Metamorphic rocks are some of the most varied and complex rocks found in nature. Because they can be formed from any other type of rock, there is almost no limit to their variety. Being able to properly identify a metamorphic rock can tell you a lot about its geologic history.
There are many different types of metamorphic rocks, and if you know what to look for it can be fairly easy to identify them and distinguish them from other rock types. Metamorphic rocks are typically classified first by their texture and then by their mineral composition and protolith (the rock type they formed from). Identifying a metamorphic rock can sometimes be very challenging, so it helps to know what metamorphic rocks typically look like.
In general, metamorphic rocks appear dense and crystalline, often with characteristic foliation or banding. Most metamorphic rocks are fairly dull in color, but they can also display more vibrant colors such as red, yellow, and green depending on their mineral content. Structures like fossils and gas bubbles are never present.
Properly identifying a metamorphic rock can be significantly more challenging than it is for sedimentary or igneous rocks. Some identifications are straightforward, but others require a very methodical approach. I’ll walk you through how to go about it and provide some useful descriptions, pictures, and tables for you to reference.
How to Identify Metamorphic Rocks
Identifying metamorphic rocks can be a tricky business. A metamorphic rock can be formed from any rock type, be it igneous, sedimentary, or even another metamorphic rock. Not only that, but the original rock can be changed into several different metamorphic rock types depending on the conditions it has been subjected to!
Metamorphic rocks form by subjecting a rock (protolith) to pressure, temperature, and chemically active fluids. When identifying metamorphic rocks, it is common for geologists to focus on specific ‘indicator minerals’ that serve as clues about the degree of metamorphism a rock has undergone.
To identify an igneous rock, first identify its texture. Then, perform basic tests on the rock observing its color, hardness, fracture tendency, and acid reaction. Look for any easily identifiable minerals such as quartz, garnet, or chlorite. Lastly, compare your findings to known metamorphic rock types.
There are many metamorphic rock types (which are described in detail below) but it helps to first have a basic understanding of how protoliths (the original rocks) change with varying degrees of metamorphism. Below is a table showing how increasing temperatures and pressures can change a single protolith type into different metamorphic rocks.
|Protolith||Very Low Grade||Low Grade||Medium Grade||High Grade|
|Felsic Igneous (Granite)||No change||No change||No change||Gneiss|
|Mafic Igneous (Basalt)||Greenschist||Greenschist||Amphibolite||Granulite|
|Sandstone||No change||No change||Quartzite||Quartzite|
Pictures & Descriptions of Metamorphic Rock Types
There are so many types of metamorphic rocks that it can be hard to keep track of them all, but thankfully things are made a little easier by dividing them into two general groups – foliated and non-foliated (or granular). These groups are based on the shape and orientation of the metamorphic rock’s grains and, in general, all of the rocks within each group will tend to have similar characteristics.
I have listed and described many varieties of metamorphic rocks below, but not everyone agrees on certain definitions or classifications. For some rock types, there are also several sub-varieties that could easily use their own descriptions. For ease of reference and identification, I have kept things fairly general while still covering all the metamorphic rock types.
Foliated metamorphic rocks are one of two major groups of metamorphic rocks. Foliation describes the general texture of a metamorphic rock as having layers or sheets formed from the preferential orientation of minerals during the metamorphic process.
There are four main types of foliation, all of which describe a specific type of texture and are an indicator of the degree of metamorphism a rock has undergone.
- Slaty – Parallel layering of fine-grained, platy minerals
- Schistose – Parallel layering of coarse-grained minerals
- Gneissic – Alternating parallel bands of platy minerals and coarse-grained minerals
- Migmatite – Extreme end of gneissic, mixed with igneous textures
|Texture||Degree of Metamorphism||Example|
|Migmatite||Extreme (almost igneous)||Migmatite|
Like all metamorphic rocks, foliated metamorphic rocks form from extreme pressures and temperatures acting on existing rocks. As pressures and temperatures increase, the degree of metamorphism increases correspondingly. The same original rock (the protolith) is therefore able to end up as several types of metamorphic rock depending on the conditions it is subjected to.
Foliated rocks are usually formed when there is a significant amount of differential pressure acting on the rock. Put simply, the rock is being squeezed much harder in one direction than in any others. The layers or banding in foliated rocks form perpendicular to the direction of maximum compression.
