3.7 Metamorphic Rocks

Charlene Estrada

Layers of rock in an outcrop are folded together like an accordion. These types of folds are possible due to metamorphism. This
Figure 3.7.1. Limestone and chert layers of rock that have been folded at high temperatures and pressures due to metamorphism.

The word “metamorphic” is Greek: meta means change; morphos means form. When rock units are buried very deeply within Earth’s , they are subjected to high temperatures and pressures. These rocks are squeezed and warped like putty in a process called ; some of these rocks grow new s and textures and others lose minerals. The result of this high temperature and pressure transformation is a rock.

Metamorphic rocks can involve high temperatures, but unlike rocks, they do not melt into . They also involve the transformation of one rock type into another, but unlike rocks, they do not reduce the original rock into fragments before reassembling. Metamorphism is a unique process that takes any type of preexisting rock (even old metamorphic units) and subjects it to heat and pressure over long periods of time until it has changed.

Metamorphism usually happens at least a couple kilometers beneath the Earth’s surface. Tectonic boundaries, in particular, cause different types of metamorphism at mountain-building cores, zones, and . The metamorphic rocks that form at elevated temperatures and pressures are categorized by both metamorphic texture and the amount of change that the original rock appears to have undergone: metamorphic grade.

Metamorphic Texture

Metamorphic texture describes the shape and orientation of mineral grains within a metamorphic rock. As the original rock is subjected to higher temperatures and pressures, some of its minerals might stretch out in a single direction, , or enlarge. Therefore, the new metamorphic rock will have a different texture than the .

Metamorphic texture is broadly categorized as either or .

Foliated texture

is a term that describes how minerals line up along a preferred direction. Some minerals, particularly s, are usually thin and planar by default. Rocks with texture look like they have their minerals stacked together as though they were pages in a book; hence the term “folia”, or leaflike.

This rock formation contains rock with wavy, stacked layers that are thin and easily break apart.
Figure 3.7.2. Foliated texture can look like very thin horizontal layers in rock that appear like they can peel apart like leaves, like in this mica-schist formation.

Why do foliated rocks form during ? At least two conditions need to be satisfied. First, the original rock must contain minerals that will easily deform or align with applied pressure into a flat plane. We know that mica minerals such as and will do this as well as ; however, stronger minerals like and will often resist most pressure in the Earth’s .

 

This metamorphic rock shows striping or white and black banding of its minerals.
Figure 3.7.3. Mineral banding in the high-grade metamorphic rock gneiss. This segregation of minerals together is a type of foliation.

The second requirement needed for a foliated texture is directed pressure called . If pressure is applied unevenly, the weak minerals in the original rock will easily deform into long planes. This type of stress could be squeezing (), stretching (), or sliding () the rock unit.

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.
Figure 3.7.4. 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.

Non-Foliated texture 

Tan-colored rock with a granular, interlocking crystalline texture
Figure 3.7.5. Non-foliated texture does not have any distinct layering or banding. Instead, the rock appears granular or crystalline, as seen in the rock quartzite.

metamorphic rocks do not have any preferential alignments of mineral grains. These rocks are also called ““, which references the tendency of the individual grains to have somewhat equal shapes and dimensions. The majority of a non-foliated metamorphic rock contains mostly one mineral. Nonetheless, has still taken place within these rocks; the mineral grains have , interlocked, and grown larger. As a consequence, non-foliated rocks are much more durable and resistant to weathering than their parent rocks.

Metamorphic Grade

Video 3.7.1. What does a rock look like as it undergoes increasing grade in metamorphism? (1:22).

Metamorphic Grade refers to the extent to which metamorphism can transform the preexisting rock. This original rock is called the , and it can undergo low-grade metamorphism (little metamorphic change) to high-grade metamorphism (significant metamorphic change).

Low-grade metamorphism begins at temperatures and pressures that are not much higher than those that form rocks. This type of metamorphism often results from rocks being buried at depths of at least 2 km. The conditions in such an environment are typically low temperature and pressure. The parent rock still transforms into a new metamorphic rock; however, it is often easy to identify visual similarities between the resulting metamorphic rock and original rock.

High-grade metamorphism requires both high temperatures and pressures. Burial depths for high-grade metamorphic rocks can be up to 35 km! Mountain-building centers and zones are prime examples of regions where high-grade metamorphism might take place. When a parent rock undergoes metamorphism at these conditions, the resulting metamorphic rock bears very few similarities to the original specimen; it often has strong or banding of minerals.

Metamorphic rocks are often not restricted to either low or high-grade. Geologists typically rank these rocks on a scale of how many (or few) similarities they bear to the parent rock. In a sequence or outcrop filled with metamorphic rocks, you might observe some units that appear similar to the parent rock and others that look nothing like it. In general, the following rocks have been ranked in order of increasing metamorphic grade:

Metamorphic Rock Field Guide

Slate

Slate interactive model
Figure 3.7.6. Slate. Click on this image to go to a 3D interactive model by rocksandminerals CC BY.

