Sedimentary Rocks

There is a story that goes behind every rock in the planet. The formation of rocks is not easy to understand. Rocks may be difficult to detect even to the students and experts. In this blog, we shall discuss some of the examples of the sedimentary rocks so that you can easily identify them as you come across them. Sedimentary rocks occupy nearly three-quarters of the landmass of the Earth, yet geologists have estimated that they constitute approximately half of the crust of the earth.

Consider it to be a mere analogy. Suppose that the crust of the Earth is a great cake. The top-most frosting is that of sedimentary rock. The frosting is also thin and therefore, it takes a very small share of the cake volume. But it nearly fills the whole top surface hence its common appearance.

What Are Sedimentary Rocks

Sedimentary is a word that is derived by the Latin term, sedimentum, which translates to settling. Sedimentary rocks are the rocks that are formed with the help of the particles of the already existing rocks and come together slowly. This is caused by weathering, erosion and deposition to produce layers of sediment.

How sedimentary rocks form

Sedimentary rocks are formed as a result of mechanical and chemical weathering. Existing rocks begin the process of weathering. The material that has been weathered is moved to other places by gravity and erosional agents where it is deposited. On transport, further disintegration of the sediments occurs and at some point they form sedimentary rock.

Principles of sedimentary rocks

 Sediment occurs in two major sources.

1. A layer of solid particles which are formed due to both mechanical and chemical weathering. These formations are referred to as detrital. Rocks that are created as a result of detrital deposits are known as detrital sedimentary rocks.

2. Material which is formed by a high percentage of chemical erosion. When the inorganic or biological method causes the precipitation of ions in solution, the substance is referred to as chemical sediment. The rocks that are formed using this material are termed as chemical sedimentary rocks.

Types of sedimentary rocks

There are two main types.

Detrital Sedimentary Rocks

The detrital sedimentary rocks are:  

•           Composed of pieces of rocks that had been formed before

•           It consists of clay minerals and quartz in the majority

•           Sorted by the size of their sediments (clastic rocks)

Examples: sandstone, shale and conglomerate.

Chemical Sedimentary Rocks

Composed of minerals which are deposited by precipitation out of the water, whether by inorganic effects or by biologic action, such as the growth of corals.

Examples Limestone, rock salt, chert, flint and rock gypsum.

Approximately 90 percent of limestones are comprised of biochemical sediments (shells/skeletons).

Examples of chemical sedimentary rocks

Limestone

•           It is made of calcite carbonate, CaCO 3

•           Marine biochemical limestones are coral reefs, coquina (broken shells), and chalk (microscopic organisms)

•           Inorganic limestones encompass travertine and oolitic limestone

Evaporites

Form Water that has been evaporated leaves behind dissolved minerals in solid form. Examples of them are rock salt and rock gypsum. 

       

Use of Sedimentary Rocks in Civil Engineering Field

• Limestone is a primary raw material for manufacturing Portland cement.

• Crushed limestone is widely used as aggregate in concrete, road bases and drainage layers.

• Lime from limestone is used for soil stabilization and improving weak clay soils.

• Sand (from sandstone weathering) is essential for concrete, mortar and asphalt.

• Sandstone is used as a durable building and decorative stone for walls, cladding and flooring.

• Shale is used to produce bricks, tiles and lightweight aggregates.

• Gypsum is used in making plaster, drywall and POP (plaster of Paris).

• Conglomerate and gravel are used in road construction and foundations as base materials.

• Sedimentary rocks help engineers assess soil layers, bearing capacity and groundwater conditions.

• Sandstone and sand layers are used in water filtration systems due to high permeability.