Rochas metamórficas formação tipos exemplos

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Rochas metamórficas: formação, tipos, exemplos
O que são rochas metamórficas?
As rochas metamórficas são um dos três principais tipos de rocha. Eles são “rochas trocadas” que começaram como
outras rochas (seja rochas ígneas, sedimentares ou outras rochas metamórficas) e sofreram uma transformação
física e química – uma metamorfose. Este processo envolve altas temperaturas e pressões. Altera a estrutura
original e/ou composição mineralógica da rocha.
Durante o metamorfismo, os minerais existentes na rocha pai re-cristalam ou formam novos minerais, levando a
mudanças na textura, como o desenvolvimento de foliação (camada) ou uma estrutura mais granular. Este processo
muitas vezes resulta em uma alteração significativa da aparência original da rocha e propriedades físicas.
Como as rochas metamórficas se formam
Você pode pegar qualquer rocha e torná-la metamórfica. Tudo o que você precisa fazer é sujeitá-lo a pressão e
temperatura suficientes que ele começa a mudar fundamentalmente.
Créditos da imagem: Educação em Siyavula.
A jornada de uma rocha metamórfica começa com uma rocha dos pais, que pode ser uma rocha ígnea, sedimentar
ou mesmo outra rocha metamórfica. Esta rocha sofre profundas mudanças físicas e químicas quando submetida a
altas temperaturas e pressões, condições muitas vezes associadas a movimentos de placas tectônicas e enterro
profundo sob a superfície da Terra.
A rocha-mãe é chamada de protolito – ou a “pedra original”. Algumas vezes, você pode dizer o que era o rock dos
pais (ou pelo menos ganhar algumas pistas sobre isso). Mas a rocha metamórfica resultante é diferente da original,
seja em textura, estrutura, composição química ou todos os itens acima.
O papel do calor e da pressão
O calor e a pressão são os principais agentes que impulsionam o metamorfismo. As temperaturas variam de 150 a
cerca de 1.000 graus Celsius – qualquer coisa mais baixa, e não haveria calor suficiente para mudar as coisas;
qualquer coisa mais alta, e você acabaria com rochas ígneas.
O outro aspecto é a pressão. Altas temperaturas e as imensas pressões de milhares de atmosferas trabalham juntas
para reorganizar os minerais na rocha dos pais. Este processo pode criar minerais e texturas inteiramente novos, um
fenômeno semelhante a uma lagarta que se transforma em borboleta.
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O calor é o principal motor do metamorfismo. Pode se originar de várias fontes, incluindo o gradiente geotérmico (o
aumento da temperatura com profundidade na Terra), o calor gerado pelo decaimento de elementos radioativos ou a
intrusão do magma quente em rochas circundantes mais frias. Conforme a temperatura aumenta, ela pode quebrar
as ligações químicas em minerais, facilitando o crescimento de novos minerais estáveis em temperaturas mais altas.
A pressão aumenta com a profundidade na Terra e pode afetar significativamente a formação rochosa. Pode ser
litostático (pressão uniforme aplicada em todas as direções) ou diferencial (pressão maior em uma direção). A
pressão diferencial, muitas vezes associada às forças tectônicas, leva ao alinhamento de minerais na rocha,
resultando em foliação, uma característica fundamental de muitas rochas metamórficas.
Os fluidos, muitas vezes água com íons dissolvidos, também desempenham um papel significativo no
metamorfismo. Eles podem facilitar o movimento de íons, auxiliando no crescimento de novos minerais e a alteração
dos existentes – mudando a química das rochas. Este metamorfismo induzido por fluido pode resultar na formação
de montagens e texturas minerais únicas.
Este metamorfismo envolve uma recristalização de estado sólido dos minerais da rocha. Este processo pode
acontecer em várias temperaturas e pressões e, portanto, há muitos tipos diferentes de rochas metamórficas. A
diversidade dessas rochas reflete a variedade de seus materiais pais e a gama de condições sob as quais eles se
formam.
Algumas das rochas metamórficas mais comuns
Before we get into the details of how metamorphic rocks form and what role they play in geology, here are some of
the most common metamorphic rocks.
Slate: Originally a shale, slate is fine-grained and known for its ability to split into flat sheets. It’s commonly
used in roofing and flooring.
Piles of natural slate. Image credits: Geograph.
Gneiss: Originating from granite or sedimentary rock, gneiss is notable for its banded appearance, created by
the segregation of different minerals into layers.
