Metamorphic Rocks – Formation, Types, Examples


Metamorphic Rocks
Metamorphic rocks start out as other rocks that undergo physical and chemical changes due to extreme conditions deep in the Earth’s crust.

Metamorphic rocks represent one of the three major classes of rocks, the others being igneous and sedimentary. Each class has its unique formation processes, characteristics, and significance in Earth’s geology.

What Are Metamorphic Rocks?

Metamorphic rocks originate from pre-existing rocks, including igneous, sedimentary, or older metamorphic rocks, which undergo physical or chemical changes due to extreme heat, pressure, or mineral-rich fluids. This transformation, occurring deep within the Earth’s crust, alters the mineral composition and structure of the rock without melting it.

Comparison with Igneous and Sedimentary Rocks

  • Igneous Rocks: Igneous rocks form from the cooling and solidification of molten magma or lava. Their key characteristics are crystalline textures and the presence of minerals like quartz and feldspar.
  • Sedimentary Rocks: Sedimentary rocks form from the accumulation and compaction of sediments. These rocks often show layering and may contain fossils. Common examples are limestone and sandstone.
  • Metamorphic Rocks: Unlike igneous and sedimentary rocks, metamorphic rocks form by transformation under pressure and heat, leading to new textures and mineral compositions.

Examples of Common Metamorphic Rocks

Here are some examples of metamorphic rocks and their properties:

  • Slate: Derived from shale, a sedimentary rock. Slate has a fine grain and ability to be split into thin sheets..
  • Schist: Characterized by its sheet-like structure and formed typically from mudstone or shale. Its platy minerals are larger than those in slate.
  • Gneiss: Has a banded or foliated appearance, usually formed from high-grade metamorphism of igneous rocks like granite.
  • Marble: Marble forms from limestone or dolomite. It finds use in sculpture and architecture.
  • Quartzite: Formed from sandstone, it’s extremely hard and resistant to weathering.
  • Phyllite: Between slate and schist in terms of metamorphic grade, with a slightly glossy sheen and larger mica flakes than slate.
  • Amphibolite: A metamorphosed basalt or gabbro, characterized by amphibole minerals (like hornblende) and plagioclase feldspar.
  • Mylonite: Formed in fault zones, it is characterized by a layered or banded appearance due to extreme deformation.
  • Greenstone: Altered basalt or gabbro, typically green due to chlorite, actinolite, and other green minerals formed during metamorphism.
  • Eclogite: A high-pressure metamorphic rock, characterized by green pyroxene (omphacite) and red garnet.

How Metamorphic Rocks Form

The transformation into metamorphic rocks involves several key processes:

  1. Heat: Heat is a primary agent in metamorphism. It facilitates the recrystallization of minerals, often without melting the rock. Heat originates from magma or the geothermal gradient, which is the increase in temperature with depth in the Earth’s crust. As temperature rises, the energy reorganizes atoms in minerals, leading to the formation of new minerals. For example, the metamorphic rock gneiss often forms from the heating of shale or granite.
  2. Pressure: Along with heat, pressure plays a crucial role in metamorphism. Its cause deformation and reorientation of minerals. It can be of two types:
    • Confining Pressure: This is uniform pressure applied in all directions, as occurs with the burial of rock beneath other rock layers. It leads to the compaction and reorientation of minerals.
    • Differential Pressure: This occurs when pressure is exerted more in one direction than in the others, often associated with tectonic forces. This type of pressure leads to foliation, a texture where minerals are aligned in planes. Slate, which forms from the metamorphism of shale under differential pressure, is a classic example.
  3. Chemically Active Fluids: Fluids, particularly water with dissolved ions, cause chemical reactions that alter the composition of the rock. They promote the growth of new minerals by enhancing ion migration. For instance, the presence of fluids facilitates the transformation of limestone to marble.
  4. Time: Metamorphism is a process that occurs over geological timescales. The longer the rock is subjected to heat and pressure, the more pronounced the metamorphic transformation.

Specific Examples of Metamorphic Processes

  1. Regional Metamorphism: This occurs over large areas and is typically associated with mountain building where rocks are subjected to high pressures and temperatures. For example, the formation of schist from mudstone or shale typically results from regional metamorphism.
  2. Contact Metamorphism: This happens when rock is heated by the intrusion of hot magma from the Earth’s interior. The rock surrounding the magma gets baked and changes form. An example is the formation of hornfels, which results from the heating of shale or clay by a nearby magma source.
  3. Dynamic Metamorphism: Associated with fault zones where rocks are subjected to high differential pressure. Mylonites, extremely deformed rocks that are often streaked and sliced, are a typical result of dynamic metamorphism.
  4. Burial Metamorphism: Occurs when rocks are buried deeply in the Earth’s crust. They are subjected to increased temperature and pressure due to the weight of overlying rocks. This forms rocks like phyllite, an intermediate-grade metamorphic rock.
  5. Hydrothermal Metamorphism: This involves the alteration of rocks by hot, mineral-rich water. This process creates a variety of metamorphic rocks, such as serpentinite, which forms from the alteration of peridotite in the presence of water.
  6. Shock Metamorphism: Happens due to the impact of meteorites. The intense pressure and heat from the impact can transform rock instantaneously. An example is the formation of shatter cones and other shock-metamorphic features in and around meteorite impact craters.

