Gabbro is an fairly common rock type, and can be found in locations all around the world. Chances are good that you have seen and touched gabbro many times in your life, whether or not you were aware of it at the time. It tends to be a very dull and uninteresting-looking rock, but it is still worth knowing about due to its prevalence and geologic significance.
The biggest reason gabbro is not as well-known as it should be is that it is often confused with other igneous rock types. This is perfectly understandable because many igneous rocks can superficially look very similar and they share many of the same physical properties. It can be difficult to know exactly what gabbro looks like and how to identify it, but it is relatively simple if you know what to look for.
Gabbro is an intrusive igneous rock composed primarily of plagioclase feldspar and large amounts of mafic minerals like pyroxene. It has a phaneritic texture, meaning its interlocking crystals can be seen with the naked eye. It is usually dark gray, black, or dark greenish, and is closely related to basalt.
Gabbro is mafic in composition, meaning it is devoid of quartz and contains a great deal of darker minerals like pyroxene and hornblende. While gabbro is a clearly defined rock type, it can sometimes be hard to distinguish it from closely related rocks – especially if you don’t have access to sophisticated testing methods. I’ll walk you through identifying gabbro, what it looks like, and where it can be found.
What Does Gabbro Look Like?
Gabbro is one of the most common igneous rocks in the world. Most people have seen it many times in their life – probably without realizing it. Gabbro is mafic in composition, containing less quartz than felsic granite and intermediate diorite, but it can sometimes be difficult to distinguish from diorite when it comes to the percentage of mafic minerals present.
In everyday life, it is relatively unimportant to be able to distinguish between gabbro and diorite, so people often just label a rock as one or the other (or don’t bother with an identification at all) and call it a day. But if you’re actually interested in geology and knowing what kind of rocks you have you probably want to be more specific than that. So, what does real gabbro actually look like?
Gabbro has a coarse-grained texture with large, interlocking crystals of white to off-white plagioclase feldspar and darker, mafic minerals like pyroxene, hornblende, and sometimes olivine. Its crystals are roughly equal-sized and it is dark in color, sometimes with a ‘salt and pepper’ appearance.
One of the defining characteristics of gabbro is its overall lack of quartz. Rocks like granite and granodiorite have quite a bit of quartz (over 20%) but true gabbro is almost completely devoid of any visible quartz grains – less than 5%.
While true gabbro meets this general description, there are some closely-related rock types that vary only slightly in their mineralogy and are usually referred to as ‘gabbroids’. For our purposes, we will lump all gabbroids in with gabbro because it is usually impossible to tell the difference between them and true gabbro without sophisticated laboratory testing.
There is quite a bit of variation in how gabbros can look from location to location. The difference in their appearance is driven by the mineralogy of each type of gabbro and, to a lesser extent, the subtle differences in their textures (crystal sizes).
Color is Driven by Mineralogy
The major mineral present in all gabbro is plagioclase feldspar. Alkali feldspars are also usually present, at up to 35% of the total feldspar content in some gabbroids. Each of these minerals can present itself in different ways. The plagioclase feldspar in true gabbro is rich in calcium, typically either species called labradorite or bytownite.
Tip: “Gabbroid” refers to rocks very closely related to true gabbro. In the field and in common practice, most gabbroids are simply referred to simply as ‘gabbro’.
Quartz is almost entirely absent from true gabbro. In some gabbroids, however, it can be present at up to 20% of the total rock volume. It usually appears in gabbro as translucent, light gray, or off-white. Some varieties may have more impurities which make the quartz crystals an opaque gray or white color. In any case, quartz is not a significant contributor to gabbro’s overall color because it simply isn’t prevalent enough.
Feldspar has much more variety than quartz. It is actually a group of minerals that can be divided into two main types: plagioclase feldspar and alkali feldspar. These two types of feldspar can look quite different from one another, and can even vary significantly in appearance themselves. Both are present in gabbro.
Plagioclase feldspar is by far the most abundant mineral in gabbro, making up over 65% of the total feldspar content. In true gabbro, this percentage is over 95%. Being able to distinguish between the two types of feldspar is perhaps the most important (and most difficult) aspect of identifying a rock as gabbro as opposed to syenite, which has more alkali feldspar than plagioclase. While this can sometimes be tricky, there are some easy-to-spot differences that can often help you see the difference and further understand your rock.
Both alkali feldspar and plagioclase feldspar can be white, but alkali feldspar is also very commonly salmon-pink or even darker pink, bordering on red. Plagioclase feldspar usually has visible striations in its crystals that may be visible if the individual crystals are large enough.