Schist: Formed from mudstone or shale, schist is characterized by its shiny, layered appearance and rich
mineral content, including garnet, mica, and quartz.
Marble: This rock starts as limestone or dolostone and transforms under heat and pressure. Known for its
beautiful veining and used extensively in sculpture and architecture.
Quartzite: Formed from quartz-rich sandstone, quartzite is extremely hard and resistant to weathering. It’s
often used in decorative stone works.
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Schist with garnets (top left; garnets are the larger darker crystals), Marble (top right), and Quartzite (bottom)
Phyllite: A fine-grained rock that evolves from slate as it undergoes further metamorphism. It has a slightly
glossy sheen and a wavy surface, often used in decorative stone works.
Amphibolite: Formed from basalt or gabbro, amphibolite is characterized by a predominance of amphibole
minerals (like hornblende) and plagioclase feldspar. It’s used in construction and as a decorative stone.
Serpentinite: Derived from the metamorphism of ultramafic rocks from the Earth’s mantle, serpentinite is
composed of minerals from the serpentine group. It has a distinctive green color and is used in decorative and
architectural applications.
Phyllite (top left), Amphibolite (top right), and Serpentinite (bottom). Adapted from Wiki Commons.
Migmatite: A mixed rock composed of metamorphic and igneous parts. It forms under high-temperature
conditions where partial melting occurs. Its complex texture reflects a mix of solid and molten rock history.
Eclogite: This is a high-pressure, high-temperature metamorphic rock formed from basalt or gabbro. It’s
characterized by a striking green and red appearance due to its main minerals: green omphacite (a type of
pyroxene) and red garnet. Eclogite forms in very specific conditions.
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Migmatite (left) and Eclogite (right). Adapted from Wiki Commons.
Types of metamorphism
Depiction of contact metamorphism. Image via Wiki Commons.
The diversity of metamorphic rocks is a direct reflection of the varying conditions under which they form. There are
three main types of metamorphism, based on the geological processes that create the rocks and minerals.
Regional Metamorphism
This is the most common type and occurs over large areas, typically associated with mountain-building processes
where tectonic plates collide. The immense pressure from the collision and the heat generated by the thickening crust
lead to widespread metamorphic changes. Regional metamorphism produces a wide range of metamorphic rocks,
from low-grade (like slate) to high-grade (like gneiss).
Contact Metamorphism
This occurs when hot magma intrudes into cooler surrounding rock, known as the country rock. The heat from the
magma raises the temperature of the surrounding rock, causing changes in its mineral structure. Contact
metamorphism usually affects a smaller area compared to regional metamorphism and often results in the formation
of non-foliated rocks like marble and quartzite.
Dynamic Metamorphism
This type occurs in fault zones where rocks are subjected to high differential pressure and shear stress. The intense
grinding and crushing along fault lines can lead to the formation of mylonites, rocks that are characteristically fine-
grained and foliated, a result of the extreme physical deformation.
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Large-scale tectonic processes are a big trigger of metamorphic processes. Here, a geologic fault captured
by NASA.
In addition, other localized phenomena can also create different types of metamorphism.
Cataclastic Metamorphism: This process involves the mechanical deformation of rocks along fault zones,
where friction generates heat and causes the rocks to crush and pulverize. It’s a relatively rare phenomenon,
confined to narrow zones where this shearing happens.
Hydrothermal Metamorphism: This occurs when rocks (especially basaltic ones) are altered by hot, high-
pressure fluids. This leads to the formation of hydrous minerals like talc, chlorite, serpentine, and others. Rich
ore deposits often form as a result of this process.
Burial Metamorphism: This happens when sedimentary rocks are buried deeply relatively quickly, reaching
temperatures over 300°C without significant stress. Under these conditions, new minerals like Zeolites form,
though the rock doesn’t seem obviously metamorphosed. This overlaps with diagenesis and can transition into
regional metamorphism under higher temperatures and pressure.
Shock Metamorphism (Impact Metamorphism): This is caused by the extreme pressures from events like
meteorite impacts or massive volcanic explosions. These pressures can create unique high-pressure minerals
like coesite and stishovite, as well as specific textures in the rocks, such as shock lamellae and shatter cones.
Here are some metamorphic rocks based on how they form and the type of metamorphism.