“SLAYTE”

Most commonly confused with: ,

A , low-grade metamorphic rock. is fine-grained and composed of s, and minerals that are usually too small to see with the naked eye. Slate displays strong foliation in thin sheets or layers that sometimes resemble bedding or the linae of . It is usually dark gray, but it can also be red, green, brown, and even blue.

Although slate often forms from shale and bears a strong resemblance to this , its foliation pattern is typically more pronounced. Slate is also sometimes confused with a higher grade metamorphic rock, , which contains visible grains of mica minerals.

Slate will easily break into sheets along its foliation planes, but the minerals that compose its fine-grained structure make it durable against processes directed perpendicular to the foliation line. Slate is therefore used as a building material for roofs and tiles in construction.

Phyllite

Phyllite interactive model
Figure 3.7.7. Phyllite. Click on this image to go to a 3D interactive model by Dr. Parvinder Sethi CC BY.

“FY-LITE”

Most commonly confused with: ,

A , low to medium-grade metamorphic rock. is fine-grained and composed mostly of , , and visible flakes of minerals. These enlarged mica crystals give phyllite a shimmering or silky appearance under light. Phyllite, like , is foliated with thin sheets that sometime separate into uneven layers that give an individual rock a wavy appearance.

The color of phyllite is often gray, black, tan, or green, and it is sometimes confused for either slate or . Phyllite can generally be thought of a version of slate that has undergone more metamorphism; it can be distinguished from slate by its visible, glittering mica minerals. By contrast, phyllite can very broadly be thought of as the lower-grade version of schist. Schist has much larger, foliated plates of mica.

Schist

Video 3.7.2. Let’s observe a garnet schist. The sample is foliated. The shiny minerals are mica while the dark red grains are garnet (0:31).

“SHIST”

Most commonly confused with:

A , medium-grade metamorphic rock. contains large, leaflike grains of such as and , that are strongly oriented into a single direction. Some varieties of schist have s, which only form at elevated temperatures and pressures, although most schists also contain and . Due to the presence of large mica plates and their flattened orientation, schist is usually shiny or under light.

Schist is often gray or brown in color, and it is sometimes confused with , which contains smaller grains of mica. Schist can also be identified by its strong foliation pattern, which is called “schistosity”. This pattern is much more pronounced than lower grade metamorphic rocks such as and phyllite.

Gneiss

Gneiss Interactive Model
Figure 3.7.8. Gneiss. Click on this image to go to a 3D interactive model by Sara Carena CC BY-NC.

“NICE”

Most commonly confused with: ,

A foliated, high-grade metamorphic rock. has a visible separation of light and dark bands, which is called . Gneiss is coarse-grained and mostly contains minerals that are resistant to high temperatures such as , , , and . The banding pattern on gneiss is usually wavy or folded, which reflects how the parent rock deformed like putty at extremely high temperatures and pressures; indeed, sometimes the rock partially melts.

Gneiss is occasionally mislabeled as the rock or . Although gneiss may contain similar silicate minerals as these rocks, it is distinctively banded, whereas the igneous rocks are granular and have no preferred orientation. Because gneiss is not easily broken into sheets, it is a useful construction material in landscaping and architecture.

Marble

Marble Interactive Model
Figure 3.7.9. Marble. Click on this image to go to a 3D interactive model by EDUROCK – EDUCATIONAL VIRTUAL ROCK COLLECTION CC BY.

Most commonly confused with:

A metamorphic rock. forms under low grade or high-grade metamorphism, although at the latter it will grow larger, more interlocked crystals that reflect its higher temperature and pressure origins. The parent rock of marble is , although marble typically appears to have more identifiable grains. Marble contains the mineral and/or , and it may fizz with dilute hydrochloric acid.

Marble is light in color, and it can be white, pink, tan, or gray. Some varieties of marble look similar to another non-foliated metamorphic rock, . However, the minerals calcite and dolomite that primarily compose marble are much softer than those found in , and marble is easier to scratch with an iron nail.

Although marble that contains calcite will react to acidic rainwater and groundwater over long periods of time, marble has traditionally been a sculpting material in art and architecture for thousands of years. Polished material remains a popular building and decorative material to this day.

Quartzite

Quartzite interactive model
Figure 3.7.10. Quartzite. Click on this image to go to a 3D interactive model by rocksandminerals CC BY.

Most commonly confused with: ,

A , high-grade metamorphic rock. only forms from the rock , and it almost exclusively contains . The high temperatures and pressures in the metamorphic environment have caused the individual quartz grains to increase in size and interlock.

Quartzite is usually light tan or pink in color and coarse-grained. Like other rocks that predominately contain , it can display , although it is very resistant to mechanical breakage. Although this rock can look similar to marble, it will not fizz in contact with dilute hydrochloric acid, and it will not easily scratch.

Quartzite is also more durable than its parent rock, sandstone, which weakly sand grains together. For example, if sandstone is hit with a rock hammer, the individual grains will remain intact and the cement would crumble. If quartzite is hit with a rock hammer, it would probably spark from the friction and cause the hammer to bounce back, and, thus, is NOT recommended!

Quartzite will resist both and . As such, this rock has been used for thousands of years as a tool, construction, manufacturing, and architectural material.