

4 - Which is the predominant color of the igneous rock? The protolith, that is.

Optional: a photo of you at the place, and on the log, will be appreciated and it proofs that you were there. Please note that the pothole dimensions should not be estimated by one looking at the photo. Thank you for your visit.

 

Rochas metamórficas

 

As duas caras

Rochas metamórficas são rochas que resultam da transformação da rocha original, o protólito. Este dá origem a uma rocha metamórfica depois de sofrer transformações químicas e físicas devido ao fato de se submeter a temperaturas e pressões elevadas e à atuação de fluidos (metassomatose) em zonas profundas da crosta terrestre, sem que, contudo, cheguem a fundir (a não ser, talvez, parcialmente). O protólito tanto pode ser uma rocha sedimentar, como uma rocha ígnea ou mesmo outra rocha metamórfica.

Podem formar-se, simplesmente, por estarem sujeitas às altas temperaturas existentes muito abaixo da superfície terrestre e à pressão provocada pelo peso das camadas de rocha superiores (pressões litostáticas). Podem também ter origem em processos tectónicos como colisões continentais que provocam pressão horizontal, fricção e deformações. Podem, ainda, formar-se graças ao chamado metamorfismo de contacto, quando a rocha, sempre no estado sólido, é aquecida pela intrusão de rocha fundida (magma) proveniente do interior da Terra. Alguns exemplos de rochas metamórficas são o gnaisse, a ardósia,o mármore, o xisto, e o quartzito.

gnaisse ardósia mármore xisto quartzito

 

As rochas metamórficas são classificadas de acordo com critérios texturais e mineralógicos. Podem dividir-se em rochas foliadas (como o xisto e o gnaisse) e não foliadas (como o mármore).

A foliação (palavra derivada do Latim folia, que significa "folhas") refere-se à disposição dos minerais das rochas metamórficas em estratos e ocorre quando a rocha é submetida a uma tensão ao longo de um eixo durante a recristalização. Este processo provoca a rotação de cristais lamelares ou alongados (como a mica ou as clorites), de modo a que os seus longos eixos se disponham perpendicularmente à orientação da tensão. Daqui resulta uma rocha foliada com lâminas a exibir as cores dos minerais que as formaram. Esta é uma foliação secundária, provocada pelo metamorfismo, diferente de outros tipos de foliação presente nas rochas sedimentares e nas rochas ígneas.

As rochas foliadas podem ser classificadas de acordo com três tipos de textura, correspondentes a diferentes graus de metamorfismo. Rochas com clivagem ardosífera (como a ardósia, correspondente a um baixo grau de metamorfismo); rochas que apresentam xistosidade (como o xisto, correspondente a um grau médio de metamorfismo) e rochas com bandado gnáissico (como o gnaisse, correspondente a um grau elevado de metamorfismo). Estas rochas formam-se, de uma forma geral, a partir de rochas constituídas por vários minerais e que foram submetidas a condições de tensão dirigida e a temperaturas crescentes.

As rochas não foliadas, à excepção das corneanas (originadas em contexto de metamorfismo de contacto), formam-se, em geral, a partir de rochas constituídas por um só mineral. As texturas das rochas metamórficas podem ser categorizadas em foliadas e não foliadas. As rochas foliadas resultam da pressão diferencial que deforma a rocha num plano, criando, por vezes, um plano de clivagem. As rochas não foliadas não apresentam padrões planares ou deformações visíveis, podendo ter um aspecto cristalino, como acontece com os quartzitos e os mármores. Entre as rochas metamórficas foliadas podemos ainda referir rochas de baixo grau de metamorfismo, como os xistos argilosos, de grau médio de metamorfismo, como os micaxistos e de grau elevado de metamorfismo, como acontece com o gnaisse.

