Metamorphic rocks are rocks that have been transformed from their original igneous or sedimentary form into new types of rock through the process of metamorphism. This process involves exposing rocks to high temperatures, often over 150 to 200°C, and high pressures, typically over 100 megapascals. The extreme heat and pressure cause the rocks’ physical and chemical structures to change profoundly, leading to recrystallization and new mineral compositions. However, metamorphic rocks remain largely in a solid state throughout the transformation.
There are various types of metamorphic rocks, including gneiss, slate, marble, schist, and quartzite. Slate and quartzite are commonly used as building materials for tiles and other construction purposes. Marble is also widely used in construction and sculpture due to its beauty when polished. On the other hand, schist poses some engineering challenges because its prominent planes of weakness can make it prone to slipping. Other examples of metamorphic rocks include soapstone, phyllite, amphibolite, and hornfels.
Metamorphic rocks differ significantly from igneous and sedimentary rocks in their textures, mineral compositions, physical properties, formation processes, and other characteristics. For instance, metamorphic rocks have a foliated or non-foliated texture, while igneous rocks are crystalline and sedimentary rocks are clastic. The differences highlight how exposure to extreme temperatures and pressures transforms rocks into entirely new forms. Metamorphic rocks provide insights into the powerful geological forces that shape the Earth’s crust.
How Metamorphic Rocks Are Formed
Metamorphic rocks form when existing rocks undergo metamorphism – a process that alters rocks due to increased levels of heat and pressure deep beneath the Earth’s surface. This can occur in various geological settings:
- During mountain building events when rocks are buried to great depths by tectonic forces. The immense pressures squeeze and fold the rocks.
- At convergent plate boundaries when continental plates collide. Huge slabs of rock are thrust on top of each other and buried deep in the crust.
- Near intrusive igneous bodies like granite plutons. The nearby hot magma bakes the surrounding rocks.
- In the contact aureoles around volcanoes due to heating from erupting magma.
- In subduction zones when oceanic crust is forced deep into the mantle.
The increased temperatures, up to 700°C or more in high-grade metamorphism, cause the minerals within the rocks to destabilize. The intense pressures, which can exceed 200 megapascals, deform and twist the rocks. These extreme conditions allow new minerals to grow and recrystallize as the rocks partially melt. The changes alter the rocks’ textures, crystal structures, and overall chemical compositions. However, metamorphic rocks remain largely in a solid state, unlike rocks that fully melt to become magma.
The Three Main Types of Rocks
There are three main types of rocks:
- Igneous rocks – formed by the cooling and solidification of molten magma or lava. They are characterized by a crystalline texture, resulting from the growth of interlocking mineral crystals as the melt solidifies.
- Sedimentary rocks – formed by the accumulation and cementation of sediment, which can include eroded rock debris, organic matter, chemical precipitates, and mineral grains. Sedimentary rocks have a clastic texture made up of fragments or grains of the accumulated sediment.
- Metamorphic rocks – formed by the transformation of existing rock types through metamorphism in the solid state under heat and pressure. Metamorphic rocks have a foliated or non-foliated texture, banded or not, depending on how the minerals recrystallize.
Types of Metamorphic Rocks
Based on texture, there are two main categories of metamorphic rocks:
Foliated Metamorphic Rocks
Foliated metamorphic rocks have a banded or layered appearance produced by aligned minerals that form in planes as the rocks compress and stretch during metamorphism. Examples include:
- Gneiss – characterized by alternating bands of light and dark minerals, with the minerals segregated into layers. Has a coarse texture.
- Schist – contains coarse flakes or layers of mica minerals like biotite and muscovite. The aligned mica flakes produce the foliation.
- Slate – has very fine-grained layers and splits easily into thin smooth sheets. Highly aligned clay minerals produce the cleavage.
- Phyllite – fine-grained rock intermediate between slate and schist. Has a sheen due to very fine-grained mica flakes.
Non-foliated Metamorphic Rocks
Non-foliated metamorphic rocks lack distinct banding or layered structure. The minerals recrystallize more uniformly in all directions during metamorphism rather than aligning along planes. Examples include:
- Quartzite – made almost entirely of interlocking quartz grains that have recrystallized from sandstone. Extremely hard and durable.
- Marble – recrystallized limestone or dolomite with a granular texture. Composed predominately of calcite, dolomite, or both.
