Igneous rocks are some of the most interesting rocks found in nature, mostly because of the spectacular way in which they are formed. All igneous rocks are created from the cooling of magma or lava, but there are almost limitless varieties based on the minerals present in the source magma and the temperature and pressure conditions when it cooled.
Because igneous rocks can be so varied it can be difficult to accurately identify them, or even to know if a rock is igneous or not. If you’re trying to identify an igneous rock it helps to know what they typically look like.
In general, igneous rocks appear dark, dull, and uniform, with no visible layering. However, they can be almost any color including very light gray, dark gray, pink, and green. The texture of igneous rocks varies from extremely coarse, large crystals to smooth and glassy depending on quickly they cool.
Properly identifying igneous rocks takes some practice and practical knowledge, but almost anyone can do it. Keep reading, and I’ll give you everything you need to recognize and identify your igneous rock and answer a lot of commonly asked questions that you may have.
How to Identify Igneous Rocks
Sometimes it can be very difficult to identify your igneous rock even with pictures and descriptions to compare it to, which is why it is usually best to take a methodical, step-by-step approach to the identification process. By going through this procedure you can almost always confidently arrive at a solid classification for your igneous rock.
To identify an igneous rock, first determine its approximate mineral composition by judging its overall color and labeling it as felsic, intermediate, mafic, or ultramafic. Next, observe its texture based on the crystal grain sizes present in the rock. Then, compare your observations to known igneous rock types.
This procedure might seem a little daunting at first, but it is actually quite simple. I’ll walk you through how to determine what type of igneous rock you have and provide some helpful procedures and descriptions to compare to.
Pictures & Descriptions of Igneous Rock Types
One of the easiest and most practical ways to identify an igneous rock is by comparing it to examples. While there can be quite a bit of variation even among the same rock type (one granite can look quite a bit different than another, for example), it can be helpful to have pictures of known rock types to compare to.
I have broken up these examples into the two major igneous rock types – intrusive and extrusive. Intrusive igneous rocks cooled slowly beneath the earth’s surface and therefore have larger crystals. Extrusive igneous rocks cooled more rapidly on the surface and have smaller crystals because they didn’t have as long to grow.
If you’re here to identify a rock you’ve found, take a look at your rock and try to make a determination about whether it is intrusive or extrusive. If in doubt, you can just browse all of the pictures and see if anything matches.
TIP: When trying to identify an igneous rock, always remember that rock types are a continuum. For example, ‘Granite’ bleeds over into ‘Granodiorite’ which bleeds over into ‘Diorite’. It is nearly impossible to draw a distinct line between the rock types, so just make your best effort.
Intrusive Igneous Rocks
Intrusive igneous rocks are formed when magma cools and slowly solidifies beneath the surface of the Earth. Since the magma is still surrounded by warm rocks and not exposed to the cool air (or water) on the surface, it takes a long time for it to cool down. This gives the minerals in the magma time to organize and create large crystals, which is a key indicator that your rock is likely an intrusive igneous rock.
Depending on how long it takes for the magma to cool and what its mineral composition is, intrusive igneous rocks may have many different ‘textures’, which refers to how the crystals look and feel in relation to one another. For example, an intrusive igneous rock may be described as ‘phaneritic’ or ‘pegmatitic’. These texture names are relatively unimportant for a casual rock enthusiast to know, but it is useful to know that not all intrusive igneous rocks have the same texture.
Granite is an intrusive igneous rock composed primarily of feldspar, quartz, and smaller amounts of mica. It has a phaneritic texture, meaning that individual crystals can be seen with the naked eye.
Because of its relatively high quartz and feldspar content, granite tends to be light in color (felsic). It is usually gray or off-white, but can also have shades of pink or red depending on the type of feldspar.
Granite comes in many varieties and is a very attractive, well-known rock type. Its texture, color, and widespread availability make it popular with collectors and for decorative use.
Gabbro is an intrusive igneous rock composed primarily of pyroxene and plagioclase feldspar. It has a phaneritic (coarse-grained) texture.
Due to its low silica content, gabbro is a mafic igneous rock, meaning it is dark in color. It is the intrusive equivalent of basalt – it has the same chemical makeup but because it cools more slowly beneath the surface it has visible crystals.
Diabase, also known as dolerite, is an intrusive igneous rock composed primarily of pyroxene and plagioclase feldspar. It is dark in color and has a phaneritic texture. It is very similar to gabbro but has slightly smaller crystal sizes.
Diabase usually forms from igneous intrusions like dikes or sills, which allows it to cool more quickly than gabbro but not as quickly as basalt. The three rock types are the same mineralogically but vary in their respective crystal sizes.
