Metamorphic rocks are type of rock that formed when pre-existing rocks (parent rocks) undergo significant physical or chemical changes under high pressure and temperature, without melting completely. This process, known as metamorphism, alters the texture, mineralogy, and composition of the original rock, creating a new rock type. Metamorphism, Literally Means "Change of Form."
Factors Influencing Metamorphism:
Temperature: Typically exceeding 150-200°C, elevated temperatures activate chemical reactions that alter the mineral composition of the parent rock.
Pressure: Often greater than 100 megapascals, pressure compacts the rock, promoting recrystallization and the formation of new structures.
Chemical Modifiers: Hot fluids carrying dissolved minerals can introduce new chemical components or remove existing ones, further modifying the rock's composition.
Time: These transformative processes occur over vast timescales, spanning millions of years, allowing for gradual and profound changes.
How Do Metamorphic Rock Formed
Metamorphism can occur through various processes, including:
Regional Metamorphism is the dominant process, responsible for the formation of vast areas of metamorphic rock. It occurs due to immense pressure and heat generated during:
Mountain Building: As tectonic plates collide, immense forces compress and uplift rock layers, subjecting them to high pressure and shearing forces.
Continental Collisions: When continents collide, the immense pressure and heat generated can lead to extensive regional metamorphism.
Subduction Zones: When oceanic plates subduct beneath continental plates, the descending plate undergoes intense heating and pressure, resulting in various metamorphic rocks.
The intense heat and pressure of regional metamorphism trigger recrystallization of minerals in the parent rock, often creating the characteristic foliated textures.
Contact Metamorphism occurs when a hot igneous intrusion comes into contact with surrounding cooler rocks. The heat from the intrusion bakes the surrounding rock, leading to its partial melting and recrystallization. This process typically results in non-foliated metamorphic rocks like marble and hornfels.
The extent of contact metamorphism depends on the size and temperature of the intrusion, as well as the composition and permeability of the surrounding rock.
Hydrothermal Metamorphism involves the interaction of hot, chemically-charged fluids circulating through rock formations. These fluids can dissolve existing minerals, precipitate new ones, and alter the overall texture and composition of the rock. Hydrothermal metamorphism is often associated with volcanic activity, geothermal areas, and near-surface mineral deposits.
The specific type of hydrothermal metamorphism depends on the composition of the fluids and the temperature and pressure conditions.
Types of Metamorphic rocks
Based on their appearance and mineral composition, metamorphic rocks are classified into two major categories: foliated and non-foliated.
These rocks possess a layered or banded structure due to the alignment of platy minerals like mica and chlorite. This alignment is a result of the pressure and shearing forces they experience during metamorphism.
Examples of foliated metamorphic rocks include:
Slate: This fine-grained rock, often grey or black, boasts a well-developed cleavage due to the alignment of aligned mica minerals that allows it to be split into thin sheets, commonly used for roofing and decorative purposes.
Phyllite: Finely crystalline rock with a silky sheen and a more pronounced foliation than slate. It is sometimes used for decorative purposes.
Schist: A coarse-grained rock with a well-developed foliation, schist is often composed of mica minerals that give it a flaky appearance. It is used in construction and for manufacturing electrical insulators.
Gneiss: The most complex type of foliated rock, gneiss is often highly banded rock with alternating layers of light and dark minerals, sometimes containing augen (large, feldspar crystals).
Non-foliated Metamorphic Rocks
These rocks lack the layered structure and have a more homogenous appearance. The absence of foliation can be attributed to different factors like the composition of the parent rock and the type of metamorphism experienced.
Examples of non-foliated metamorphic rocks include:
Marble: A rock formed from recrystallized limestone composed primarily of calcite. It is also used in construction and for decorative purposes.
Quartzite: A rock formed from recrystallized sandstone composed primarily of quartz. Quartzite is a very hard and durable rock. It is often used for countertops, flooring, and in construction.
Hornfels: A fine-grained, non-foliated rock formed by contact metamorphism. Hornfels is often dark colored and has a fine-grained texture. It is used in construction and for paving.
Novaculite: A hard, non-foliated, fine-grained rock formed from the metamorphism of chert. Novaculite used for whetstones and abrasives.
Skarn: A non-foliated rock formed at the contact zone between igneous intrusions and carbonate rocks. Skarn is often rich in iron and other metals. It is used as a source of iron ore and other metals.
Occurrence of Metamorphic rocks
Metamorphic rocks are abundantly found on Earth, making up a significant portion of the continental crust and covering roughly 12% of the land surface. They occur in various geological settings, each with its own unique characteristics:
These are areas where mountain ranges are formed due to plate tectonics. Metamorphism occurs due to intense pressure and heat generated during collisions between continental plates and subduction zones.
Examples include the Appalachian Mountains, the Himalayas, and the Alps.
Aureoles of igneous intrusions
When hot magma intrudes into pre-existing rocks, it can cause contact metamorphism in the surrounding area. The intensity of metamorphism depends on the size and temperature of the magma body.
Examples include the contact zones around batholiths and granite intrusions.
Metamorphism can occur on the seafloor due to the high pressure of overlying rock and hydrothermal fluids circulating through the ocean crust.
Examples include serpentinized peridotite found near mid-ocean ridges and subduction zones.
