Dacite is an extremely common rock type, but many people have never even heard of it. Chances are good that you have seen and touched dacite many times in your life, whether or not you were aware of it at the time. It comes in many varieties, and it is worth learning more about due to its prevalence and geologic significance.
Dacite 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 dacite looks like and how to identify it, but it is relatively simple if you know what to look for.
Dacite is an extrusive igneous rock composed primarily of feldspar, quartz, and smaller amounts of mafic minerals. It has an aphanitic texture, meaning its interlocking crystals cannot be seen with the naked eye. Dacite comes in many colors but is usually off-white, light gray, or brown and is often porphyritic.
While dacite is a clearly defined rock type, it can be hard to distinguish from rocks like rhyolite and andesite – especially if you don’t have access to sophisticated testing methods. I’ll walk you through how to identify dacite, what it looks like, and where it can be found.
What Does Dacite Look Like?
Dacite is a very common rock. Most people have seen it many times in their life – often without realizing it. Dacite is felsic in composition, containing more quartz than andesite and more plagioclase feldspar than rhyolite. It can often be very difficult to distinguish dacite from these closely-related rock types.
In everyday life, it is relatively unimportant to be able to distinguish between dacite and other fine-grained igneous rocks, so people often just dismiss the exercise entirely. 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 dacite actually look like?
Dacite has a fine-grained texture with minute crystals of white to off-white plagioclase feldspar and quartz, and less than 15% dark, mafic minerals like hornblende. The crystals are too small to see with the naked eye. Dacite is usually light gray or brown and often contains larger crystals called phenocrysts.
Large crystals are often present in the fine-grained groundmass of dacite. These are known as ‘phenocrysts‘ and are usually made of quartz, plagioclase feldspar, hornblende, or pyroxene. They occur when dacitic magma has time to crystallize minerals with higher melt points before being erupted to the surface.
While all dacite meets this general description, there is a pretty wide spectrum of dacites that can look significantly different from one another. The difference in their appearance is driven by the mineralogy of each type of dacite and, to a lesser extent, the subtle differences in their textures (crystal sizes).
Color is Driven by Mineralogy
The two major minerals present in all dacites are quartz and plagioclase feldspar. Alkali feldspar is also very common, but in smaller amounts than plagioclase. Each of these minerals can present itself in different ways. Feldspars, in particular, can look quite different from rock to rock depending on the specific type of feldspar that is present.
Quartz is abundant in dacite as translucent, light gray, or off-white crystals. The crystals in the groundmass are too small to see individually, but you can often clearly see some quartz in the form of phenocrysts. Some varieties may have more impurities which make the quartz crystals an opaque gray or white color. In any case, quartz is a significant contributor to dacite’s overall light color.
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 usually present in dacite.
Plagioclase feldspar is somewhat more abundant in dacite than is alkali feldspar, but both are common. Being able to distinguish between the two is perhaps the most important (and most difficult) aspect of identifying a rock as dacite as opposed to rhyolite. 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.
In general, dacite will be more white or gray, while rhyolite will be pink or reddish. 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. If your rock has large feldspar crystals you may be able to see the striations, especially with the aid of a hand lens.
Some of the most common accessory minerals (minerals found in relatively low amounts) in dacite are mica, pyroxene, and hornblende. The mica (usually biotite, a dark variety of mica), forms very flaky ‘books’ as crystals in the rock. Hornblende and pyroxene will almost always look like dark, blocky crystals interspersed throughout the granite.
The mafic accessory minerals in dacite control how dark the rock appears. By definition, dacite contains less than 15% mafic minerals so it tends to be very light colored.
Texture of Dacite
One of the defining features of dacite’s appearance is its texture. All dacite is fine-grained, meaning that you cannot see the individual crystals in the rock. This texture is known as ‘aphanitic’ and it occurs when magma cools rapidly, preventing crystals from having enough time to grow before becoming completely solidified.
While all dacites 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, dacite 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 before it erupts to the surface and the entire melt rapidly crystallizes around them.
Dacite that is not porphyritic may often be described as ‘equigranular‘, which means that all of the crystals are approximately the same size. So, a dacitic rock may be described as ‘aphanitic and equigranular’ if it is made up of minute crystals with no visible phenocrysts.
How to Identify Dacite
As common as dacite 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 dacite.
To identify dacite, you should first ensure that you cannot see the crystals in the groundmass with the naked eye. Dark, mafic minerals should make up 0-15% of the rock by volume. Some larger, visible crystals may be present. Dacite is usually light gray or light brown, but can also be white, green, or reddish.
A rock must meet all of these requirements to be considered a dacite:
- Igneous – Formed from cooling magma, with interlocking crystal grains.
- Fine-grained – Aphanitic texture with groundmass crystals not visible to the naked eye
- Felsic Mineralogy – Over 20% quartz content and 0-15% dark, mafic minerals. The rest is mostly plagioclase feldspar
If your rock meets all of those criteria, it is likely a dacite, or at least something very closely related. Remember that dacite 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.