Metamorphic
Rock
Type of
Metamorphism Parent Rock Characteristics
Slate Regional Shale Fine-grained, splits easily,
usually gray
Schist Regional Various, including shale and
igneous rocks
Medium to coarse-grained,
layered, contains mica
Gneiss Regional Various, including granite
and sedimentary rocks Banded, coarse-grained
Marble Regional/Contact Limestone or Dolomite Crystalline, various colors,
often veined
Quartzite Regional Quartz sandstone Extremely hard and resistant,
glassy luster
Phyllite Regional Shale or Mudstone Fine-grained, lustrous sheen,
foliated
Serpentinite Hydrothermal/Contact Ultramafic rocks like
peridotite
Smooth, green color, often
veiny
Hornfels Contact Various, often shale or clay-
rich rocks
Dense, fine-grained, non-
foliated
Types of metamorphic rocks
Metamorphic rocks come in an array of types, each with unique characteristics and formation stories. There are
multiple ways of classifying metamorphic rocks. Here are the most common ones.
Texture-Based Classification
There are two main types of metamorphic rocks that dominate the scene: foliated and non-foliated metamorphic
rocks.
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Foliated rocks (on the left) vs non-foliated rocks (on the right). Images adapted via Wiki Commons.
Foliated Metamorphic Rocks are known for their layered or banded appearance, a result of the alignment of mineral
grains under directional pressure. This category includes rocks like slate, formed from shale; schist, which often
begins as phyllite; and gneiss, derived from granite or volcanic rock. Each of these rocks reveals the intensity of
metamorphism they’ve undergone, from mild to extreme.
Non-Foliated Metamorphic Rocks lack a layered structure. These rocks, such as marble (originating from
limestone) and quartzite (originating from sandstone), form under conditions where pressure is applied equally from
all directions, or where the parent rock’s composition doesn’t allow for layering.
Examples of Foliated Metamorphic Rocks
These rocks have a layered or banded appearance, resulting from the alignment of minerals under directional
pressure. Examples include:
Slate: Very fine-grained, results from the metamorphism of shale.
Phyllite: Slightly coarser than slate, with a sheeny surface.
Schist: Medium to coarse-grained, characterized by prominent schistosity due to large mica flakes.
Gneiss: Coarse-grained, distinguished by its banded appearance, resulting from high-grade metamorphism.
Examples Non-Foliated Metamorphic Rocks
These rocks lack a layered structure, often formed without significant pressure or from minerals that don’t exhibit
alignment. Examples include:
Marble: Formed from limestone or dolomite, it’s primarily composed of calcite or dolomite crystals.
quartzite: Formed from sandstone and dominated by quartz.
Hornfels: Formed by contact metamorphism, characterized by a dense, hard texture.
Mineral Composition-Based Classification
The other main way to classify metamorphic rocks is by looking at the minerals they comprise of.
Mafic Metamorphic Rocks
These rocks contain a high proportion of iron and magnesium-rich minerals. These are generally darker-hued
minerals such as amphibole, plagioclase, or olivine. Examples:
Greenschist: Contains chlorite, actinolite, and other green minerals, typically formed under low-grade
metamorphic conditions.
Amphibolite: Higher-grade than greenschist, predominantly composed of amphibole and plagioclase.
Pelitic Metamorphic Rocks
Derived from mudstone or shale, these rocks are rich in aluminum silicate minerals. Examples:
Slate, Phyllite, Schist: These can be pelitic if they originate from shale.
CalcareousMetamorphic Rocks
Originating from limestone or dolomite, these are rich in calcite or dolomite. Examples:
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Marble: The most common calcareous metamorphic rock.
Quartzo-feldspathic Metamorphic Rocks
Derived from sandstones or granitoid rocks, rich in quartz and feldspar. Examples:
Gneiss: If it originates from granitoid rocks or high-grade sandstones.
Metamorphic facies
If we want to truly understand metamorphic rocks, we need to consider the concept of a facies.
A metamorphic facies is a set of mineral assemblages in metamorphic rocks formed under a similar range of
pressures and temperatures. The facies concept helps geologists interpret the metamorphic history of rocks based on
their mineralogical composition. The facies is typically described in the form of a pressure-temperature chart.
Each facies represents a specific set of physical conditions, and the transitions between them can often be gradual.
The facies concept is integral in understanding the tectonic processes and environmental conditions that shape the
Earth’s crust. Different facies are associated with different tectonic settings such as subduction zones, continental
collision zones, and areas affected by igneous intrusions.