- Hornfels – fine-grained, with no foliation. Formed by contact metamorphism near igneous intrusions.
- Novaculite – a type of high-purity recrystallized chert with microcrystalline quartz.
Other common metamorphic rocks include mica schists, soapstone, amphibolite, and granulite.
Comparison of Some Common Metamorphic Rocks
|Metamorphic Rock||Description||Texture||Mineral Composition||Uses|
|Gneiss||Banded rock with alternating layers of light and dark minerals||Foliated||Quartz, feldspar, mica, hornblende||Ornamental stone, construction aggregate|
|Schist||Medium to coarse-grained rock with aligned mica flakes||Foliated||Mica, quartz, feldspar, garnet||Roofing, decorative stone|
|Slate||Fine-grained metamorphosed shale, splits into thin sheets||Foliated||Quartz, muscovite mica||Roofing, flooring, blackboards|
|Marble||Recrystallized limestone or dolomite||Non-foliated||Calcite and/or dolomite||Sculpture, architecture, monuments|
|Quartzite||Metamorphosed quartz sandstone||Non-foliated||Quartz||Concrete aggregate, industrial uses|
|Phyllite||Fine-grained rock with sheen from mica flakes||Foliated||Quartz, mica, chlorite||Roofing, decorative stone|
|Soapstone||Soft metamorphosed talc and magnesite||Non-foliated||Talc, magnesite, chlorite||Sculpture, countertops, sinks|
|Hornfels||Fine-grained contact metamorphic rock||Non-foliated||Varies, quartz, feldspar||Construction aggregate, road material|
|Migmatite||Partially melted metamorphic rock||Foliated||Quartz, feldspar, mica||Decorative building stone|
The Process of Metamorphism
The process of metamorphism occurs within the solid state under intense heat and pressure. There are three main factors that influence the process:
- Temperatures between 150-200°C are needed to metamorphose rocks in the lowest grades of metamorphism.
- Higher temperatures result in more intense metamorphism at higher grades as minerals become increasingly unstable.
- In high-grade metamorphism above 650°C, partial melting may occur, resulting in migmatites, which are mixtures of melted and solid rocks.
- Heat drives metamorphism by providing the energy needed for minerals to destabilize and recrystallize into new forms.
- Immense pressures are needed to alter rocks at depth – usually over 100 megapascals.
- Pressure squeezes, folds, and deforms the rocks. This causes realignment of platy or elongated minerals into foliated metamorphic fabrics.
- In low-pressure regional metamorphism, new minerals grow slowly and large crystals form. In high-pressure contact metamorphism, recrystallization is rapid and crystals are small.
- Pressure greatly lowers the melting point of rocks, facilitating metamorphism and partial melting at depth.
- Hydrothermal fluids or water vapor carry heat and minerals dissolved from surrounding rocks.
- As hot fluids permeate the rocks, chemical reactions occur that introduce new elements and alter compositions.
- Common alteration minerals formed by hot fluids include chlorite, epidote, sericite, and talc.
- Fluids lower the necessary temperature and pressure conditions for metamorphic reactions.
Metamorphism typically happens across a gradient, progressing in stages from low to high intensity:
- Diagenesis – compaction and cementation at low temperatures and pressures.
- Anchizone – initial recrystallization below about 200°C.
- Greenschist Facies – widespread formation of green minerals like chlorite and epidote, typical of low-grade regional metamorphism.
- Amphibolite Facies – higher grade metamorphism between 500-700°C, characterized by amphibole and plagioclase.
- Granulite Facies – formation of granulites above about 700°C and 5-10 kb pressure.
- Eclogite Facies – high pressure metamorphism of basaltic rocks forming eclogite.
Uses of Metamorphic Rocks
Due to their durable physical properties, metamorphic rocks have a variety of uses:
- Construction – Slate and marble are commonly used for tiles, flooring, walls, countertops and facing stone. The durability of quartzite and gneiss make them suitable aggregates and dimension stone.
- Sculpture – Marble and soapstone are prized for their carvability and aesthetic qualities when polished, making them ideal media for sculptures and monuments.
- Engineering – Foliated rocks like schist can pose challenges for engineering projects like roads and tunnels due to the planes of weakness between mineral layers.
