Foliated Metamorphic Rocks

Foliated metamorphic rocks are a type of metamorphic rock that has a banded or layered appearance. This banding is caused by the alignment of mineral grains in the rock, which is a result of the high pressure and directed stress that the rock was subjected to during metamorphism.

The formation of foliated metamorphic rock is a complex process that involves the transformation of existing rock under intense heat, pressure, and sometimes hot mineral-rich fluids. This process, known as metamorphism, occurs deep within the Earth's crust or along the boundaries of tectonic plates, where the conditions are favorable for rock alteration.

When a rock is acted upon by pressure that is not the same in all directions, or by shear stress (forces acting to “smear” the rock), minerals can become elongated in the direction perpendicular to the main stress. The pattern of aligned crystals that results is called foliation.

Foliation can develop in a number of ways. Minerals can deform when they are squeezed, becoming narrower in one direction and longer in another.

Foliated Metamorphic Rocks: Slate --- Phyllite --- Schist --- Gneiss --- Migmatite

Regional metamorphism covers large areas of continental crust typically associated with mountain ranges, particularly those associated with convergent tectonic plates or the roots of previously eroded mountains.

Conditions producing widespread regionally metamorphosed rocks occur during an orogenic event. The collision of two continental plates or island arcs with continental plates produce the extreme compressional forces required for the metamorphic changes typical of regional metamorphism.

These orogenic mountains are later eroded, exposing the intensely deformed rocks typical of their cores. The conditions within the subducting slab as it plunges toward the mantle in a subduction zone also produce regional metamorphic effects, characterised by paired metamorphic belts.

The techniques of structural geology are used to unravel the collisional history and determine the forces involved. Regional metamorphism can be described and classified into metamorphic facies or metamorphic zones of temperature/pressure conditions throughout the orogenic terrane.

Foliation that develops when minerals are squeezed and deform by lengthening in the direction perpendicular to the greatest stress (indicated by black arrows). Left- before squeezing. Right- after squeezing. Photo: Steven Earle

If the parent rock (see Figure , above) was composed of several mineral types and the agents of metamorphism included directed and/or shear stress, a planar fabric called foliation will usually be present in the resulting metamorphic rocks.

Foliation may be expressed as (1) alternating layers of differing mineral composition or (2) parallel alignments of platy minerals. Foliation is usually developed during metamorphism by directed stresses; either in the compressional mode (perpendicular) or as shear (parallel).

In order to understand the development of metamorphic foliation, let's consider what would happen to a shale undergoing increasing temperature and pressure in a convergent plate boundary, for example. Recall that shale is a sedimentary rock , which consists mainly of clays. These clays have formed from the chemical weathering of many of our rock-forming minerals that we have examined in the igneous rocks. The clays are deposited as flat layers of microscopic clay flakes.

As metamorphism begins, the clay particles or flakes are slowly heated and "squished" by the increasing pressure. Generally the pressure imposed on the clay minerals is stronger in two directions than in others, and a compressional force begins to affect the minerals. Lower pressures cause only a slight change in the clay flakes in that they are compressed very close together and become much more densely packed.

Some of the clay flakes may begin to recrystallize into very small crystals of mica minerals. Any water remaining in the clay is driven off. During this process, a more dense platy metamorphic rock called slate will form. This rock very closely resembles shale; the difference can be determined by the fact that slate will "clink" like fine china when it is dropped, and only the shale will smell "muddy" when it is wet. Because the clays have begun to recrystallize into micas in slate, this rock has a microcrystalline texture.

As the process of metamorphism continues, additional pressure and temperature will cause the minerals to continue to recrystallize and become larger. The next stage in this transition will be the formation of a phyllite, which has slightly larger (but still microscopic) mica crystals than the slate. Because of the slightly larger crystals, the microcrystalline phyllite will display a characteristic "sheen" that resembles the shine of satin.

Environments of metamorphism in the context of plate tectonics: (a) regional metamorphism related to mountain building at a continent-continent convergent boundary, (b) regional metamorphism of oceanic crust in the area on either side of a spreading ridge, (c) regional metamorphism of oceanic crustal rocks within a subduction zone, (d) contact metamorphism adjacent to a magma body at a high level in the crust, and (e) regional metamorphism related to mountain building at a convergent boundary

Continuing to increase the temperatures and pressures will allow the minerals to recrystallize into still-larger crystals. At this stage, the mica crystals will become visible on the surface of the rock (as may other minerals), and the rock will be called a schist and have what we describe as a crystalline texture.

