Sedimentary Rocks: Types, Formation, Characteristics, and Sample Questions

Sedimentary rocks along with metamorphic rocks and igneous rocks are the three types of rocks that make up the surface of the Earth. Land, air, and water are the three elements that make up our world. Now, if we look at what truly makes up landmasses, we can see that they are the solid landmasses that make up the earth's outer crust, which is largely made up of rocks. Igneous rocks, metamorphic rocks, and sedimentary rocks are all aggregate masses of natural minerals separated into three groups. Sedimentary rocks are the ones that are formed on or near the Earth's surface. As a result, it differs from metamorphic and igneous rocks, which are formed deep beneath the Earth. Erosion, weathering, dissolution, precipitation, and lithification are the most important geological processes that result in the production of these rocks. In this article, we will have a look at sedimentary rocks, their characteristics, classification, and uses.

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What are Sedimentary Rocks?

Sedimentary rocks make up 73% of the earth's land surface. They are, however, the least abundant form of rock in the entire earth's crust, accounting for only 8% of its total volume. Igneous and metamorphic rocks make up the majority of the earth's crust, which are subjected to years of weathering, attrition, and erosion as a result of temperature variations and other meteorological conditions. As a result, the dregs of these rocks dissolve and are deposited on the earth's surface. These deposits compress under environmental pressure after years of deposition. Sedimentary rocks are the rocks that form as a result of this process.

 

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How are Sedimentary Rocks Formed?

Sedimentary rocks are named as such because they are produced from the sediments of pre-existing rocks. The creation of sedimentary rocks is accomplished through a geological process known as lithification, which entails multiple processes: 

• To begin, existing rocks are eroded and weathered, which entails the physical breakdown of the rocks and the subsequent transfer of the debris away from the original rock.
• Sediments are frequently created as a result of chemical degradation caused by rainfall or dissolution. The movement of debris is the next step in the development of sedimentary rocks.
• As most sediments are carried by wind or flowing volumes of water such as rivers or glacial ice, gravity plays a significant role in assisting transportation.
• The sediments begin to deposit once they have been transported. When subjected to an appropriate temperature and air pressure, slit deposition can also occur as a result of precipitation. Sedimentary rocks like organic reefs are one example.
• Once the sediments begin to deposit, layers of these sediments begin to pile up on top of one another in a process known as burial.
• Through the process of lithification, these sediments become compacted and cemented into solid rocks once they reach a particular depth.Sedimentary rocks are created in this manner. Chalk, clay, flint, and shale are some of the most prevalent sedimentary rocks.

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What is Sedimentation?

Sedimentation refers to the combined process that causes the mineral and the organic particles to settle. The particles that help in the formation of sedimentary rocks are known as sediments. These are formed due to weathering and erosion processes and are then transported to the place where they are deposited by the wind, ice, water, and other agents of denudation.

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Types of Sedimentary Rocks

Sedimentary rocks can be divided into subgroups based on their formation mechanism or mineral makeup. Sedimentary rocks are classified as clastic, biochemical, or chemical rocks depending on their origin.

• Clastic Sedimentary Rocks: Clastic sedimentary rocks are sedimentary rocks formed from pre-existing rocks. Weathering loosens the bits of these rocks, which are subsequently carried to a basin, where the deposit of the sediment produces sedimentary rocks.
• Biochemical Sedimentary Rocks: Biochemical sedimentary rocks are made up of rocks that have been created by organic processes. Snail or clam calcium carbonate shells, for example, can aid in the formation of limestone.
• Chemical Sedimentary Rocks: Chemical sedimentary rocks, such as halite and gypsum, occur when minerals in solutions become supersaturated and so inorganically precipitate.