Slate is a foliated, fine-grained metamorphic rock with a slaty texture of thin, flat sheets. It has a dull luster, and is most often gray, green, black, brown, or red. Slate is formed when clay-rich shale or mudstone is subjected to low degrees of metamorphism.
Interestingly, the orientation of the slaty cleavage beds in slate are usually not the same as those of the original shale. Instead, the slaty beds are formed perpendicular to the direction of maximum stress during metamorphism.
At higher degrees of metamorphism due to increased heat and pressure, slate grades in phyllite.
Phyllite is a foliated, fine-grained metamorphic rock with slaty or weakly schistose texture. It has a shiny luster and its crystals may or may not be large enough to see with the naked eye. It is typically gray, green, black, brown, or red.
Phyllite is formed from clay-rich shales or mudstones when slate undergoes further metamorphism. It is usually composed of quartz, mica, and chlorite, and much of its appearance is the result of the fine-grained mica flakes aligning parallel to one another.
At higher degrees of metamorphism, phyllite grades into schist.
Schist is a foliated, coarse-grained metamorphic rock with a schistose texture, meaning it appears scaly or flaky and can be easily broken into thin sheets. It is primarily composed of micas, quartz, and feldspar, but may also contain visible amounts of accessory minerals such as staurolite, garnet, and kyanite.
The mineral grain sizes and distributions of schist give it a fairly uniform appearance when compared to rocks like gneiss. While you can easily see flakes or plates in the rock, there is very little or no visible banding.
At higher degrees of metamorphism, schist grades into gneiss.
Gneiss is a foliated, coarse-grained metamorphic rock with characteristic bands of parallel oriented minerals known as gneissic texture. These bands are easily visible to the naked eye, but lack the schistose tendency to break along the parallel planes.
Gneiss is composed of micas (typically biotite), quartz, and feldspar. The gneissic banding is a result of the lighter colored minerals (quartz and feldspar) separating from the mica into parallel layers.
Interestingly, gneiss can actually form from two protolith types. It can be formed from the continued metamorphism of schist (with a shale protolith) or from felsic volcanic rocks such as granite. Both scenarios require a high degree of metamorphism.
Blueschist is a fine-grained foliated metamorphic rock with fibrous crystals and a characteristic blue color, sometimes grading to purple. It may also display some schistose texture. The blueish color is a result of a high glaucophane and/or lawsonite content along with chlorite.
Blueschist forms from the metamorphism of mafic rocks such as basalt. It may rarely (as pictured) display a porphyritic texture with larger crystals of minerals like garnet.
Greenschist is a medium-to-coarse-grained foliated metamorphic rock with a schistose texture, meaning it has scales or flakes that can be easily broken apart. It typically contains large amounts of minerals of chlorite, epidote, and serpentine which give it its characteristic greenish color.
Unlike other schists, greenschist forms from low-grade metamorphism of basaltic igneous rocks. At higher degrees of metamorphism, it grades into amphibolite.
Amphibolite is a weakly foliated metamorphic rock, but may also be granular. It is characterized by the mottled ‘salt and pepper’ appearance of its contrasting light and dark minerals.
Amphibolite is dominated by the mineral amphibole, but often contains other minerals such as feldspar, epidote, and garnet.
Most amphibolite forms from the further metamorphism of greenschist, meaning their protolith is basaltic igneous rock. However, with the proper rock chemistry it is also possible to form amphibolite from a protolith of dolomite or marl.
Granulite is a coarse-grained metamorphic rock composed primarily of quartz and feldspar with very little mica. It is often spotted with phenocrysts (larger crystals) of garnets or pyroxenes. While granulite is usually foliated, it may also appear gneissic or even granular.
Granulite forms when mafic igneous rocks like basalt are exposed to high degrees of regional metamorphism deep in the earth. At lower degrees of metamorphism it grades into amphibolite.
Granulite can be a very attractive rock and is often used for countertops (pictured). Unfortunately, it is usually mislabeled as ‘granite’ when being sold to consumers.
Non-foliated metamorphic rocks are one of two major groups of metamorphic rocks. Non-foliated metamorphic rocks appear granular, with no apparent banding or layering. They tend to form when the protolith is being squeezed equally from all sides.
The mineralogy of non-foliated rocks tends to be simpler than that of foliated rocks. Most non-foliated metamorphic rocks are primarily comprised of just one or two minerals.
