Breccia is a very unique rock type, with a highly variable appearance. It doesn’t get enough credit for how beautiful it can be, and many people aren’t even familiar enough with it to be able to readily identify it.
It is easy to confuse breccia for other rock types, but it is a well-defined and fairly widespread rock that can be found in locations all over the world. It’s worth knowing what it is and how it is identified.
Breccia is a clastic sedimentary rock made from many different sizes of angular grains, many of which are gravel-sized or larger. These grains are fragments of other rocks, bound together by cement which is typically silica or calcite. Its angular or subangular grains differentiate it from conglomerate.
While breccia is a clearly defined rock type, there are many different varieties and it can easily be confused for similar, closely related rocks. I’ll walk you through how to identify breccia, what different varieties look like, and where it can be found.
What Does Breccia Look Like?
Breccia is made up of bits and pieces of other rocks and minerals, which means that its appearance is, in large part, determined by what those original rocks looked like. Because every breccia’s source rocks are different, there is almost unlimited variety in breccia’s appearance.
Despite this variability, all breccia varieties have a lot in common that makes them easily identifiable if you know what to look for.
Breccia looks like many poorly-sorted rock fragments that have been mixed up and cemented together. The grains range in size from sand to pebbles or even cobbles, often with varying colors due to multiple source rocks. The grains are angular or subangular, with sharp points and jagged edges.
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While all breccia meets this general description, there is a pretty wide spectrum of breccias that can look significantly different from one another. Their differences in appearance are primarily driven by the rock fragments from which they form and their method of deposition.
Color is Driven by Source Rocks
Since breccia is made from relatively large pieces of other rocks, it’s only natural that it inherits the colors of those source rocks (parent rocks). In most cases, breccia will be made up of pieces of many different parent rocks, giving them a very colorful and disorganized appearance.
For reasons we will get into later, breccia tends to be made up of rock and mineral fragments that are fairly hard. Therefore, colors associated with these harder materials are more common in breccias.
Quartz and feldspar are some of the common and widespread minerals you’ll find in many types of rocks, and breccia is no exception. These minerals are fairly hard and resistant to weathering, so they can travel great distances before reaching their final resting place before lithification (turning into another rock).
Quartz tends to be a milky white or off-white color, while feldspar can be much more variable. Depending on the type of feldspar it may be whiteish, pink, red, or brownish.
While these harder parent rocks are very common, almost any rock can be a parent rock to a breccia. This means that any color you see in rocks can also likely be found in a breccia, given the right conditions.
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Some of the coloration in breccia comes from its cement. There are three primary cement types: silica, calcite, and iron oxide.
Silica and calcite are general very light in color or even colorless. They are also the most common types of cement. Iron oxide, however, is rust-red and often stains the breccia to match.
Texture of Breccia
One of the defining features of breccia’s appearance is its texture. All breccia is clastic and coarse-grained, meaning that it is made up of rock and mineral fragments and that the largest of those fragments are bigger than 2 mm across.
For a sedimentary rock to be considered a breccia, a large portion (typically over 30%) of its total volume must consist of large clasts. The rest of the rock is usually made up of sand, silt, or clay-sized particles which help to fill the space between the larger grains and cement the rock together.
The larger clasts must also be angular or subangular. This means that they have jagged edges or points, as opposed to being smooth and rounded. These angular fragments retain their original shape after being broken off from their parent rock and didn’t have time to be smoothed during transport before redeposition. This differentiates breccia from conglomerate, a closely-related rock type.
How to Identify Breccia
Breccia is so distinct looking and relatively recognizable that you might think that identifying it is a trivial matter. While that is often the case, that mindset often leads people to misidentify other rock types as breccia. As with any rock, it is important to take a more systematic approach when identifying breccia.
To identify breccia, first look for many different sizes of rock or mineral fragments mixed and cemented together. These fragments may range in size from sand to cobbles and be many different colors. Then, observe the shape of the larger grains to ensure they are angular to subangular and not rounded.
A rock must meet all of these requirements to be considered breccia:
- Clastic Sedimentary – Formed from the cementing together of rock and mineral fragments
- Coarse-grained – A significant portion of the rock consists of large (over 2 mm) grains
- Poorly Sorted – Highly variable grain sizes, ranging from sand to pebbles or cobbles
- Angular Grains – The larger grains are relatively jagged, with points and/or sharp edges.
If your rock meets all of those criteria then it is very likely a breccia, or at least something very closely related. There are, however, a few closely related rocks (and non-rocks) that people sometimes confuse for breccia.
Tip: This article is part of my sedimentary rock identification series. To read more about how to identify all igneous rocks, check out my article here.
Conglomerate is breccia’s closest relative in the rock world. It is essentially the exact same thing as breccia, but with rounded or sub-rounded grains instead of angular ones. This just means that the large rock fragments are well-worn and smoothed down, indicating that they were transported a great distance before deposition.
Metaconglomerate is a metamorphic rock type that forms when conglomerate or breccia undergoes low-grade metamorphism. While uncommon, you may call a metamorphosed breccia a Metabreccia. The low temperatures and pressures it is subjected to aren’t enough to fully transform the rock as you see in many other metamorphic rocks, but it is enough to change how the rock behaves structurally.
The easiest way to tell the difference between breccia and metaconglomerate is how the rock breaks. If it breaks cleanly through the larger rock fragments then it is metaconglomerate, but if it breaks around them then it is just regular breccia.