Some of the most common accessory minerals (making up to 40-90% of the total rock volume) in gabbro are pyroxene, hornblende, and olivine. Hornblende and pyroxene will almost always look like dark, blocky crystals interspersed throughout the gabbro. Olivine is usually easy to spot because it is noticeably green, much like the color of a green olive.
The accessory minerals in gabbro sometimes give it a ‘salt and pepper’ appearance. There is usually about twice as much black as there is white, but the ratio can sometimes look about even.
Texture of Gabbro
One of the defining features of gabbro’s appearance is its texture. All gabbro is coarse-grained, meaning that you can see the individual crystals in the rock. This texture is known as ‘phaneritic’, and it forms when magma cools slowly and allows the crystals enough time to grow before becoming completely solidified.
While all gabbros are phaneritic, it is also possible (and even common) to describe their texture in other ways. Rocks can be described as having more than one texture as long as the texture types are not mutually exclusive.
Most gabbro is ‘equigranular‘, which means that all of the crystals are approximately the same size. So, a gabbroic rock may be described as ‘phaneritic and equigranular’ if you can see similarly-sized crystals with the naked eye.
Some gabbro can have a ‘porphyritic‘ texture. This indicates that there are larger crystals called ‘phenocrysts’ in a groundmass of smaller crystals. So if a gabbro has large pyroxene crystals surrounded by smaller (but still visible) plagioclase feldspar crystals you could describe it as ‘porphyritic and phaneritic’.
How to Identify Gabbro
Gabbro has a reasonably distinctive look, but it is often confused for closely-related rock types. If you aren’t overly familiar with how it’s defined and what it looks like it can be easy to misidentify it as something else entirely. As with any rock, it is important to take a systematic approach when identifying gabbro.
To identify gabbro, first look to see if you can make out individual crystals with the naked eye. Try to identify large amounts of whitish plagioclase feldspar. Dark, mafic minerals like pyroxene hornblende should make up 40-90% of the rock by volume. The overall color of the rock is usually gray or dark gray.
A rock must meet all of these requirements to be considered a gabbro:
- Igneous – Formed from cooling magma, with interlocking crystal grains.
- Coarse-grained – Phaneritic texture with crystals visible to the naked eye
- Mafic Mineralogy – 40% to 90% of the rock is dark, mafic minerals. The rest is mostly plagioclase feldspar with very little quartz
- Massive – No internal structures or layering
If your rock meets all of those criteria then it is very likely a gabbro, or at least something very closely related. Rocks with mineralogies very similar to gabbro are considered ‘gabbroids’, and many people simply group them all together when a specific identification isn’t overly important.
Tip: This article is part of my igneous rock identification series. To read more about how to identify all igneous rocks, check out my article here.
When observing and identifying a rock like gabbro it can often be useful to use a hand lens like this one from Amazon. This allows you to see the individual crystal grains more clearly and often helps you identify the specific species of minerals present in a rock.
What Is Gabbro Made Of?
As I mentioned above, gabbro is largely defined by a specific mineralogy. All rocks are made from one or more minerals, and in order to fully understand a rock like gabbro you have to know what those minerals are.
The mineral composition of gabbro consists of large crystals of 40-90% mafic minerals like pyroxene and hornblende, with the rest of the rock containing 0-20% quartz and 65-100% of the total feldspar being plagioclase. Other accessory minerals like olivine may also be present in small quantities.
This definition is very specific, but without sophisticated methods for measuring mineral types and percentages it is impractical (or even impossible) for us to determine if a gabbro-like rock falls into those specific percentage ranges.
The best you can do is to first estimate the percentage of dark, mafic minerals in the rock. If it is over 40% then your rock is mafic, which helps you rule out diorite as a possibility. Then, if the amount of quartz is between 0 and 20% then you know it is probably gabbro. If there is noticeably more whitish plagioclase than pinkish alkali feldspar then you can confidently call your rock ‘gabbro’.
It is useful to know what some closely-related rock types are called and what their mineralogy looks like. These rocks all form in similar ways but vary in the relative proportions of their minerals.
- Diorite – Quartz-deficient with more plagioclase than alkali feldspar, up to 40% mafic minerals
- Syenite – Quartz-deficient with very more alkali feldspar than plagioclase
- Granodiorite – Quartz-rich with more plagioclase than alkali feldspar
Where Is Gabbro Found?
Gabbro is a fairly widespread igneous rock and is found all around the world. You might think that it must be easy to find, but it is actually not all that common on continental crust, where we live.
Gabbro is most commonly found in the deeper portions of the oceanic crust. It can also be found on the continental crust where large basaltic lava flows and magma bodies cool very slowly. It is exposed on the surface where erosion has stripped away shallower rock to reveal the gabbro beneath.