Diorite is an intrusive igneous rock with a mineral composition between granite and gabbro. It primarily consists of plagioclase feldspar, biotite, pyroxene, and hornblende. Diorite, by definition, contains very little quartz. Chemically, diorite is the intrusive equivalent of andesite.
Diorite can be difficult to differentiate from gabbro without laboratory equipment but in the field, it is usually done by judging the color. Because of its intermediate composition, it is lighter than gabbro and darker than granite.
Pegmatite is a type of intrusive igneous rock formed during the last stages of magma crystallization. Its crystals are usually larger than 1 cm in size, and often much larger.
The large crystal sizes in pegmatite are a result of that part of the magma body taking so long to cool, giving the crystals plenty of time to grow.
Because pegmatite is defined by its method of formation and crystal size, its composition can be highly variable. However, most pegmatites are mineralogically similar to quartz, consisting of quartz, feldspar, and mica.
Peridotite is a phaneritic intrusive igneous rock consisting primarily of olivine and pyroxene. Due to its low quartz content it is considered ultramafic. Peridotite generally originates in the Earth’s mantle. It is usually dark gray to green in color.
Peridotite contains large amounts of olivine which often manifests itself in the form of bright green crystals. The aesthetic appeal and relative rarity of peridotite on the Earth’s surface make it popular with collectors.
Anorthosite is an intrusive igneous rock composed almost entirely of plagioclase feldspars, usually labradorite and, more rarely, bytownite. Depending on the presence and abundance of accessory minerals, anorthosite is usually off-white to gray in color.
Like other intrusive igneous rocks, anorthosite is made of large crystals due to the relatively slow cooling of its source magma. It can be identified by its large feldspar crystals and relatively light color compared to most other intrusive igneous rocks.
Dunite is an intrusive, phaneritic igneous rock consisting almost entirely of olivine. It is a type of peridotite, distinguished by its exceptionally high (>90%) concentrations of olivine. Minor amounts of darker minerals like pyroxenes are usually present.
The relatively large and vibrantly green olivine crystals in dunite make it very attractive and popular with collectors. Because dunite, like other peridotites, forms in the mantle it is relatively rare to find on the Earth’s surface.
Porphyry is a type of intrusive igneous rock that refers to a specific type of texture rather than mineralogy. Porphyry consists of large crystals (typically quartz or feldspar) contained within a mass of smaller crystals.
The larger crystals in porphyry are referred to as phenocrysts, and they form during a period of slower cooling before the rest of the magma solidifies. The surrounding rock (the matrix) cooled more rapidly which resulted in an aphanitic texture, meaning the crystals are not visible to the naked eye.
Since porphyry refers specifically to a rock texture, porphyry can be almost any color since the minerals present can vary wildly. However, in common use, porphyry is most often used to describe pink or red rocks with porphyritic texture.
Syenite is an intrusive igneous rock consisting primarily of feldspar and mafic minerals like hornblende. It is very similar to granite, but notably almost entirely devoid of any quartz.
Because of the high concentrations of feldspar (usually an alkali feldspar such as orthoclase), syenite is usually intermediate in color. It is often easily recognizable by its mottled off-white and black coloring thanks to the contrast between the feldspar and mafic minerals.
Extrusive Igneous Rocks
Extrusive igneous rocks are formed by the cooling of lava on the Earth’s surface. They are characterized by their small individual crystal sizes when compared to intrusive igneous rocks. The magma’s exposure to the relatively cold air or water causes it to cool quickly, which doesn’t give the minerals in the magma enough time to organize and form large crystals.
It is usually not possible to see the individual crystals of extrusive igneous rocks with the naked eye, but you may be able to see them with a good hand lens like this one from Amazon.
Small, uniform crystal sizes in igneous rocks is referred to as ‘aphanitic texture’. This texture is very common in extrusive igneous rocks. Other textures include ‘glassy’ in obsidian and ‘porphyritic’ which means that there are some larger crystals contained within a matrix of smaller crystals. This happens when there is a period of time where some minerals are able to crystalize before the rest of the magma around them cools more quickly.
Basalt is an extrusive igneous rock consisting primarily of plagioclase feldspar and pyroxene minerals. It is fine-grained (aphanitic) and dark in color, often with visible voids formed from gas bubbles. It is the extrusive equivalent of gabbro.
Basalt is the most common igneous rock on the Earth’s surface. It is found on the ocean floor and in many locations on land where large lava flows erupted to the surface. Because basaltic magma is exposed to cool air or water during an eruption, the crystals form rapidly and are therefore very small.
