Nearly all sedimentary rocks can be categorized as clastic, but what does that mean to us?
Simply, clastic sedimentary rocks are rocks that have formed from particles of other rocks.
In the article that follows, we’ll explain what it means to be clastic, and give you some prime examples of clastic sedimentary rocks you are likely to come across.
Clastic Sedimentary Rocks Examples
What Is A Clastic Sedimentary Rock?
Clastic sedimentary rocks typically form when rock fragments are cemented together.
Previous to their cementation, these fragments eroded from larger rocks.
After erosion, the rock fragments were transported to an area, typically bowl-shaped, where they were compressed together.
We can identify a variety of different clastic sedimentary rocks, including Breccia, Conglomerate, Sandstone, Siltstone, and Shale.
The smallest sedimentary rocks are microscopic, while the largest we’ve discovered are boulder-sized.
Each type is unique in both characteristics and appearance.
For example, breccia rocks are often rather rounded and bulky, while shale rocks are known for their distinctive flat structure.
These differences are caused by the size of the materials that make up each rock.
This is seen in sedimentary rocks containing sand or silt that are less likely to be rounded than those with large clasts.
Let’s examine each clastic sedimentary rock type and how to identify them with the naked eye!
The first of our clastic sedimentary rocks is Breccia, one of the largest types.
Breccia rocks are characterized by their angular clasts and asymmetrical shape.
They come in any color, while their chemical composition depends on each fragment’s original rock composition.
During formation, the clasts in each breccia rock are cemented together with either mineral cement or a similar material.
The best way for us to identify breccia rocks is through their clast shape.
The pointy and often lopsided clasts are exclusive to this rock type.
This is in contrast with the clasts in our next rock type, conglomerate, which are known for their round shape.
The clasts in a breccia rock will be at least two millimeters in diameter, similar to about twenty sheets of paper stacked together.
Our second type of clastic sedimentary rock is conglomerate, known for looking similar to concrete.
Although conglomerate rocks are similar in size to breccia rocks, they are formed quite differently.
Conglomerate rocks are assembled when water deposits fragments of other rocks in a rounded shape.
The most critical difference in the formation of conglomerate rocks and breccia rocks is the presence of water.
We’ve identified several main types of conglomerate clasts, such as quartz, granite, or limestone.
In between the clasts, the rock will be composed of materials like sand or mud.
Because of the circular shape of the clasts, we can infer that the rocks were discovered quite far from their original deposition site.
This is due to the amount of time it takes to tumble the clasts into their signature shape and the flow of the water.
Our third clastic sedimentary rock type is sandstone, unique due to the presence of organic materials in its composition.
These organic materials consist primarily of shell pieces in the sand.
Like conglomerate rocks, sandstone rocks can be formed through fragments being deposited by water.
However, sandstone can also be formed by wind deposition.
With sand being the primary element of sandstone, we see it used frequently in construction or glass manufacturing.
The rock is also cemented together with silt and clay, as well as other minute materials.
It is known for its characteristic striped layers and often tan or reddish appearance.
This type of clastic rock is one of the most common types and looks quite similar to our next type: siltstone.
The main difference between the stones is the central element of their composition.
Like conglomerate rocks, sandstone rocks can be made of quarts, but they differ in shape and size quite a bit.
While conglomerate rocks tend to be rounded, sandstone rocks are flat.
Siltstone rocks are primarily composed of their namesake, silt, and similar particles.
Silt itself is made from feldspar, quartz, or clay-like minerals.
For a siltstone rock to be formed, the small grains of previous rocks are deposited by rain, wind, or ice, and then cemented together.
These rocks are much less common than sandstone or shale, despite the similarities between the types.
To identify if a specimen is siltstone, we must observe the size and structure of the grains themselves.
We do this by removing a small piece of the rock and separating the grains.
A siltstone rock will be composed of particles that are finer than sand.
The color varies, but it is most commonly observed as gray or brown.
Because of their formation process, siltstone rocks are usually fairly flat and thin.
Our final clastic rock type is the most common type of all five: shale rocks.
These rocks are the thinnest type and are identified as sedimentary rocks that have been laminated and composed of fine-grained materials.
Like many of the other types, quartz is a common element in shale rocks.
They are formed through water transportation in light currents and then cemented.
Because of this process, shale rocks are frequently found on the ocean floor or in similar environments.
A unique aspect of shale rocks is their tendency to break into flat layers.
These rocks can be any color, although the presence of organic material tends to darken the shade.
Shale is regularly used in the ceramic industry, as well as cement, as a form of raw material.
It is similar in composition to siltstone and sandstone, despite its differences in the method of formation.
Clastic sedimentary rocks vary in the method of formation and composition, but they share the common trait of being comprised of fragments of other rocks.
Whether made of sand, quartz, or organic materials, our clastic rocks include the most common types of sedimentary.
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