Tipos de metamorfismo

Metamorfismo de contacto

Metamorfismo de contacto, também conhecido como metamorfismo termal é o nome dado às mudanças que ocorrem quando há uma intrusão de magma numa rocha preexistente. Estas mudanças são tanto maiores quanto a proximidade à zona de contacto com o magma, devido às altas temperaturas aí registadas, e menos significativas à medida que a massa rochosa se vai afastando da intrusão magmática. Em torno da rocha ígnea que resulta do arrefecimento do magma regista-se, então, uma zona metamorfizada designada de auréola metamórfica, que apresenta vários graus de metamorfismo desde a superfície de contacto até à rocha encaixante não metamorfizada.

Metamorfismo regional

Metamorfismo regional, também conhecido como metamorfismo dinamotermal, é o nome dado às alterações em grandes massas rochosas presentes numa área extensa. As rochas podem ser metamorfizadas pelo simples fato de se encontrarem a grande profundidade abaixo da superfície terrestre, onde são submetidas a elevadas temperaturas e às extremas condições de pressão causadas pelo peso das camadas de rocha que se encontram acima delas. Grande parte da crosta continental inferior continental é metamórfica, à excepção de intrusões ígneas recentes.

Metamorfismo cataclástico

O metamorfismo dinâmico ou metamorfismo cataclástico ocorre devido à ação do atrito, em longas faixas e na adjacência de falhas, onde pressões de grande intensidade causam movimentações e rupturas na crosta. Movimentos tectónicos horizontais como a colisão de continentes dão origem a cinturões orogenéticos onde as rochas são submetidas a grandes deformações em decorrência das altas temperaturas e pressões.

O ponto zero fica em cima

Fontes:

Rocha metamórfica - wikipedia

 

Metamorphic rocks

 

Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form".The original rock (protolith) is subjected to heat (temperatures greater than 150 to 200 °C) and pressure (150 megapascals (1,500 bar) causing profound physical or chemical change. The protolith may be a sedimentary, igneous, or existing metamorphic rock.

Metamorphic rocks make up a large part of the Earth's crust and form 12% of the Earth's land surface.They are classified by texture and by chemical and mineral assemblage (metamorphic facies). They may be formed simply by being deep beneath the Earth's surface, subjected to high temperatures and the great pressure of the rock layers above it. They can form from tectonic processes such as continental collisions, which cause horizontal pressure, friction and distortion. They are also formed when rock is heated by the intrusion of hot molten rock called magma from the Earth's interior. The study of metamorphic rocks (now exposed at the Earth's surface following erosion and uplift) provides information about the temperatures and pressures that occur at great depths within the Earth's crust. Some examples of metamorphic rocks are gneiss, slate, marble, schist, and quartzite.

 

gneiss slate marble schist quartzite

The layering within metamorphic rocks is called foliation (derived from the Latin word folia, meaning "leaves"), and it occurs when a rock is being shortened along one axis during recrystallization. This causes the platy or elongated crystals of minerals, such as mica and chlorite, to become rotated such that their long axes are perpendicular to the orientation of shortening. This results in a banded, or foliated rock, with the bands showing the colors of the minerals that formed them.

Textures are separated into foliated and non-foliated categories. Foliated rock is a product of differential stress that deforms the rock in one plane, sometimes creating a plane of cleavage. For example, slate is a foliated metamorphic rock, originating from shale. Non-foliated rock does not have planar patterns of strain.

Rocks that were subjected to uniform pressure from all sides, or those that lack minerals with distinctive growth habits, will not be foliated. Where a rock has been subject to differential stress, the type of foliation that develops depends on the metamorphic grade. For instance, starting with a mudstone, the following sequence develops with increasing temperature: slate is a very fine-grained, foliated metamorphic rock, characteristic of very low grade metamorphism, while phyllite is fine-grained and found in areas of low grade metamorphism, schist is medium to coarse-grained and found in areas of medium grade metamorphism, and gneiss coarse to very coarse-grained, found in areas of high-grade metamorphism. Marble is generally not foliated, which allows its use as a material for sculpture and architecture.