Here’s an overview of the main metamorphic facies (though keep in mind that there are a few others):
Zeolite Facies: Represents the lowest grade of metamorphism, typically found in oceanic crust and associated
with low temperatures and low pressures.
Greenschist Facies: Characterized by the presence of minerals like chlorite, actinolite, and epidote. This
facies forms under low to moderate temperatures and pressures.
Blue Schist Facies: Identified by the presence of blue amphibole (glaucophane), it forms under high pressures
and relatively low temperatures, often associated with subduction zones.
Eclogite Facies: Represents very high-pressure conditions, typically found in subduction zones. It’s
characterized by an assemblage of garnet and omphacite (a type of pyroxene).
Amphibolite Facies: This is a facies with medium to high-grade metamorphic rocks. The amphibolite facies is
notable for the presence of amphiboles (like hornblende) and plagioclase. It forms under moderate to high
temperatures and pressures.
Granulite Facies: Represents the highest grade of metamorphism, characterized by high temperatures and
pressures. Rocks in this facies often contain pyroxenes, garnet, and silicate minerals devoid of water.
Hornfels Facies: Associated with contact metamorphism, this facies forms due to the heating of rocks by an
igneous intrusion. The resulting rocks are fine-grained and non-foliated. This type of rock has a variety of
mineral assemblages depending on the original rock composition.
Metamorphic rocks in geology
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This type of “changed” rocks aren’t just fascinating geological phenomena; they hold crucial clues about Earth’s
history and processes.
By studying metamorphic rocks, geologists can decipher the conditions that prevailed deep within the Earth at
different geological times. This knowledge helps scientists understand the movement and interaction of tectonic
plates, the formation of mountains, and the history of our planet’s crust.
Metamorphic Rocks and Plate Tectonics
Tectonic processes can do things like this to rocks and metamorphose them. Image in public domain.
Metamorphic rocks can be found in many places around the planet. The distribution of metamorphic rocks is
intimately linked to the theory of plate tectonics. The movement and interaction of tectonic plates create the
conditions necessary for metamorphism. Subduction zones, where one plate dives beneath another, are hotspots for
metamorphic activity, leading to the formation of high-pressure, low-temperature rocks like blueschist.
Mountain building events, such as the formation of the Himalayas, are prime examples of regions where regional
metamorphism is prevalent. The immense pressure exerted during these collisions results in the widespread
transformation of rocks, giving rise to some of the most spectacular metamorphic formations.
Uses for metamorphic rocks
Metamorphic rocks have various uses in economic and industrial contexts. In construction, slate and gneiss are used
for their durability and aesthetic appeal, suitable for roofing, flooring, and landscaping. Marble, another metamorphic
rock, is notable for its beauty and versatility, widely used in sculptures, buildings, and decorative applications,
exemplified by the Taj Mahal and Michelangelo’s David.
In industry, talc from metamorphic rocks like soapstone is vital for its lubricating properties, used in machinery, baby
powder, and food additives. Garnet, known for its hardness and abrasive qualities, is employed in sandblasting,
waterjet cutting, and sandpaper. Metamorphic rocks also source precious gemstones like rubies and sapphires,
significant for the jewelry industry and collectors.
Environmentally, metamorphic rocks play a role in soil formation by contributing minerals that enhance fertility and
support ecosystems. They also shape landscapes and influence climate patterns, such as mountain ranges affecting
wind and precipitation.
A constant geological metamorphosis
In conclusion, metamorphic rocks play a vital role in shaping our understanding of Earth’s geological processes and
history. These rocks, formed through intense heat, pressure, and fluid interaction, provide a unique window into the
dynamic forces that shape our planet’s crust. The variety of metamorphic rocks, from slate and marble to gneiss and
schist, is a testament to the complexity and diversity of geological conditions experienced over Earth’s history.
These rocks have significant economic, cultural, and environmental impacts. They are integral in construction, art,
and industry, providing materials like slate for roofing, marble for sculpture and architecture, and minerals like talc and
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garnet for various industrial applications. Furthermore, gemstones such as rubies and sapphires, formed in
metamorphic environments, have considerable economic appeal.
Moreover, metamorphic rocks contribute to soil formation, impacting agriculture and ecosystems. Their weathering
releases minerals that enrich soils, supporting plant growth and biodiversity. The formation and distribution of
metamorphic rocks are closely tied to the movement of tectonic plates, offering insights into the ongoing evolution of
the Earth’s surface.
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