Basalt is an extremely common rock type, and can be found in locations all around the world. Chances are good that you have seen and touched basalt 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.
Basalt can sometimes be fairly difficult to identify, partly because it comes in so many varieties and partly because it is sometimes confused with other 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 basalt looks like and how to identify it, but it is relatively simple if you know what to look for.
Basalt is an extrusive igneous rock composed primarily of plagioclase feldspar and large amounts of mafic minerals like pyroxene. It has an aphanitic texture, meaning its interlocking crystals cannot be seen by the naked eye. It is usually dark gray, black, or dark greenish, and is often vesicular or porphyritic.
Basalt is mafic in composition, meaning it is devoid of quartz and contains a great deal of darker minerals like pyroxene and hornblende. While basalt 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 basalt, what it looks like, and where it can be found.
What Does Basalt Look Like?
Basalt is, by far, the most common igneous rock in the world. Most people have seen it many times in their life – often without realizing it. Basalt is mafic in composition, containing less quartz than felsic rhyolite and intermediate andesite, but it can sometimes be difficult to distinguish from andesite when it comes to the percentage of mafic minerals present.
There are many different varieties of basalt, each with its own unique properties and appearance, but all varieties of basalt have quite a bit in common when it comes to their overall look. So, what does basalt look like?
Basalt has a fine-grained texture with very small, interlocking crystals of white to off-white plagioclase feldspar and darker, mafic minerals like pyroxene. The crystals are too small to see with the naked eye. Basalt is usually gray, brown, or black, and often contains features like vesicles and phenocrysts.
The vesicles in basalt are the remnants of gas bubbles that became trapped in the lava as it cooled. They are usually oval in shape and can range in size from very small to golf ball sized or larger.
Large crystals are often present in the fine-grained groundmass of basalt. These are known as ‘phenocrysts’ and are usually made of plagioclase feldspar or augite. They occur when basaltic magma has time to crystallize minerals with higher melt points before being erupted to the surface.
One of the defining characteristics of basalt is its overall lack of quartz. Rocks like rhyolite and dacite have quite a bit of quartz (over 20%) but true basalt is almost completely devoid of any visible quartz grains – less than 5%.
There are many variations of basalt that can look quite different from one another. These differences are due to subtle variations in mineralogy, the gas content of the lava, and how the lava cooled. Even small differences in these conditions can have a drastic impact on what basalt looks like.
Color is Driven by Mineralogy
The major minerals present in all basalt are pyroxene plagioclase feldspar. Alkali feldspars are also usually present, at up to 35% of the total feldspar content. Each of these minerals can present itself in different ways. The plagioclase feldspar in true basalt is rich in calcium, typically a species called labradorite.
Quartz is almost entirely absent from basalt. In some basalts, however, it can be present at up to 20% of the total rock volume. Of course, it is usually impossible to estimate the percentage of quartz in a basaltic rock (if any is present at all) because the crystals are too small to see. In any case, quartz is not a significant contributor to basalt’s overall dark 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 basalt.
Plagioclase feldspar is often the most abundant mineral in basalt, making up over 65% of the total feldspar content. There are several species of plagioclase feldspar, but in basalt it is usually the calcium-rich end member known as ‘labradorite’.
The ratio of plagioclase to darker, mafic minerals is the primary determining factor of how dark a piece of basalt appears. As a general rule, basalt is fairly dark and will have a ‘color index’ of at least 40, indicating that at least 40% of the minerals present are dark, mafic minerals.
Some of the most common mafic minerals (making up to 40-90% of the total rock volume) in basalt are pyroxene, hornblende, and olivine. Hornblende and pyroxene are very dark minerals and contribute significantly to basalt’s overall dark gray or black coloration. Olivine, while considered a mafic (or ultramafic) mineral, has a characteristic green color that sometimes gives basalt a greenish tinge if present in large enough quantities.
The groundmass (the main, fine-grained part of the rock) of basalt is almost always dark gray or black, but large, differently-colored crystals (phenocrysts) are very common. For example, plagioclase feldspar phenocrysts will look like white splotches or even very well-defined white crystals in a mass of fine-grained dark gray rock.
Texture and Morphology of Basalt
One of the defining features of basalt’s appearance is its texture. All basalt is fine-grained, meaning that you cannot see the individual crystals in the rock. This texture is known as ‘aphanitic’, and it forms when magma cools rapidly, preventing crystals from having enough time to grow before becoming completely solidified.
While all basalts are aphanitic, 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.
As I mentioned above, basalt is very often ‘porphyritic’. This texture describes large, visible crystals surrounded by a fine-grained groundmass. It occurs when minerals with a higher melt point have time to crystallize in the magma melt before the magma erupts to the surface and the entire melt rapidly crystallizes around them.
It is also very common for basalt to have a vesicular texture. As magma erupts to the surface it becomes depressurized, releasing a great deal of gas – very similar to opening a can of soda. Bubbles form in the lava, and the lava cools around them before the bubbles have time to completely escape. The remnants of these bubbles are called vesicles, and often become filled in with secondary minerals like agate.
