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

Metamorphic rocks can involve high temperatures, but unlike igneous rocks, they do not melt into magma. They also involve transforming one rock type into another, but unlike sedimentary 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 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, subduction zones, and spreading centers. 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, recrystallize, or enlarge. Therefore, the new metamorphic rock will have a different texture than the parent rock.

Metamorphic texture is broadly categorized as either foliated or non-foliated.

Foliated texture

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

Why do foliated rocks form during metamorphism? 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 muscovite and biotite will do this as well as amphibole; however, stronger minerals like quartz and feldspar will often resist most pressure in the Earth’s crust.

The second requirement needed for a foliated texture is directed pressure called differential stress. 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 (compression), stretching (tensional), or sliding (shear) the rock unit.

Non-Foliated texture 

Non-foliated metamorphic rocks do not have any preferential alignments of mineral grains. These rocks are also called “Granoblastic”, which references the tendency of the individual grains to have somewhat equal shapes and dimensions. The majority of non-foliated metamorphic rock contains mostly one mineral. Nonetheless, metamorphism still occurs within these rocks; the mineral grains have recrystallized, 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. 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 parent rock, 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 sedimentary 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 subduction zones are prime examples of regions where high-grade metamorphism might take place. When a parent rock undergoes metamorphism under these conditions, the resulting metamorphic rock bears very few similarities to the original specimen; it often has strong foliation or banding of minerals.

Metamorphic rocks are often not restricted to either low or high grades. Geologists typically rank these rocks based on 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

“SLAYTE”

Most commonly confused with: shale, phyllite

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

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

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

Phyllite

“FY-LITE”

Most commonly confused with: slate, schist

A foliated, low to medium-grade metamorphic rock. Phyllite is fine-grained and composed mostly of quartz, feldspar, and visible flakes of mica minerals. These enlarged mica crystals give phyllite a shimmering or silky appearance under light. Phyllite, like slate, 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 schist. Phyllite can generally be thought of as 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. 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: phyllite

A foliated, medium-grade metamorphic rock. Schist contains large, leaflike grains of mica such as muscovite and biotite, that are strongly oriented in a single direction. Some varieties of schist have garnet, which only forms at elevated temperatures and pressures, although most schists also contain quartz and feldspar. Due to the presence of large mica plates and their flattened orientation, schist is usually shiny or vitreous (glassy) under light.

Schist is often gray or brown in color, and it is sometimes confused with phyllite, 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 slate and phyllite.

Gneiss

“NICE”

Most commonly confused with: granite, diorite

A foliated, high-grade metamorphic rock. Gneiss has a visible separation of light and dark bands, which is called lineation. Gneiss is coarse-grained and mostly contains silicate minerals that are resistant to high temperatures such as quartz, feldspar, biotite, and garnet. 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 igneous rock granite or diorite. 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

Most commonly confused with: quartzite

A non-foliated metamorphic rock. Marble 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 limestone, although marble typically appears to have more identifiable grains. Marble contains the mineral calcite and/or dolomite, which 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, quartzite. However, the minerals calcite and dolomite that primarily compose marble are much softer than those found in quartz, 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, 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

Most commonly confused with: marble, sandstone

A non-foliated, high-grade metamorphic rock. Quartzite only forms from the sedimentary rock sandstone, and it almost exclusively contains quartz. 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 silica, it can display conchoidal fracture, 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 cements 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!