Another important mechanism of metamorphism is that of chemical reactions that occur between minerals without them melting. In the process atoms are exchanged between the minerals, and thus new minerals are formed. Many complex high-temperature reactions may take place, and each mineral assemblage produced provides us with a clue as to the temperatures and pressures at the time of metamorphism.

Metasomatism is the drastic change in the bulk chemical composition of a rock that often occurs during the processes of metamorphism. It is due to the introduction of chemicals from other surrounding rocks. Water may transport these chemicals rapidly over great distances. Because of the role played by water, metamorphic rocks generally contain many elements absent from the original rock, and lack some that originally were present. Still, the introduction of new chemicals is not necessary for recrystallization to occur.

Types of metamorphism

Contact metamorphism

Contact metamorphism is the name given to the changes that take place when magma is injected into the surrounding solid rock (country rock). The changes that occur are greatest wherever the magma comes into contact with the rock because the temperatures are highest at this boundary and decrease with distance from it. Around the igneous rock that forms from the cooling magma is a metamorphosed zone called a contact metamorphism aureole. Aureoles may show all degrees of metamorphism from the contact area to unmetamorphosed (unchanged) country rock some distance away. The formation of important ore minerals may occur by the process of metasomatism at or near the contact zone.

When a rock is contact altered by an igneous intrusion it very frequently becomes more indurated, and more coarsely crystalline. Many altered rocks of this type were formerly called hornstones, and the term hornfels is often used by geologists to signify those fine grained, compact, non-foliated products of contact metamorphism. A shale may become a dark argillaceous hornfels, full of tiny plates of brownish biotite; a marl or impure limestone may change to a grey, yellow or greenish lime-silicate-hornfels or siliceous marble, tough and splintery, with abundant augite, garnet, wollastonite and other minerals in which calcite is an important component. A diabase or andesite may become a diabase hornfels or andesite hornfels with development of new hornblende and biotite and a partial recrystallization of the original feldspar. Chert or flint may become a finely crystalline quartz rock; sandstones lose their clastic structure and are converted into a mosaic of small close-fitting grains of quartz in a metamorphic rock called quartzite.

If the rock was originally banded or foliated (as, for example, a laminated sandstone or a foliated calc-schist) this character may not be obliterated, and a banded hornfels is the product; fossils even may have their shapes preserved, though entirely recrystallized, and in many contact-altered lavas the vesicles are still visible, though their contents have usually entered into new combinations to form minerals that were not originally present. The minute structures, however, disappear, often completely, if the thermal alteration is very profound. Thus small grains of quartz in a shale are lost or blend with the surrounding particles of clay, and the fine ground-mass of lavas is entirely reconstructed.

By recrystallization in this manner peculiar rocks of very distinct types are often produced. Thus shales may pass into cordierite rocks, or may show large crystals of andalusite (and chiastolite), staurolite, garnet, kyanite and sillimanite, all derived from the aluminous content of the original shale. A considerable amount of mica (both muscovite and biotite) is often simultaneously formed, and the resulting product has a close resemblance to many kinds of schist. Limestones, if pure, are often turned into coarsely crystalline marbles; but if there was an admixture of clay or sand in the original rock such minerals as garnet, epidote, idocrase, wollastonite, will be present. Sandstones when greatly heated may change into coarse quartzites composed of large clear grains of quartz. These more intense stages of alteration are not so commonly seen in igneous rocks, because their minerals, being formed at high temperatures, are not so easily transformed or recrystallized.

In a few cases rocks are fused and in the dark glassy product minute crystals of spinel, sillimanite and cordierite may separate out. Shales are occasionally thus altered by basalt dikes, and feldspathic sandstones may be completely vitrified. Similar changes may be induced in shales by the burning of coal seams or even by an ordinary furnace.