There are many varieties of basalt, and it often manifests itself in impressive fashion in formations like hexagonal basalt columns. The void spaces in basalt are the source of most agates, making it of particular interest to rock collectors.
Obsidian is an extrusive igneous rock formed from the rapid cooling of magma as it interacts with air or water. It is made of felsic minerals, with a mineral composition similar to rhyolite or granite.
Obsidian is easily recognizable by its glassy texture – a result of extremely rapid cooling. It also has a characteristic conchoidal fracture, creating sharp edges that made it useful for prehistoric arrowheads and knives.
While obsidian is usually black, it can actually come in many colors. Dark red coloring is fairly common, and ‘snowflake’ obsidian has white spots on black. Some obsidian can even display an iridescent effect known as ‘rainbow obsidian’.
Pumice is an extrusive igneous rock formed from the rapid expulsion and cooling of lava in a volcanic eruption. As the lava depressurizes, if its gas and water content is high enough the gases expand and make the lava frothy. As the lava rapidly cools the gas bubbles are trapped, giving pumice its characteristic appearance and texture.
Pumice is easily identifiable by its light weight, vesicular texture (meaning it is full of holes), and by the fact that it famously floats in water. It can sometimes be mistaken for its cousin scoria, which tends to have larger vesicles, be denser over all, and darker in color.
Scoria is an extrusive igneous rock that, like pumice, forms from the rapid expulsion and cooling of lava in a volcanic eruption. Scoria’s composition is usually basaltic and therefore tends to be black or dark brown in color.
Like pumice, scoria is highly vesicular (it has many holes formed from gas bubbles). However, unlike pumice, all scoria will sink in water. Testing to see if a vesicular volcanic rock sinks or floats is the easiest way to determine if it is scoria or pumice.
Rhyolite is an extrusive igneous rock with a very high silica content, usually off-white to reddish in color. It is the extrusive equivalent of granite, with a very similar mineralogical makeup.
Rhyolite often displays a porphyritic texture, meaning that there are some large crystals visible to the naked eye contained within a mass of smaller crystals. In rhyolite, the larger crystals are quartz and plagioclase feldspar.
It is also common for rhyolite to have some void spaces created by gas bubbles at the time of cooling. These voids often contain glassy, microcrystalline minerals like opal or agate.
Andesite is an extrusive igneous rock with a mineral composition between basalt and rhyolite. It has an aphanitic texture with most crystals not visible to the naked eye. Chemically, it is the extrusive equivalent of diorite.
Andesite is typically light gray or dark gray as a result of its mineral makeup, which is mostly plagioclase feldspar, biotite, and pyroxenes. It is common for weathered to andesite to appear brown, so exposing a fresh surface with a hammer is often helpful for identification purposes.
Dacite is an intrusive igneous rock composed primarily of plagioclase feldspar, biotite, and quartz. Mineralogically, it falls in the spectrum between rhyolite and andesite. It has a higher quartz content than andesite and more plagioclase feldspar than rhyolite.
Dacite is usually porphyritic, with relatively large quartz phenocryst crystals in a matrix of plagioclase feldspar and biotite. It is usually light gray or light brown in color, but can also be reddish.
Tuff is an extrusive igneous rock composed of rock fragments and volcanic ash. It is formed during violent volcanic eruptions in which large amounts of rock and ash are ejected into the air and subsequently deposited surrounding the volcano. This mixture of ejecta and ash is then solidified into solid rock via lithification.
In many ways, tuff can also be considered a sedimentary rock since it is formed from the consolidation of other rock fragments. However, in most cases, tuff would be categorized and described as an igneous rock because it formed during a volcanic eruption and the rock matrix consists of volcanic ash.
How are Igneous Rocks Classified?
Classifying an igneous rock may seem daunting if you don’t have much experience doing it. There are so many types of igneous rocks and many of them can appear very similar at first glance. However, identifying an igneous rock is usually fairly straightforward and simple once you know how it’s done.
In general, igneous rocks are classified first by their mineral composition and then by their texture or crystal size. In common practice, estimates of mineral composition are usually done by judging the overall color of the rock, and crystal size can be sufficiently characterized by visual inspection.
While sophisticated laboratory testing is often necessary for nuanced classification, identification for most practical purposes can be done visually in the field or at home.
The first step towards classifying your igneous rock is to estimate its mineral composition. Igneous rocks are classified into four categories based on their percentage of ferromagnesian minerals. In practice, this just means that they are classified based on their percentage of dark-colored minerals. Look at your rock (preferably a fresh, non-weathered surface) and make a determination about its color.