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Characteristics of Sedimentary Rocks

Let us now look at the distinct features of sedimentary rocks, now that we understand how they are generated:

• Porosity in sedimentary rocks ranges from one percent to fifty percent. This is because the particles in both organic and clastic sedimentary rocks, as well as chemical sedimentary rocks, are not always equally packed during the formation process.
• Porosity is estimated at 27 percent in more compact sedimentary rocks, but it can reach above 47 percent in more loosely packed rocks. While sandstones can have a porosity as low as 5%, clays can have a porosity as high as 50%.
• Even though some sedimentary rocks are porous, they are largely impervious to water. If they have joints or cracks, only a few types of sedimentary rocks are pervious.
• The sphericity of sedimentary rocks varies depending on where they travel during the transit process. The further sedimentary rocks are from their source rock, the more rounded they become and the greater their sphericity. 
• Similarly, sediments that have traveled longer distances have more sorted debris than sediments that have traveled lesser distances.
• Sedimentary Rocks have mud cracks which refers to the fissures created when the water that is present in the sediments dry out.
• These rocks house beddings that are of different sizes ranging from thin laminas to the thick stratas. These are primarily found in horizontal settings as a result of deposition of various agents.
• Sedimentary rocks have more fossils i.e. the remains of living organisms in their rock layers.
• They have a distinctive color which primarily ranges from white to black to red.
• The raindrops leave marks on sedimentary rocks known as rain prints. Similarly, the rocks that travel with water feature ripple marks.
• They have a cross-bedding which is a normal and a shorter one formed as a result of strong and whimsical currents.
• Sedimentary rocks also have rill marks that are the marks formed by waves that return back to the ocean after they hit the shore.

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Properties of Sedimentary Rocks

The properties of sedimentary rocks are as given below:

• Colour

Iron, a metal having two primary oxides: iron(II) oxide and iron(III) oxide, is frequently responsible for the color of these rocks. Only low oxygen (anoxic) conditions cause iron(II) oxide (FeO) to develop, giving it a grey or greenish color.

These rocks are in direct contact with the atmosphere in arid continental regions, and oxidation is a major process that causes the rock to turn red or orange. The presence of organic substances in the rock might cause it to turn black or grey.

• Texture

Although a rock's texture is a small-scale attribute, it determines many of the rock's large-scale qualities. The density, porosity, and permeability of the rock, for example. Chemical sedimentary rocks are made up of crystals and have a non-clastic structure.

The texture of these stones is mostly determined by factors such as clay matrix, detrital grain size and sorting, and particle roundness.

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• Mineralogy

Quartz or calcite make up the majority of these rocks. Sedimentary rocks differ from igneous and metamorphic rocks in that they typically contain a wide variety of important minerals. To put it another way, the origins of minerals in these rocks are frequently more complicated than in igneous rocks.

The minerals in these rocks may have formed as a result of precipitation during the sedimentation process. Mica, feldspar, and other less stable minerals have been converted to clay minerals such as kaolinite, illite, and smectite in most of these rocks.

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Examples of Sedimentary Rocks

Sedimentary rocks include:

• Limestone
• Chalk
• Clay
• Sandstone
• Shale

These rocks make about 75% of the earth's entire surface area

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Uses of Sedimentary Rocks

Sedimentary rock is used in almost all constructions and monuments. Some of its uses are as given below:

• Cement is made from limestone.
• Building stones are made of limestone and sandstone.
• Quartz is a sedimentary rock that is used in the production of glass.
• Plaster is made from rock gypsum.
• Sedimentary rocks are where natural gas, oil, coal, uranium, and other energy resources are created and extracted.

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Things to remember

• The term "sedimentation" refers to all of the processes that cause organic and mineral particles to settle. As a result, sediment refers to the particles that aid in the creation of sedimentary rocks.
• Sedimentary rocks cover the majority of the Earth's rocky surface, yet they account for only a small portion of the planet's crust.
• They frequently contain millions of-year-old plant and animal fossils.
• Limestone is frequently formed from the fossilized remains of marine creatures that perished millions of years ago.
• Sedimentary Rocks have mud cracks which refers to the fissures created when the water that is present in the sediments dry out.
• The texture of these stones is mostly determined by factors such as clay matrix, detrital grain size and sorting, and particle roundness
• These rocks create strata, which are frequently seen in exposed cliffs..

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