What Are Sedimentary Rocks – Clastic, Biogenic, Chemical
Sedimentary rocks are comprised of varying sizes of grains of rock or organic fragments which are called sediment. They are set apart into three different types of sedimentary rock; clastic, biogenic, and chemical. These types of sedimentary rocks are made different by the way they are formed and their composition.
What are Clastic Sedimentary Rocks?
Clastic sedimentary rocks are pre-existing rocks being weathered down to sand, gravel, or even clay fragments. These fragments, which are called clasts, get cemented together by pressure. Clastic fragment sizes can range from very small grain sizes to giant boulders.
Weathering will loosen rock fragments making them available for further transport to a final resting place. Smaller fragments will be more easily carried by wind while larger fragments can be more easily carried by rivers.
Clastic grain sizes of sedimentary rocks are;
- Boulder – Large rocks, and often too heavy to carry.
- Cobble – Are rocks capable of being picked up in your hands.
- Pebble – Smaller rocks.
- Granules – Small grains of rock, like sand.
- Silt – Rock that is too small to actually be visible. Clay is a good example of this
What are Biogenic Sedimentary Rocks?
Biogenic sedimentary rocks are rocks created from organisms. Organic contents can include animal skeletal fragments and remains, plants, and ocean life remains such as corals, molluscs, radiolarian, and diatoms. Sometimes called organic sedimentary rocks, these organic elements collect in layers and are cemented together by pressure.
Some common examples of biogenic sedimentary rock are chert and limestone. These rocks are formed from the remains of the ocean life as mentioned.
Large amounts of dead plants that are compressed as a sediment can be found in layers and is known as peat. These plant layers that were compressed for longer amounts of time and deeper in the earth form what we know today as coal.
What are Chemical Sedimentary Rocks?
Chemical sedimentary rocks are rocks formed from chemical reactions of highly charged positive and negative ions within saturated water. The process of these negative and positive ions naturally attracting to form bonds of solid mineral crystals is called precipitation. This process give us rocks like halite (rock salt), sylvite, barite, and gypsum.
How are Sedimentary Rocks Formed?
Sedimentary rocks are formed on ocean floors or in other bodies of water. They form when particles of rocks or organic material are accumulated and cemented together. These particles, or what is called sediment, come about from the process of erosion of other rocks.
As sediment is built up, it will be pressurized into solid layers as each new layer compacts down on the previous layer. Layers are commonly known as bedding or stratification.
Weather plays an important role in transporting these sediments to its resting layers. Wind will transport small debris of rocks through the air, water will move rocks along a river, freezing and thawing will break rocks apart for easier transportation. This type of weathering is commonly called mechanical weathering.
There is also something called chemical weathering. This is when minerals come in contact with elements that will cause a chemical reaction. This reaction can create a new mineral or byproduct.
Sedimentary Rocks are the most Common Type of Rock
The most common type of sedimentary rock is shale, which is a clastic sedimentary mudrock. It is the most common of the mudrocks and mudrocks make up about 65% of all sedimentary rocks.
Sandstones will make up about 20% to 25% of all sedimentary rocks and carbonate rocks will be at about 10% to 15% of all sedimentary rocks.
Sedimentary rocks are the most common type of rock on our earth’s surface. In general, they make up roughly 75% of the rocks you see around you.
The difference between sandstone and mudrocks are; when there are sand sized fragments, it is considered sandstone. If there is silt and clay sized fragments, then they are mud rocks. For the determination of carbonate rocks, there will be calcite, aragonite, or dolomite.
Identifying Sedimentary Rocks
To identify sedimentary rocks you need to look at the mixture of the rock material. Any mixture of rocks, silt, sand, or clay clumped together, will be defined as a sedimentary rock. Conglomerate rocks have many larger rocks cemented together. Sandstone has many fine grained particles cemented together. Some organic mixtures with shells and other plant life will be considered as a sedimentary rock.
Sedimentary rocks are found wherever they have been exposed on the earth’s surface. They can be found where there is little water to very saturated climates. Sedimentary rocks can be found in both high and low altitudes.
Sedimentary Rocks are Important
Sedimentary rocks are important as they can contain earths organic history through the fossilization of many creatures. These rocks are also important as they will hold liquids such as water, oil, and other important minerals.
We extract many minerals as well as oil and water from sedimentary rocks for our daily uses. We even used sedimentary rocks as building materials in many of our building structures.
Sedimentary rocks are very important in the overall rock cycle. They take part in the continual renewal and use of our earths minerals and resources. Renewal in the rock cycle can take quite a bit of time, for more information on the rock cycle you can read about it here “Diagram of the Rock Cycle Explained“
If you have some further interest in sedimentary rocks you may be interested in seeing some examples of various types of sedimentary rocks. Here is a great article showing some example pictures and a short description of some popular sedimentary rocks. “Several Examples of Sedimentary Rocks”