It can sometimes be very difficult to distinguish dacite from closely related rock types like andesite. The best you can do is to first estimate the percentage of dark, mafic minerals in the rock. If it is under 15% then your rock is felsic, which helps you rule out andesite as a possibility. The presence of plagioclase phenocrysts is also a great indication that your rock is dacite as opposed to rhyolite, which contains more alkali feldspar.
When observing and identifying a rock like dacite 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.
Like most rocks, dacite 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 Dacite Made Of?
As I mentioned above, dacite 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 dacite you have to know what those minerals are.
Dacite is primarily made from large amounts of quartz and plagioclase feldspar, with smaller amounts of alkali feldspar and mafic minerals like amphibole and hornblende. The mineral composition of dacite consists of 20% to 60% quartz, and 35% to 90% of the total feldspar must be plagioclase.
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 dacite-like rock falls into those specific percentage ranges – especially since the crystals are too small to see with the naked eye.
The best you can do is to first estimate the percentage of quartz in the rock. If it is between 20 and 60% quartz then you know it is probably either dacite or rhyolite. Then, estimate the percentage of the feldspar that is plagioclase vs alkali feldspar. In the field or classroom setting this is best done by observing the color of the rock. If there is noticeably more whitish plagioclase than pinkish alkali feldspar then you can confidently call your rock ‘dacite’.
It is useful to know what some closely-related rock types are called and what their mineralogy looks like.
- Rhyolite – Quartz-rich, with more alkali feldspar than plagioclase
- Andesite – Quartz-deficient, with more plagioclase than alkali feldspar and 15-40% mafic minerals
- Granodiorite – Same mineralogy as dacite, but coarse-grained
Where Is Dacite Found?
Dacite is a fairly common igneous rock that can be found in locations spread all around the world. However, it isn’t as widespread as rocks like basalt and is typically constrained to specific geologic settings.
In general, dacite is found on the continental side of subduction zones where partial melting of basaltic crust results in stratovolcanoes forming over large areas. It is also sometimes associated with hot spots and any other geologic environments where basaltic magma mixes with granitic magma.
By far the most common geologic environment in which this occurs is convergent plate boundaries with subduction zones. I’ll explain more in detail why this is the case in the next section of this article. In the U.S., dacite is most commonly found in the western states – particularly in the Pacific Northwest in association with large stratovolcanoes such as Mount Rainier and Mt. Saint Helens.
Dacite is composed of relatively hard minerals (primarily feldspar and quartz) which make it incredibly durable. Once it is exposed to the surface it takes a long time for it to break down. Pieces of dacite can survive in a river for great distances, which is why you can often find pieces downriver from its source.
You can look for dacite 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.
How Does Dacite Form?
We’ve learned all about what dacite 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 dacite is a fascinating process that always follows a few simple rules but can vary significantly in the details, which is the reason dacite can look so different from one sample to another.
Dacite forms when viscous, felsic lava rapidly cools at or near the Earth’s surface, negating the opportunity for the formation of large crystals in the groundmass. This most commonly occurs in volcanic arcs associated with subduction zones but also occurs in hot spots and other settings.
The creation of dacite really begins with oceanic plates, which are basaltic in composition. At subduction zones, oceanic plates sink below continental plates which are generally made of more granitic rock. Once the oceanic plate reaches a certain depth and temperature it begins to melt, forming basaltic magma.
This partial melting causes the basaltic magma to work its way up through the granitic continental crust where it undergoes a series of changes, fundamentally changing the chemistry of the magma. Quartz and sodium are added to the melt as it continues to the surface where it eventually erupts, forming dacite.
The phenocrysts that are so commonly seen in dacite form before an eruption. As the intermediate magma mixture sits below the surface, it sometimes has enough time for larger crystals of quartz, plagioclase feldspar and other minerals to form. Then, when the magma erupts to the surface, the rest of the mixture rapidly cools into smaller crystals surrounding the larger ones.
Dacitic lava flows slowly when compared to basaltic lava because it has a higher viscosity. It tends to erupt more violently and form steeper volcanos with large blocks of rock. As the lava comes into contact with the air 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 dacite 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 dacite finally cools into its solid form.
As the individual crystals of dacite’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 Dacite Used For?
Chances are good that you have seen plenty of dacite in your day-to-day life, but it can be easy to miss if you’re not paying attention. Dacite has been in use for thousands of years because of its durability, strength, and relative abundance.
Dacite is most commonly used as aggregate in exterior building applications. Highly polished dacite is sometimes used for countertops, while rough-cut dacite is a common component of buildings, bridges, and other structures.
Like most rocks, dacite 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.
What really sets dacite apart is its ability to accept a polish and its resistance to weathering. The highly polished ‘granite’ countertops you’re used to seeing in modern buildings are sometimes actually made of dacite. They are made possible by the fact that quartz and feldspar are easily brought to a high shine.
One of the biggest reasons dacite 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.