Finally, temperatures and pressures may increase to the point that the mica crystals can recrystallize into higher temperature and pressure minerals such as feldspars and amphiboles. These minerals will occur as alternating layers of black and white visible crystals. This type of rock may also form from the metamorphic alteration of a felsic igneous rock. In either case, the rock is called a gneiss (pronounced "nice") and it displays a crystalline texture.

Types of Foliation

Slaty Cleavage

• Appearance: Fine-grained, with wavy or parallel alignment of minerals, often giving the rock a slate-like appearance.
• Formation: Occurs in low-grade metamorphism, with oriented mica and clay minerals creating the banding.
• Significance: Indicates the direction of compressional stress and reveals the early stages of metamorphic transformation.
• Example: Slate, used for roofing, flooring, and decorative purposes.
  

Schistosity

• Appearance: Well-developed foliation with prominent mineral bands, allowing the rock to be easily split into thin sheets.
• Formation: Develops under moderate to high metamorphic grades, with larger crystals of mica, amphibole, and chlorite contributing to the distinct layering.
• Significance: Reveals the intensity of metamorphism and provides clues about the rock's original composition.
• Example: Schist, found in mountain ranges like the Himalayas and Alps, used for building and decorative purposes.
  

Gneissic Foliation

• Appearance: Coarse-grained with alternating bands of light and dark minerals, often with complex folding and contortions.
• Formation: Characteristic of high-grade metamorphism, where recrystallization and segregation of minerals create pronounced banding.
• Significance: Offers insights into the complex geological history of the rock, including potential multiple metamorphic events.
• Example: Gneiss, forming the bedrock of regions like Scandinavia and Canada, used for building and dimension stone.
  

Lineation

• Appearance: Not a true foliation, but a secondary feature with elongated mineral grains or aggregates arranged in a preferred direction.
• Formation: Can occur alongside any type of foliation due to shearing stress, creating streaks or striations on the rock surface.
• Significance: Provides additional information about the direction of movement and deformation during metamorphism.

Other Foliation Types

• Augen Gneiss: Gneiss with rounded augen (eyes) of feldspar surrounded by darker mafic minerals.
• Milonitic Foliation: Fine-grained and strongly sheared foliation developed during high-strain, low-temperature metamorphism.
• Phyllitic Foliation: Transitionary stage between slaty cleavage and schistosity, with larger mica crystals but still fine-grained.

Examples of Foliated Metamorphic Rocks

Foliated metamorphic rocks are found worldwide, adorning mountain ranges, ancient shield regions, and even the cores of orogenic belts.

Some common examples include:

Slate

The finest-grained, often with a smooth, slate-like appearance.

• Formation: Slate commonly arises from the low-grade metamorphism of shale or volcanic ash. The intense pressure aligns the fine-grained clay and mica particles, creating a wavy or parallel arrangement.
• Characteristics: Slate is typically fine-grained, with a dark grey or black color. It breaks easily along the plane of foliation, making it ideal for roofing, flooring, and decorative purposes.
• Examples: The iconic Penrhyn Slate Quarry in Wales is a testament to the beauty and utility of this rock. Roofing slates from Vermont and Pennsylvania have adorned historical buildings for centuries.

Schist

A coarser-grained rock with a pronounced banding due to the aligned minerals.

• Formation: Schist forms from a variety of parent rocks, including shale, carbonates, and mafic igneous rocks. Under moderate to high-grade metamorphism, minerals like mica, chlorite, quartz, and hornblende align, creating prominent bands or foliation.
• Characteristics: Schists are typically medium- to coarse-grained, with a sparkly appearance due to the presence of mica. They exhibit a well-developed foliation, allowing them to be easily split into thin sheets.
• Examples: The Scottish Highlands are renowned for their diverse schist formations, including the iconic Moine Schist. The Himalayas and Alps also boast impressive schist outcrops, often containing valuable minerals like garnets and kyanite.

Gneiss

A coarse-grained rock with banded gneissosity,

• Formation: Gneiss originates from high-grade metamorphism of various rock types, including igneous and sedimentary rocks. Intense pressure and heat cause complex mineral segregation and recrystallization, leading to alternating bands of light and dark minerals.
• Characteristics: Gneiss is typically coarse-grained with a banded or streaky appearance. The foliation can be complex, with folds and contortions often evident. Gneiss is a durable rock, widely used for building materials and dimension stone.
• Examples: The Canadian Shield, a vast Precambrian rock formation, is dominated by gneissic rocks. The Lewisian Gneiss of Scotland is another notable example, showcasing the intricate patterns and textures of this rock type.