Slate is one of the most common and recognizable metamorphic rocks in the world. Chances are good that you have seen and touched slate many times in your life, whether or not you were aware of it at the time. It has many applications in our day-to-day lives and comes in a surprising array of colors.
Slate can sometimes be fairly difficult to identify, partly because some people aren’t overly familiar with it and partly because it is often confused for other rock types. This is perfectly understandable because many rocks can superficially look very similar and share many of the same physical properties. It can be difficult to know exactly what slate looks like and how to identify it, but it is relatively simple if you know what to look for.
Slate is a very fine-grained foliated metamorphic rock primarily made of quartz and platy clay minerals like illite and chlorite. Accessory minerals like hematite and pyrite are common. Slate forms from regional metamorphism of mudstones and displays characteristic slaty cleavage, easily splitting into thin slabs.
While slate is a clearly defined rock type, there are many different varieties and closely related rocks that can appear quite different from one another. I’ll walk you through how to identify slate, what it looks like, and where it can be found.
What Does Slate Look Like?
It can often be very difficult for people to identify slate, even with pictures and descriptions to compare to. This is because slate isn’t primarily defined by its mineralogy, but rather by its texture.
In everyday life, it is relatively unimportant to be able to distinguish between slate and other similar-looking rocks, so people often just dismiss the exercise entirely. But if you’re actually interested in geology and knowing what kind of rock you’re looking at you probably want to be more specific than that. So, what does real slate actually look like?
Slate is very fine-grained, made of minuscule mineral crystals invisible to the naked eye. It displays slaty cleavage and breaks easily along flat surfaces. It is usually light to dark gray but comes in many colors including green, reddish, and purple. Larger crystals of minerals like pyrite may be present.
Slate can be composed of many different types of minerals but is primarily made of quartz and platy minerals like clays and micas. Slate is not defined by its mineralogy so much as it is by its slaty texture.
While slate doesn’t have a strictly defined mineral composition, in practice there are some minerals that are almost always found in abundance. Quartz and platy minerals like muscovite, illite, chlorite, and graphite are the most common minerals in slate, but the mineral grains are far too small to see with the naked eye. However, you may be able to see some individual grains with the aid of a powerful hand lens like this one.
While all slates meet this general description, there is a pretty wide spectrum of slates that can look significantly different from one another. The difference in their appearance is driven by the mineralogy of each type of slate.
Texture of Slate
Since a slate is primarily defined by its texture, it first helps to know what slaty texture looks like.
Slaty texture is the lowest grade of metamorphic foliation. It describes the parallel orientation of very small crystals of platy minerals in a rock, making it easy to break the rock along those planes.
I’ll dive deeper into how slate forms later in this article, but slaty texture occurs when very fine-grained mudrocks like shale are compressed during low-grade metamorphism.
One often-overlooked aspect of slaty texture is crystal size. In order for a rock to be considered slate, you shouldn’t be able to see individual crystals with the naked eye. If you can see the crystals then you may very well be looking at schist, which forms from higher degrees of metamorphism.
Similarly, slate has not yet developed visible alternating bands of minerals like quartz and feldspar. A banded texture like that is known as gneissic and is the next phase of metamorphism beyond schist. Slate looks like a homogenous mass of rock with no visible layers or banding.
Color is Driven by Mineralogy
The variation in the mineralogy of slate means that it can come in a wide variety of colors. The dominant platy mineral often has the biggest impact on color, but it is also affected by the amount of quartz, feldspar, and other accessory minerals present in the rock.
Most slate is light or dark gray. It gets this color from the mica minerals from which it is usually formed – namely, muscovite and illite. When combined with tiny quartz crystals, which are usually off-white or translucent, the rock ends up with a rather nondescript gray color.
Chlorite has a distinct green color and is sometimes a major component of slate. Many other colors are possible including off-white (talc) and dark silver (graphite). Slate can take on many colors depending on the relative abundance of these, and other, minerals. Even a small change in their relative amounts can have a drastic impact on the slate’s color.