Properties of Metamorphic Rocks

Since metamorphic rocks start out as other rocks, they share some common properties with their source material. But, they are usually harder than the parent rocks and may or may not have similar mineral composition:

  1. Texture: Can be foliated (layered) or non-foliated.
  2. Hardness: Generally harder than their parent rocks.
  3. Mineral Composition: Varies depending on the parent rock and the conditions of metamorphism.

Classification of Metamorphic Rocks

The classification of metamorphic rocks mainly relates to texture and mineral composition. The texture is either foliated, showing a layered or banded appearance, or non-foliated, where the rock does not exhibit layers.

Two Types of Metamorphic Rocks

  1. Foliated Metamorphic Rocks:
    • Form under directed pressure, leading to the alignment of minerals in layers.
    • Examples include slate, schist, and gneiss.
  2. Non-Foliated Metamorphic Rocks:
    • Form without directed pressure, thus lacking a layered texture.
    • Examples include marble and quartzite.

Textures of Metamorphic Rocks

Textures are in comparison to slate, schist, or gneiss:

  • Slaty Texture: Fine-grained, allows the rock to be split into thin slabs.
  • Schistose Texture: Characterized by larger, visible minerals.
  • Gneissic Texture: Exhibits distinct bands of different minerals.

Uses of Metamorphic Rocks

The uses of metamorphic rocks are diverse, including construction, decorative arts, and industrial applications. For example:

  • Slate: Slate finds wide use for roofing and flooring tiles and writing surfaces due to its durability and natural cleavage into thin sheets. It is also common in outdoor landscaping for walkways and wall cladding.
  • Marble: Prized for its beauty and elegance, marble is important in sculpture and building construction. It’s a favored material for countertops, flooring, and decorative panels in both interior and exterior architecture. Historical buildings like the Taj Mahal and the Parthenon are examples of marble’s use in architecture. Michelangelo’s David consists of marble.
  • Quartzite: Known for its hardness and resistance to weathering, quartzite finds use in road construction, as railway ballast, and in countertops and flooring where durability is a necessity.
  • Soapstone: A type of talc schist, soapstone is popular for countertops, sinks, and stoves due to its heat-resistant properties.

Where to Find Metamorphic Rocks

Metamorphic rocks form primarily at convergent plate boundaries and in mountain ranges due to the immense pressures and temperatures present. They also form at the contact between magma and existing rock (contact metamorphism). While they are widespread, they constitute about 15% to 25% of the Earth’s upper crust.

How to Identify Metamorphic Rocks

Here are some tips for distinguishing them from igneous or sedimentary rocks, as well as for identifying specific types of metamorphic rocks:

Distinguishing Metamorphic Rocks from Igneous and Sedimentary Rocks

  1. Texture and Fabric: Metamorphic rocks often have a foliated or layered texture that is not typical in igneous rocks. Sedimentary rocks might also have layers, but these are usually more uniform and less distorted than those in metamorphic rocks.
  2. Mineral Alignment: Look for minerals that align in a particular direction, which is a common feature in many metamorphic rocks (like schist) due to pressure during formation.
  3. Grain Size: Metamorphic rocks can have a wide range of grain sizes, but the grains are often aligned, unlike the random grain orientation in igneous rocks.
  4. Absence of Fossils: Unlike sedimentary rocks, metamorphic rocks rarely contain fossils because the high pressure and temperature conditions usually destroy any organic material.
  5. Evidence of Deformation: Metamorphic rocks sometimes show signs of bending, folding, or other deformation, which is not typical in igneous rocks.

Identifying Specific Types of Metamorphic Rocks

A hand lens, some hydrochloric acid, and a good field guide make metamorphic rock identification easier:

  1. Slate: Identify by its fine grain and ability to split into thin, flat sheets. It’s usually dull in appearance and ranges in color.
  2. Schist: Look for its shiny surface and visible mineral grains, often mica. Schist is identifiable by its foliated texture and ease of splitting along mineral layers.
  3. Gneiss: Notable for its banded appearance with alternating light and dark mineral layers. It’s harder than schist and does not split as easily.
  4. Marble: Identify by its crystalline texture and hardness. It reacts with dilute hydrochloric acid, effervescing (fizzing) due to the presence of calcite.
  5. Quartzite: Extremely hard and resistant, it has a characteristic glassy luster and grainy texture. It doesn’t scratch easily.
  6. Phyllite: Recognizable by its fine grains and slightly glossy sheen. It’s generally finer than schist but coarser than slate.
  7. Amphibolite: Identify by its dark color and the presence of hornblende and plagioclase minerals. It often appears in long, needle-like crystals.
  8. Mylonite: Look for a well-developed foliation and a streaked appearance due to extreme deformation.
  9. Greenstone: Its green color, due to chlorite and other minerals, is a key identifying feature.
  10. Eclogite: It contains bright green and red minerals – green pyroxene and red garnet.

References

  • Haldar, S. K. (2013). Introduction to Mineralogy and Petrology. Elsevier Science. ISBN 9780124167100.
  • Liu, Liang; Zhang, Junfeng; Green, et al. (2007). “Evidence of former stishovite in metamorphosed sediments, implying subduction to >350 km”. Earth and Planetary Science Letters. 263 (3–4): 181. doi:10.1016/j.epsl.2007.08.010
  • Wicander, R.; Munroe, J. (2005). Essentials of Geology. Cengage Learning. ISBN 978-0495013655.
  • Yardley, B. W. D. (1989). An Introduction to Metamorphic Petrology. Harlow, Essex, England: Longman Scientific & Technical. ISBN 0582300967.