Another issue I encounter fairly frequently is people confusing concrete for breccia or conglomerate. This is understandable, since concrete is essentially a manmade conglomerate – a mixture of aggregate (rock fragments) and cement. However, by definition, rocks are non-manmade.
You can usually tell the difference between concrete and breccia by looking at the distribution of rock fragments and the color of the cement. If there is a noticeable lack of intermediately sized grains and the cement is the characteristic gray color, you almost certainly have a piece of cement.
What Is Breccia Made Of?
It is difficult to define what breccia is made from because it can literally be comprised of fragments of a virtually unlimited selection of one or more parent rocks. This means that breccia isn’t defined by mineralogy, but rather by its depositional setting, grain size, grain sorting, and grain shape.
However, that doesn’t mean that we can’t make some broad generalizations about what breccia is made from. As a clastic sedimentary rock, breccia is, by definition, composed of rock and/or mineral fragments. These fragments are broken pieces of older rocks that have been transported to their new place of deposition (likely very nearby) to form a new rock.
Breccia is generally comprised of 30% or more of rock fragments measuring more than 2 mm in size, with sand, silt, and clay making up the remainder of the rock volume. These grains are cemented together by silica, calcite, or iron oxide. The rock fragments can be made of any previously existing rock(s).
It is possible to find fossils in breccia, especially if they are still embedded in their original parent rock. For example, a fossiliferous limestone may break apart, become deposited, and then lithify into a breccia with other rock fragments. Any fossils in those pieces of limestone are likely to be preserved in the original structure of the limestone, provided the remaining rock fragments are big enough.
In general, breccia has less variation in its large rock fragments than conglomerate does. It is common for a breccia to be formed in a singular event like a landslide where the parent rock is limited to a few or even one rock type. This is in contrast to conglomerates which may form from many different rock types far upriver from the final place of deposition.
Where Is Breccia Found?
Breccia is actually a fairly uncommon sedimentary rock. I wouldn’t go so far as to call it rare or exotic, but it isn’t anywhere near as common as its clastic sedimentary brothers shale and sandstone, for example.
You can find breccia in several geologic settings where existing rocks are likely to undergo sudden trauma, with the resulting rock fragments undergoing rapid redeposition.
In general, breccia is found in areas with a history of sudden, high-energy events such as volcanic eruptions, fault zones, impact events, or, most commonly, in debris flows associated with events like landslides. Breccia forms very close to these events, without further transportation of the rock fragments.
Because breccia can be formed in so many distinct geological settings, it can be found in specific locations all over the world. Sometimes it isn’t obvious that the rock formation you’re looking at is a breccia. It may be easy to dismiss a rock layer as merely a jumbled-up mess of a bunch of other rocks, but that is precisely what breccia is. Once it consolidates and cements together, breccia is a new and separate rock, even when the rock fragments are boulder-sized or larger.
How Does Breccia Form?
We’ve learned all about what breccia 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 breccia is a fascinating process that can vary a bit from setting to setting but always follows a few simple rules.
Breccia forms when large, freshly-broken rock fragments are deposited in a high-energy environment, often as part of a singular debris flow. The large rock fragments are intermingled with smaller grains and later compacted and lithified (cemented) together to form a new, singular rock unit.
The rock fragments must not have had the opportunity to become rounded and smoothed. Since that process usually occurs as rock fragments abrade against one another in a river or stream bed, breccias usually form very close to their parent rock where there simply has been no time or opportunity for that abrasion to occur.
Sedimentary Breccia forms when rock fragments are deposited en masse through processes like erosion of a rocky mountain face or debris flows in water. The rock fragments break off of their host rock and are deposited very close nearby after falling a short distance.
Some breccia forms at the contact of fault surfaces. Fault breccia occurs when existing rock is crushed and broken apart by stresses in the Earth and then resolidified as new rock.
Pyroclastic breccia forms when a volcanic eruption breaks existing rock apart (often in a violent explosion) and the subsequent rock fragments accumulate and solidify into new rock. These breccias are often cemented together by volcanic ash (called tuff). Pyroclastic breccia can be classified as an igneous rock since it is created through volcanic processes.
Over time, broken rock fragments become compacted together by the weight of rocks and/or water above them. Then, cement such as silica or calcite precipitates out of the water seeping through the rock fragments to cement them together. The cement slowly grows between the rock fragments until they are fused together, completing the transformation from ‘sediment’ to ‘sedimentary rock’.
What Is Breccia Used For?
One of the reasons that you might not be overly familiar with breccia (at least before reading this article!) is that it isn’t widely used in practical applications. It is uncommon to see it used in construction or even as decoration.
Breccia is used as a filler material or crushed to make aggregate. In rare cases where it is reliably strong it was sometimes used for construction in the past, but is almost never used today. It is also sometimes used for prospecting as an indicator of rocks and minerals that may be found in the area.
There is plenty of history of breccia being used as both a construction material and as decoration in ancient times. The Greeks, Romans, and Egyptians all used breccia in parts of buildings that we can still see today. Brecciated varieties of jasper and marble are very popular with collectors even today. They can be fashioned into beautiful lapidary pieces displaying their broken and fragmented appearance, often with striking contrasts in color.
One example of conglomerate being used in construction is the Montserrat monastery in Spain, which I pictured more closely above. This is relatively rare because conglomerate, like breccia, is unpredictable and highly variable, and therefore can’t be relied upon for strength. It is also hard to cut into specific dimensions, making it difficult to build with.
This article is part of my rock identification series. To learn more about identifying rocks, check out my full in-depth guide here.