Because gabbro forms beneath the surface of the earth (and most of it below the ocean), the vast majority of it isn’t visible or accessible to us. But sometimes very large basaltic lava flows like those near the Columbia River in the Pacific Northwest produce a large enough volume of lava that the deeper portions of the flow cool very slowly, producing gabbro. These upper layers of these basaltic lava flows can be eroded away to reveal the gabbro beneath.
It is very unusual to find gabbro which formed as part of the oceanic crust on land. Oceanic plates, which are basaltic in composition, are generally much heavier than continental plates that are more granitic. When they meet at subduction zones, the heavier oceanic plates sinks beneath the continental plates. The gabbro is then forced downwards towards the mantle where it is eventually melted and destroyed.
Gabbro is composed of relatively hard minerals (primarily feldspar and pyroxene) which make it incredibly durable. Once it is exposed to the surface it takes a long time for it to break down. Pieces of gabbro can survive in a river for great distances, which is why you can sometimes find pieces downriver from its source.
You can look for gabbro 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 gabbro and other rocks.
How Does Gabbro Form?
We’ve learned all about what gabbro 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 gabbro is a relatively straightforward process that always follows a few simple rules but can vary significantly in the details.
Gabbro forms when basaltic magma, rich in magnesium and iron, cools slowly in large masses beneath the surface of the Earth, allowing for the formation of large crystals. This most commonly occurs at mid-ocean ridges in oceanic crust, but also occurs in large magma bodies intruding into continental crust.
The creation of gabbro usually begins at oceanic ridges at the boundary of two oceanic plates. Basaltic magma erupts along the ridge, forming new oceanic crust which gradually spreads out from the ridge in both directions. Oceanic plates are commonly described as being made of basalt, but that is only partially true.
Oceanic plates are basaltic in composition, but only the uppermost part of the rock comes into contact with the water, causing it to cool rapidly and become basalt. The rest cools slowly beneath the surface, allowing for larger crystals to form into gabbro. Gabbro is, mineralogically, the coarse-grained equivalent of basalt which is why oceanic plates are usually just described as being ‘basaltic’.
Most of the gabbro that we find on the surface today formed not as oceanic crust, but as an intrusion or large basaltic flow on continental crust. Large basaltic magma bodies sometimes intrude upwards into more granitic rock or even erupt onto the surface in the form of large lava flows. When the interior of these basaltic bodies is far enough removed from cool surroundings there is often enough time for large crystals to form and creat gabbro instead of basalt.
Because the composition of the source magma is never exactly the same from one gabbro to another and the surrounding environment is always different, there can be quite a lot of variation in how the magma cools and the crystals form.
If the surrounding rock is relatively cool then the crystals formed on the edges of the igneous intrusion are likely to be smaller. Similarly, if the intrusion is very large then the crystals in the center of the magma body are likely to become very large because they have such a long time to cool down and crystalize.
As the individual crystals of gabbro’s various minerals cool down, they eventually run into each other and run out of room. Crystallization will continue until all of the magma is solidified, leaving behind interlocking crystals with virtually no porosity between them.
Because gabbro forms as one solid mass, it contains no internal structures like bedding or banding. It looks the same from all directions and you can’t tell which direction was ‘up’ at the time of crystallization. This is called a ‘massive’ rock.
What Is Gabbro Used For?
Chances are good that you have seen plenty of gabbro in your day-to-day life, but it can be easy to miss if you’re not paying attention. Because gabbro is similar in many respects to granite it should come as no surprise that they are used in many of the same applications. In fact, gabbro is sometimes referred to as ‘black granite’, even though it is not a granite at all. Gabbro has been in use for thousands of years because of its durability, strength, and striking appearance.
Gabbro is most commonly used for interior and exterior building applications. Highly polished gabbro is a very popular material for countertops, while rough-cut gabbro is often used as aggregate and is one of the most prevalent components of buildings, bridges, and other structures.
Like most rocks, gabbro is very strong in compression. This means it can withstand a great deal of ‘squeezing’ without breaking, making it ideal for use in construction projects with large overburden stresses. It is, however, a very hard rock that can be difficult to work with compared to some other igneous rocks.
What really sets gabbro (and granite) apart is its ability to accept a polish and its resistance to weathering. If you see a dark, highly-polished ‘granite’ countertop it is probably actually made of gabbro. They are made possible by the fact that feldspar and pyroxene are easily brought to a high shine.
One of the biggest reasons gabbro and similar rocks are so popular as a building material is because it doesn’t break down in the rain, ice, and wind as easily as other rocks.
This article is part of my rock identification series. To learn more about identifying rocks, check out my full in-depth guide here.