Eclogite is a non-foliated, predominantly bi-mineralic metamorphic rock consisting of garnet and pyroxene. It is easily identifiable by its relatively large, red garnet crystals set in a matrix of fine-grained, light-green pyroxene. It may also have smaller amounts of accessory minerals like kyanite, rutile, and quartz.
Eclogite forms when mafic igneous rocks such as basalt are subjected to extremely high pressures are lower temperatures compared to amphibolites and granulites. It is relatively rare to find on the Earth’s surface.
Quartzite is a non-foliated metamorphic rock consisting almost entirely of interlocking quartz crystals. It is usually off-white or gray, but may instead be shades of pink, red, green, blue, or yellow due to the presence of impurities like hematite.
Because quartzite is made up almost entirely of quartz crystals it has a shiny, glassy appearance when a fresh surface is exposed. It feels rough and gritty to the touch. It will scratch glass in a hardness test.
Quartzite is formed from medium to high-grade regional metamorphism of sandstone. The quartz grains (and any accessory minerals) in the sandstone partially melt and recrystallize to form interlocking crystals, all approximately equal in size. This process eliminates virtually all porosity and sedimentary structures from the original sandstone.
Hornfels is a group of non-foliated, massive metamorphic rock with a wide range of mineral compositions. It is almost always dull gray, black, or brown in color. It may display some color banding from varying rock types in the protolith, not as a result of metamorphism.
Hornfels forms at high temperatures and very low pressures, almost always during contact metamorphism when magma intrudes towards the surface and comes into contact with rocks like sandstone, shale, and limestone.
Hornfels is usually fine-grained and very dense. Even though it often forms from sedimentary rocks like sandstone and shale (and may retain coloring from those sedimentary layers) it tends to break conchoidally, not along any previously existing bedding surfaces.
Serpentine is a non-foliated metamorphic rock composed primarily of serpentine minerals which give the rock its distinct, characteristic yellowish-green color. It feels greasy to the touch, and is softer than glass but harder than a fingernail.
Serpentinite is formed at relatively low temperatures when ultramafic rocks like dunite are hydrated and oxidized to form minerals like brucite and magnetite. This set of circumstances occurs most often at oceanic ridges and hydrothermal vents.
Because serpentine is so unique looking, attractive, and relatively soft, it is a popular rock for artists to make sculptures and carvings.
Soapstone is a fine-grained, non-foliated metamorphic rock composed almost entirely of talc with smaller amounts of chlorite and amphibole. It is usually white but can be gray, yellow, orange, or brown depending on any impurities.
Soapstone has a soapy feel from which its name is derived, and it is so soft (Mohs hardness scale of ~1) that can be scratched with a fingernail.
Soapstone can be created either through the metamorphism of ultramafic rocks like dunite or the hydrothermal metamorphism of certain types of dolomite.
Because soapstone is soft yet durable it has traditionally been very popular for carving and sculpture.
Marble is a coarse-grained, non-foliated metamorphic rock primarily composed of carbonate minerals. It is usually white or off-white, but if impurities are present it may also contain colors like pink, gray, blue, or green.
Marble is formed from the low-grade metamorphism of limestone, and has a very similar mineralogy consisting primarily of calcium carbonate. The original sedimentary structures of the limestone are usually entirely destroyed during metamorphism, leaving only tightly fused crystals in the marble.
Because marble is primarily made of calcium carbonate, it readily fizzes and reacts with a hydrochloric acid test. It is harder than a fingernail but softer than glass.
Dolomitic Marble is a coarse-grained, non-foliated metamorphic rock composed primarily of dolomite. It is usually white or off-white, but if impurities are present it may also contain colors like pink, gray, blue, or green.
Dolomitic Marble is formed from the low-grade metamorphism of dolomite. Any existing sedimentary structures in the dolomite are usually entirely destroyed during metamorphism, leaving only tightly fused crystals in the marble.
Because dolomitic marble is primarily made of dolomite, it will weakly fizz with a hydrochloric acid test, but only after being scratched or powdered. It is harder than a fingernail but softer than glass.
How are Metamorphic Rocks Classified?
The classification of metamorphic rocks can be a daunting task but, like with sedimentary and igneous rocks, a methodical approach simplifies the process and will usually take you to the right conclusion.
Classifying a metamorphic rock may seem daunting if you don’t have much experience doing it. There are so many types of metamorphic rocks and they can be difficult to distinguish from one another, or even from other gock types. However, identifying a metamorphic rock is usually fairly straightforward and simple once you know how it’s done.