There is also a tendency for metasomatism between the igneous magma and sedimentary country rock, whereby the chemicals in each are exchanged or introduced into the other. Granites may absorb fragments of shale or pieces of basalt. In that case, hybrid rocks called skarn arise, which don't have the characteristics of normal igneous or sedimentary rocks. Sometimes an invading granite magma permeates the rocks around, filling their joints and planes of bedding, etc., with threads of quartz and feldspar. This is very exceptional but instances of it are known and it may take place on a large scale.

Regional metamorphism

Regional metamorphism tends to make the rock more indurated and at the same time to give it a foliated, shistose or gneissic texture, consisting of a planar arrangement of the minerals, so that platy or prismatic minerals like mica and hornblende have their longest axes arranged parallel to one another. For that reason many of these rocks split readily in one direction along mica-bearing zones (schists). In gneisses, minerals also tend to be segregated into bands; thus there are seams of quartz and of mica in a mica schist, very thin, but consisting essentially of one mineral. Along the mineral layers composed of soft or fissile minerals the rocks will split most readily, and the freshly split specimens will appear to be faced or coated with this mineral; for example, a piece of mica schist looked at facewise might be supposed to consist entirely of shining scales of mica. On the edge of the specimens, however, the white folia of granular quartz will be visible. In gneisses these alternating folia are sometimes thicker and less regular than in schists, but most importantly less micaceous; they may be lenticular, dying out rapidly. Gneisses also, as a rule, contain more feldspar than schists do, and are tougher and less fissile. Contortion or crumbling of the foliation is by no means uncommon; splitting faces are undulose or puckered. Schistosity and gneissic banding (the two main types of foliation) are formed by directed pressure at elevated temperature, and to interstitial movement, or internal flow arranging the mineral particles while they are crystallizing in that directed pressure field.

Cataclastic Metamorphism

Cataclastic metamorphism occurs as a result of mechanical deformation, like when two bodies of rock slide past one another along a fault zone. Heat is generated by the friction of sliding along such a shear zone, and the rocks tend to be mechanically deformed, being crushed and pulverized, due to the shearing. Cataclastic metamorphism is not very common and is restricted to a narrow zone along which the shearing occurred.

Ground zero is upstairs

Source

Metamorphic rock

Types of metamorphism

 

Nota - Note

 

Embora o acesso ao ponto zero seja possível sem ser por canyoning todo o cuidado é pouco pois o terreno não é o mais simpático. Aqui há todas as informações sobre como aceder descendo o rio com material e conhecimentos adequados. Estude apropriadamente e previamente como aceder ao ponto zero.

Although ground zero could be reached not using canyoning means one should be extremely careful. This is not a forgiving terrain. Here there is all the needed information about canyoning in this river. Please study thoroughly your path before to reach ground zero.

Um agradecimento especial aos MitoriGeikos, por não colocarem objecções em criar aqui esta earthcache e aos companheiros de jornada neste dia, anjomaco, Joca.Sara e pbrandao. Sem vocês não teria sido possível a descoberta deste local. Não deixe de visitar as caches vizinhas: esta e esta.

 

Por favor não partilhe as respostas. Para que continuem a existir earthcaches junte ao registo uma fotografia desse dia no ponto zero. Assim ajuda a acabar com as visitas fantasmas a lembrar o Walter Mitty.
Please do not share the answers. To make sure that earthcaches endure append to your log a photo of the day at ground zero. It helps to end ghost visits that resemble Walter Mitty.

 

Additional Hints (Decrypt)


Aãb rfdhrpre yrine n sbgb-fcbvyre dhr vaqvpn b dhr cebphene. Qb abg sbetrg gur fcbvyre gung cbvag jung gb frnepu.

Decryption Key

A|B|C|D|E|F|G|H|I|J|K|L|M
-------------------------
N|O|P|Q|R|S|T|U|V|W|X|Y|Z

(letter above equals below, and vice versa)