Lower and intermediate crust
Deep within the Earth's crust, rocks are subjected to high temperatures and pressures over long periods, leading to regional metamorphism.
These metamorphic rocks form the "basement" of continents and are often exposed in areas of deep erosion.
In some cases, rocks from the upper mantle can be brought up to the surface through tectonic processes and exposed as metamorphic rocks.
These rocks are often ultramafic in composition and contain minerals like garnet and pyroxene.
Specific examples of occurrences
- Marble: It's commonly found in Italy, Greece, and Vermont (USA).
- Slate: Found in Wales, Pennsylvania (USA), and Germany.
- Quartzite: Abundant in the Appalachian Mountains and Brazil.
- Gneiss: Found in Scotland, Canada, and Greenland.
- Schist: Occurs in the Alps, Himalayas, and Scotland.
Metamorphic Rocks Example
Here are some examples of metamorphic rocks:
Migmatite: Migmatite A composite rock formed by the partial melting of a pre-existing rock, typically a gneiss or schist, followed by the intrusion of molten rock material (typically magma or fluid) into the partially melted rock. This results in a mixed rock with a distinct banding or patchy appearance.
Eclogite: Eclogite is a high-grade -high-pressure, high-temperature- metamorphic rock characterized by the presence of garnet and omphacite (a high-pressure pyroxene). These minerals indicate that the rock has undergone significant burial and compression within the Earth's crust or mantle. Eclogites often have a greenish-gray color and a coarse-grained, equigranular texture. Eclogite is important for understanding the processes that occur in the subduction zones. It is often found in association with diamond deposits.
Granofels: Granofels is a non-foliated, granoblastic metamorphic rock. It is characterized by a coarse-grained texture with interlocking crystals of quartz and feldspar, often with additional minerals like garnet, pyroxene, or amphibole. The lack of foliation indicates that the rock has undergone high-grade metamorphism without significant shearing or deformation. Granofels can be classified as light or dark depending on the dominance of felsic or mafic minerals.
Soapstone: Soapstone is a soft, non-foliated metamorphic rock composed mainly of talc. It is heat-resistant and easy to carve, making it suitable for fireplaces, countertops, and sculptures.
Serpentinite: A metamorphic rock composed primarily of serpentine minerals, often formed from ultramafic rocks.
Charnockite: A high-grade metamorphic rock characterized by the presence of orthopyroxene. It is often found in Precambrian terrains.
Blue schist: A rare and beautiful rock formed under high pressure and low temperature conditions. Blue schist is characterized by its blue color due to the presence of the mineral glaucophane.
Mylonite: Mylonite is a fine-grained, foliated metamorphic rock formed by intense shearing and crushing of rocks. It is often found in fault zones.
Cataclasite: Cataclasite is a brecciated metamorphic rock formed by intense mechanical grinding of rocks. It is often found in fault zones.
Pseudotachylyte: Pseudotachylyte is a glassy, black-colored metamorphic rock formed by frictional melting during fault movement. It is a rare rock type but can be used to study past earthquake events.
Uses of Metamorphic rocks
Metamorphic rocks, which form from the alteration of pre-existing rocks under high temperature and pressure conditions, have various uses across different industries. Some common uses of metamorphic rocks include:
Building Stones: The strength and durability of metamorphic rocks like granite, gneiss, and quartzite make them excellent choices for building materials. They are resistant to weathering, fire, and wear, ensuring the longevity of structures.
Flagstones and Paving Stones: Metamorphic rocks like slate and schist are often split into thin slabs, ideal for creating attractive and durable flagstones and paving stones for sidewalks, patios, and walkways.
Aggregates: Crushed metamorphic rocks are used as aggregates in concrete and asphalt, providing a strong and stable foundation for roads, bridges, and other infrastructure projects.
2. Art and Decoration:
Sculptures and Monuments: The natural beauty and unique textures of metamorphic rocks like marble and soapstone have inspired artists for centuries. These rocks are readily carved and polished, creating stunning sculptures, monuments, and decorative elements.
Countertops and Vanities: The elegance and durability of marble, granite, and quartzite make them popular choices for countertops and vanities in kitchens and bathrooms, adding a touch of sophistication to any space.
Jewelry and Gemstones: Some metamorphic rocks harbor hidden treasures! Gemstones like jade, garnet, and kyanite, found within these rocks, are prized for their beauty and durability, adorning jewelry and captivating collectors.
Abrasives: Certain metamorphic rocks like quartzite and garnet are crushed and used as abrasives in grinding, polishing, and sandblasting applications. Their hardness and sharp edges make them effective for various industrial processes.
Refractory Materials: Metamorphic rocks like soapstone and kyanite are heat-resistant, making them suitable for use in linings for furnaces, kilns, and other high-temperature industrial settings.
Fillers and Pigments: Finely ground metamorphic rocks are used as fillers in plastics, paints, and other products. They can also be used as pigments, adding color and texture to various materials.
Metamorphic rocks provide important information about the Earth's tectonic history, as the processes leading to their formation are often associated with significant geological events.
While these are some common uses of metamorphic rocks, their versatility and durability make them valuable across various applications in both construction and industrial contexts.
Understanding these properties helps us choose the right metamorphic rock for the job.