One of my favorite things about basalt is the many shapes and sizes it comes in. It has many unique forms, referred to as morphology. Depending on where the basalt erupts and how it cools it can have drastically different looks.
Some of the most distinct and well-known basaltic morphologies are aa and pahoehoe lava flows. My favorite, however, is columnar basalt which can be seen in many famous locations all over the world. These distinct shapes are only clear when looking at the entire rock formation – you won’t be able to see them if you have just found a broken piece of basalt.
How to Identify Basalt
As common as basalt is, it can still 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. There are sometimes very few defining features on which to base an identification. As with any rock, it is important to take a systematic approach when identifying basalt.
To identify basalt, you should first ensure that you cannot see the crystals in the groundmass with the naked eye. Dark, mafic minerals should make up 40-90% of the rock by volume. Some larger crystals and vesicles may be present. Basalt is usually dark gray or black, but can also be brownish or greenish.
A rock must meet all of these requirements to be considered a basalt:
- Igneous – Formed from cooling magma, with interlocking crystal grains.
- Fine-grained – Aphanitic texture with groundmass crystals not 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
If your rock meets all of those criteria then it is very likely a basalt, or at least something very closely related. Remember that basalt comes in many different morphologies and that one specimen can look very different from the next.
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 basalt 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 Basalt Made Of?
As I mentioned above, basalt 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 basalt you have to know what those minerals are.
The mineral composition of basalt consists of minute 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 basalt-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 andesite as a possibility. The presence of plagioclase phenocrysts or vesicles is also a great indication that your rock is basalt.
Vesicular basalt is well-known for containing collectible minerals like agates. The vesicles in the basalt are gradually filled with microcrystalline quartz as mineral-laden water seeps through the rock formation.
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 (fine-grained, extrusive igneous rocks) but vary in the relative proportions of their minerals.
- Dacite – Quartz-rich, with more plagioclase than alkali feldspar
- Trachyte – Quartz-deficient, with more alkali feldspar than plagioclase
- Rhyolite – Quartz-rich, with more alkali feldspar than plagioclase
- Gabbro – Mineralogical equivalent of basalt, but coarse-grained
Where Is Basalt Found?
Basalt is, by far, the most common igneous rock found on the surface of the Earth, making up over 90% of all volcanic rocks. It is prevalent in locations all around the world in some form or another.
In general, basalt is found in regions where the Earth’s crust is relatively thin, most commonly the upper portion of oceanic plates. It is also associated with volcanic islands like Hawai’i and Iceland. It can be found on continental crust where flood basalts covered thousands of square miles.
Basalt is perhaps most well-known for forming oceanic tectonic plates at mid-ocean ridges. Basaltic magma erupts along the ridges and forms pillow basalt when it comes into contact with ocean water and rapidly cools.
You can get basaltic lava eruptions almost anywhere where the Earth’s crust is relatively thin. This is the case for hot spot islands like the Faroe Islands and Hawai’i where basaltic lava flows are still very common and continue to create more and more land.
Ancient basaltic lava flows are also fairly common on the continental crust. These lava flows can be enormous in size, covering many thousands of square miles. Famous examples include the Deccan Traps in India and the Siberian Traps in Russia. In the U.S., the best example is the Columbia River Plateau in the Pacific Northwest.
Basalt 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 basalt can survive in a river for great distances, which is why you can sometimes find pieces downriver from its source.
You can look for basalt 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 basalt and other rocks.
How Does Basalt Form?
We’ve learned all about what basalt 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 basalt is a relatively straightforward process that always follows a few simple rules but can vary significantly in the details.
Basalt forms when basaltic magma, rich in magnesium and iron, rapidly cools at or near the surface of the Earth, negating the opportunity for the formation of large crystals. This most commonly occurs at mid-ocean ridges in oceanic crust but also occurs in hot spots, volcanic arcs, and large igneous provinces.
The creation of basalt 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 basalt that we find on the surface today formed not as oceanic crust but as part of volcanic eruptions at the surface. Large basaltic magma bodies sometimes intrude upwards into more granitic rock or even erupt onto the surface in the form of large lava flows.
Basaltic lava flows very quickly because it is low in silica, allowing it to spread great distances like a flowing river. As the lava spreads out it cools very quickly, which doesn’t allow very much time for the nucleation and growth of large crystals.
Because the composition of the source magma is never exactly the same from one basalt to another and the surrounding environment is always different, there can be quite a lot of variation in how the lava cools and the crystals form. This is how you get different morphologies when the basalt finally cools into its solid form.
As the individual crystals of basalt’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.
What Is Basalt Used For?
Chances are good that you have seen plenty of basalt in your day-to-day life, but it can be easy to miss if you’re not paying attention. Because is very similar to other igneous rocks, it should come as no surprise that they are used in many of the same applications. Basalt has been in use for thousands of years because of its durability, strength, and abundance.
Basalt is most commonly used for interior and exterior building applications. Rough-cut basalt is often used as aggregate and is one of the most prevalent components of buildings, bridges, and other structures. High-quality varieties of basalt are also very popular for sculptures.
Like most rocks, basalt 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.
One of the biggest reasons basalt 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.