- Felsic – Very light in color
- Intermediate – Somewhat light in color
- Mafic – Dark in color
- Ultramafic – Extremely dark, or green due to large amounts of olivine
Tip: There are some exceptions to determining mineral makeup purely by color. For example, obsidian is usually black, but is formed from felsic lava.
After determining the basic mineral composition of your rock (based on its overall color) the next step is to observe and describe its crystal sizes and texture.
The crystals in igneous rocks are sometimes referred to as ‘grains’. It is important to make the distinction between these crystal ‘grains’ and the grains you would describe in sedimentary rocks. The crystals in igneous rocks are melded together with virtually no space between them. They form in and around one another, fused together during the cooling process. In sedimentary rocks, the grains are formed independent of one another and then cemented together, often leaving some space (porosity) between the grains.
There are several texture types that are typically used to describe igneous rocks. The names of these textures are themselves not all that important for someone wanting to identify their rock, but the textures they describe are essential for classification. They help tell the story of the rock and how it formed, which is essential to a proper identification.
Igneous Rock Textures:
- Phaneritic – All crystal grains visible to the naked eye
- Aphanitic – Grains are mostly too small to be seen with the naked eye
- Porphyritic – Large crystals in a matrix of smaller crystals
- Glassy – No grains at all, completely smooth (like obsidian)
- Vesicular – Voids and bubbles throughout the rock
- Pyroclastic – Pieces of rock contained in ash from an eruption
- Equigranular – Grains are all approximately the same size
- Spiroflex – Criss-crossing mineral grains, very rare
- Poikilitic – Large crystals display smaller crystals inside of them
- Pegmatitic – Consisting of exceptionally large crystal grains
Tip: An igneous rock can display more than one texture type. For example, a rock can be phaneritic and equigranular if you can see all of its grains and they are all approximately the same size.
Once you have determined your rock’s texture you can use that description, along with its general mineral composition, to properly identify your rock. Compare your rock to the pictures and descriptions I listed above, or to the table below. You should be able to find a very close match! Keep in mind that there is a high degree of variability even among rocks of the same type, so it’s okay if your rock looks a little different than pictures you may find to compare it to.
|Pumice||Felsic, Intermediate||Vesicular, Aphanitic|
|Scoria||Mafic, Intermediate||Vesicular, Aphanitic|
|Syenite||Intermediate, Felsic||Phaneritic, Equigranular|
Igneous Rock FAQs
What Color are Igneous Rocks?
One of the easiest and most useful identifying characteristics of igneous rocks is their color. Igneous rocks are often grouped into four classes based on their general color. These groups and their respective colors are a result of their silica content (see above).
While most igneous rocks are light gray to black, they can be almost any color depending on their mineralogy. For example, large olivine crystals are green, some feldspars can be pink or red, and obsidian can be many colors or even described as ‘rainbow obsidian’.
Do Igneous Rocks Have Layers?
In general, igneous rocks do not contain layers. Igneous rocks are deposited as one unit in a lava flow or a magma intrusion, and therefore there is no opportunity for distinct layers to form.
You may encounter outcrops of distinctly different lava flows with differing rock types, but those would be considered separate strata and not ‘bedding’ as you commonly see in sedimentary rocks.
Do Igneous Rocks Have Fossils?
Igneous rocks do not contain fossils. Igneous rocks are formed from the cooling of hot liquid magma, so any fossils that would have been present in or around the magma would have been destroyed. Once the magma has cooled and solidified there is no opportunity for fossils to be deposited in the rock.
Do Igneous Rocks React With Acid?
In general, igneous rocks will not react with acid. Geologists commonly use a diluted HCL acid solution that reacts to calcite or limestone, which is not common in igneous rocks. One rare but notable exception is carbonatite, which is an igneous rock composed of over 50% carbonate that will react with acid.
Do Igneous Rocks Have Gas Bubbles?
Igneous rocks commonly contain gas bubbles, and are the only type of rock for which this is the case. Gas pockets or bubbles form in flowing lava and are subsequently trapped as the lava cools and solidifies. These gas bubbles can make up a significant portion of the rock’s total volume as seen in pumice and scoria.
It is common for some gas bubbles in igneous rocks to be subsequently filled with minerals over many years. As water mineral-laden seeps through the rock, the minerals (usually silica) precipitate out and solidify in the empty space of the gas bubbles. This process is very common in basalt and rhyolite flows, and is how agate nodules and geodes are formed.
Are Igneous Rocks Hard or Soft?
In general, igneous 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 igneous rocks are interlocking and fused together, making them very competent and difficult to break.
It is common for geologists to refer to igneous (and metamorphic) 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.