How to Identify Slate
Even with a clear picture in your mind of what slate is supposed to look like, it can sometimes be difficult to identify. I think this is because it tends to be a fairly bland and uninteresting rock – at least at first glance. It can feel like there are very few defining features on which to base an identification. As with any rock, it is important to take a systematic approach when identifying slate.
To identify slate, first ensure that you cannot see individual mineral grains with the naked eye. If possible, see if you can break off thin, parallel sheets from the rock or look for spots where sheets have already broken off. Slate is usually light or dark gray, but may also be greenish, reddish, or purple.
A rock must meet all of these requirements to be considered a slate:
- Metamorphic – Formed from physical and chemical changes caused by heat and pressure
- Very Fine-grained – The grains of the rock matrix are not visible to the naked eye and difficult to see with a hand lens
- Slaty Texture – Foliated, with parallel orientation of minerals and breaks into flat sheets
If your rock meets all of those criteria then it is slate, or at least something very closely related.
Tip: This article is part of my metamorphic rock identification series. To learn about how to identify all metamorphic rocks, check out my article here.
Always bear in mind that slate can occur in a wide variety of colors. Shades of gray are the most common due to high mica content, but colors like green, blue, and rust-red are common. Don’t assume that a rock isn’t slate just because it isn’t gray.
When observing and identifying a rock like slate it can often be useful to use a hand lens like this one from Amazon. This allows you to see the small features in the rock more clearly and often helps you identify the specific species of minerals present in many rock types.
Even with these clear criteria for identifying slate, it is common for people to confuse it with different rocks types. There are a few rocks that look close enough to slate that they can easily confuse people, so it is useful to know what some of those closely-related rock types are called and what they look like.
Rocks that are often misidentified as slate:
- Shale – A very fine-grained, layered sedimentary rock made of clay minerals. Turns into slate when subjected to low-grade metamorphism.
- Mudstone – Very similar to shale but with no layering. Turns into slate when subjected to low-grade metamorphism
- Phyllite – A foliated metamorphic rock that forms from further metamorphism of slate, but is fine-grained and often has visible mineral flakes.
Like most rocks, slate can become very weathered when exposed to the elements. This can make it hard to identify because the colors can change significantly and many of the features become more difficult to see. When making any rock identification it is usually best to break off a piece of the rock (if you’re able) and look at a fresh surface.
What Is Slate Made Of?
Even though slate is primarily defined by its texture, its mineralogy is usually fairly consistent from one type to another. Slate forms only in a certain metamorphic setting and the end result always looks pretty much the same.
In general, slate is made from minuscule crystals of quartz and platy minerals like muscovite, chlorite, and graphite. Smaller amounts of bulky minerals like magnetite, hematite, and pyrite are common, sometimes as relatively large inclusions. Fossils are sometimes still preserved and present in slate.
The vast majority of the total volume of slate is quartz and platy minerals – usually as much as 95%. The thin sheets of these platy minerals make up the matrix (the main part of the rock) that binds any other minerals together.
The crystal grains in slate are far too small to see, as are the cleavage planes along which it breaks. However, under powerful magnification, you would be able to see that the rock is made up of very thin, alternating sheets of clay particles and quartz. It is along these boundaries that slate breaks when properly struck.
It is common to do an acid test on rocks during the identification process to check for the presence of calcium carbonate. A weak hydrochloric acid solution will react and fizz on the surface of a rock if calcium carbonate is present. Slate will not react with acid because you will almost never find a slate with enough calcite in its composition. If your rock reacts with hydrochloric acid, it is not slate.
Where Is Slate Found?
Slate is a very common rock in the Earth’s crust, and one of the most abundant metamorphic rocks you can find. It is created by the regional metamorphism of very common rock types, so there is no shortage of material from which it can form. Still, slate isn’t present just anywhere. If you are looking for it, it helps to know the type of geologic setting to look in.