In general, metamorphic rocks are classified first by their texture as either foliated or non-foliated and then by their mineral composition. In common practice, texture is judged by the overall appearance and feel of a rock, and mineralogy can be estimated by visual inspection and basic field tests.
While sophisticated laboratory testing or modeling is often necessary for nuanced classification, identification for most practical purposes can be done visually in the field or at home.
The first step to classifying your metamorphic rock is to determine if it is foliated or non-foliated. Look for any signs of parallel sheeting, layering, banding, or orientation of crystals.
If your rock is foliated, I suggest following the very helpful flowchart found here. Compare your rock to the pictures and descriptions I have listed above.
Similarly, if your rock is non-foliated I would recommend following this flowchart to arrive at a classification.
Metamorphic Rock FAQs
What Color are Metamorphic Rocks?
It is common for aspiring amateur geologists to want to identify metamorphic rocks (or any rock, really) by their color. While color is certainly very useful for classifying metamorphic rocks, it is far from the only relevant factor. It can be helpful to know what colors are common in metamorphic rocks to help get you started.
In general, metamorphic rocks are usually off-white or dull shades of gray, black, or brown. However, they may also be bright shades of blue, green, pink, and many other colors. Many metamorphic rocks are characterized by alternating colors caused by the separation of different minerals.
Do Metamorphic Rocks Have Layers?
Metamorphic rocks are some of the most interesting-looking rocks in nature thanks in large part to the prevalence of banding and foliation. One common question that many people have is whether or not metamorphic rocks have layers.
Some foliated metamorphic rocks may appear to have layers due to schistose or gneissic banding and alternating colors. These bands are not layers in the depositional sense, but rather a more two-dimensional separation of minerals. Non-foliated metamorphic rocks do not appear to have layers.
Do Metamorphic Rocks Have Fossils?
You might be wondering if it is possible for metamorphic rocks to contain fossils. After all, since metamorphic rocks form from the alteration of other rocks it might seem to make sense that some of them would retain the fossils of their protoliths.
In general, metamorphic rocks do not contain fossils. Some metamorphic rocks form from the alteration of fossil-bearing sedimentary rocks, but any fossils are destroyed through the intense heat and pressure of the metamorphic process. In rare cases, some rough evidence of fossils may still be discernible.
Fossils are only found in sedimentary rocks. When sedimentary rocks are subjected to extreme heat and pressure and begin to become metamorphosed, the sedimentary structures (including fossils) are wiped away.
Do Metamorphic Rocks React With Acid?
An acid test with a hydrochloric acid solution is a common test performed by geologists do aid in rock identification. It is meant to detect the presence of calcium carbonate, since it readily reacts and fizzes with the acid. But do metamorphic rocks react with acid?
In general, metamorphic rocks do not react with acid because they are composed of minerals other than calcium carbonate. The notable exception is marble (metamorphosed limestone) which will readily react with acid since it is primarily made of calcium carbonate. Dolomitic Marble will also react with acid if scratched.
Do Metamorphic Rocks Have Gas Bubbles?
Many people closely associate metamorphic rocks with igneous rocks. Since some igneous rocks like pumice, scoria, and basalt contain visible gas bubbles, it is natural to wonder if some metamorphic rocks contain gas bubbles as well.
Metamorphic rocks do not contain gas bubbles. Metamorphic rocks are formed under extreme heat and pressure, leaving no room for gas bubbles to form or solidify during recrystallization. Any gas bubbles present in the protolith would be destroyed by the metamorphic process.
Are Metamorphic Rocks Hard or Soft?
In general, metamorphic rocks are very hard. They usually have high quartz and feldspar content which are a 7 and 6 on Mohs hardness scale, respectively. The crystals of metamoprhic rocks are interlocking and fused together, making them very competent and difficult to break.
It is common for geologists to refer to metamoprhic (and igneous) rocks as ‘hard rocks’, while sedimentary rocks are called ‘soft rocks’. These names are just a result of volcanic and metamorphic rocks generally being harder than most sedimentary rocks, but there are exceptions. If someone says they are a ‘hard rock’ geologist it means that they focus on igneous or metamorphic rocks, often related to gemology or mining.
This post is part of my rock identification series. If you want to keep reading about how to identify more rock types, this post should be next on your list.