- Industry – Quartzite’s high silica content makes it useful as an industrial mineral. Phyllite serves as roofing slate. Garnet-mica-schist is used to manufacture commercial abrasives.
- Decorative – The beautiful banding and patterns of gneiss and migmatite are popular for landscaping stones, wall cladding, and similar decorative purposes.
Other specialized uses include novaculite for whetstones, soapstone for stoves and sinks, and tremolite asbestos (banned in most countries for health reasons).
Metamorphic Rocks in Nature
Metamorphic rocks play an integral role in the rock cycle. Existing igneous and sedimentary rocks can all be converted into metamorphic rocks when subjected to varying grades of heat and pressure over great spans of geological time.
Some national parks and landscapes that showcase metamorphic rock formations include:
- Death Valley National Park – Home to striking examples of metasedimentary and metaigneous rocks, including marble, migmatite, gneiss, and schist.
- Grand Teton National Park – Prominent peaks like the Grand Teton consist of metamorphosed gneiss and schist 1.4 billion years old.
- Glacier National Park – Metamorphic rocks like argillite and quartzite from the Belt Supergroup were transformed 50-100 million years ago and sculpted into scenic peaks.
- Great Smoky Mountains – Miles of metamorphic rocks like metagraywacke were deposited as ancient sea sediments and later metamorphosed.
- Shenandoah National Park – The Blue Ridge Mountains contain diverse metamorphic bedrock dating back over 1 billion years.
In addition, metaigneous rocks like gneiss, schist, and migmatite, form the cores of most major mountain ranges, including the Himalayas, Alps, Andes, and Sierra Nevada. These provide visible evidence of the powerful tectonic forces within the Earth.
The diverse metamorphic rock landscapes found across the globe provide insights into the dynamic geological processes that have shaped the planet over its long 4.5 billion year history. Specific metamorphic textures and mineral assemblages offer clues to uncovering the pressure, temperature, and deformational conditions that altered the rocks millions of years ago. Careful study of metamorphic rocks can help geologists reconstruct details of Earth’s distant past.
Frequently Asked Questions about Metamorphic Rocks
What are metamorphic rocks?
Metamorphic rocks are rocks that have undergone physical and/or chemical changes due to exposure to high heat, high pressure, and/or hot mineral-rich fluids. Existing rocks are transformed into new types of rocks through the process of metamorphism.
How do metamorphic rocks form?
Metamorphic rocks form when existing igneous, sedimentary or even other metamorphic rocks are subjected to increased temperature, pressure, and chemically active fluids associated with events like mountain building, subduction, and contact with magma bodies. The metamorphism causes the rocks to recrystallize and realign their minerals.
What are some examples of metamorphic rocks?
Common metamorphic rocks include slate, schist, gneiss, marble, quartzite, soapstone, phyllite, and others. They can form from a variety of protoliths or parent rocks.
What is foliation in metamorphic rocks?
Foliation refers to the banded or layered structure in some metamorphic rocks caused by the realignment of platy or elongated minerals like mica during metamorphism. Schist and gneiss are examples of foliated metamorphic rocks.
How do you identify metamorphic rocks?
Identifying metamorphic rocks involves observing their texture, mineral composition, and features like foliation or banding. Understanding the geological context where the rock is found also aids identification and determining the likely protolith.
What are some uses for metamorphic rocks?
Metamorphic rocks have a variety of uses including: construction materials like slate, marble, and schist; industrial uses like quartzite; abrasives from schist; sculptures carved from marble; and decorative dimension stone.
Where can metamorphic rocks be found?
Metamorphic rocks are found in mountainous areas where rocks have undergone high-grade regional metamorphism. They also occur in contact metamorphic zones around intrusive igneous bodies.
Metamorphic rocks form when existing rocks undergo profound changes in their physical structure, chemical composition, and mineralogy due to extreme heating and compression associated with burial, uplift, and deformation within the Earth’s crust. The process of metamorphism can transform any rock type into new metamorphic forms with unique textures, mineral assemblages, fabrics, and physical properties suited to the environmental conditions that created them. There are diverse classifications of metamorphic rocks based on texture, chemistry, grade of metamorphism, and original protolith. Understanding how these rocks form provides key insights into the dynamic geologic forces and history of the planet. Additional resources can help readers learn more about these fascinating products of planetary forces that shape the continents and oceans.