Slate is found in mountainous regions where metamorphic rock has been exposed on the surface. The continental side of convergent plate boundaries are ideal, where rocks are exposed to heat and pressure and thrust upwards. If schist or phyllite is present, slate is also likely to be present nearby.
In order to find slate, you have to have a sense of the geologic settings around you. If you’re in an area with only young sedimentary rocks exposed on the surface, you have very little chance of finding slate. However, if there are areas near you with hills or mountains you are much more likely to be successful.
Slate is usually fairly high in quartz content and its minerals are tightly packed and fused together from the metamorphic process. This means that it is pretty hard and resistant to weathering. Once it is exposed to the surface it takes a long time for it to break down. Pieces of slate can survive in a river for great distances, which is why you can often find pieces downriver from its source.
You can look for slate formations near you using this excellent interactive map from the USGS. I have a video about how to use this tool in my Practical Rock Identification System, plus even more information on how to identify dacite and other rocks.
You can also use your knowledge and intuition! If you see schist or phyllite, chances are that if you follow that formation one way or the other it will gradually grade into slate.
In the U.S., you can find slate in places like the Appalachian Mountains and in the Pacific Northwest. These areas have at one time or another undergone significant compression and mountain building which is ideal for creating the metamorphic environment necessary to make slate.
How Does Slate Form?
We’ve learned all about what slate looks like, what it is composed of, and generally where it’s found, but I have only briefly touched on how it’s actually formed. The creation of slate is a fascinating process that takes a specific combination of circumstances to occur.
Slate forms from the regional metamorphism of clay-rich rocks like shale, mudstone, and volcanic tuff. These rocks are compressed, losing up to 50% of their original volume, and gradually metamorphose into slate as their minerals rotate and reorient themselves perpendicular to the direction of maximum stress.
The first step in the creation of slate is the presence of a clay-rich protolith. In most cases, this is a sedimentary rock like shale or mudstone, but sometimes it can be a volcanic tuff where the volcanic ash forms a thick layer of bentonite. These clay minerals have the necessary chemistry to be transformed by heat and pressure into platy minerals like mica and chlorite. These clay-rich rocks are usually buried under many more layers of rock over millions of years of deposition.
Note: A ‘protolith’ is the original rock from which a metamorphic rock forms.
The next step to creating slate is compression. Tectonic forces in the Earth cause huge masses of rock to be pushed and squeezed together, creating enormous amounts of stress and heat. These conditions allow for the original minerals of the protolith to gradually reorient and transform into minerals that are more stable at higher pressures and temperatures.
Slate gets its distinct texture from the preferential orientation of its platy minerals. This orientation is a result of the regional compression the rock is enduring. The platy minerals form perpendicularly to the direction of maximum stress because that is the orientation in which they are the most stable. It’s like if you took a box of loose confetti and then squeezed the confetti together. The bits of paper would flatten and align themselves perpendicular to the direction in which you are squeezing them.
This process is gradual, and if subjected to further pressure and heat the slate would continue to metamorphose into phyllite and then schist. The process of metamorphism will take a clay-rich protolith and turn it into very fine-grained slate, then medium-grained phyllite, and finally medium-grained schist. With further metamorphism, the schist will become gneiss.
What Is Slate Used For?
Slate has seen widespread use over the course of history and continues to see extensive use in many practical applications even today. Its unique ability to be easily split into flat sheets makes it an ideal choice for a variety of uses.
Slate is most commonly used in construction, particularly as roofing shingles that provide a durable, beautiful, and low-maintenance roof. It also saw extensive use for blackboards, writing tablets, and as interior flooring.
Many historic buildings have slate roofs, and they are often still the original. While initially somewhat difficult and expensive to install, slate remains perhaps the most durable and attractive roofing material. It is resistant to weathing and frost damage and is fire-resistance while also being a good insulator and energy efficient. It often lasts for hundreds of years with little to no maintenance.
This article is part of my rock identification series. I invite you to keep reading about how to identify rocks with my full in-depth guide here.