Sedimentary Rocks

Sedimentary rocks are formed from pre-existing rocks or the remains of once-living organisms, this means they contain fossils of animals and plants and are non-crystalline.

Sedimentary rocks vary in texture, they may be made of fine or coarse particles depending on the nature of the materials they are composed of. They are soft and easy to penetrate, therefore, sedimentary rocks are not resistant to erosion.

Mode of Formation:

Sedimentary rocks are formed from sediments deposited by either water, wind or ice. These sediments form layers or strata one on top of the other, over time these layers cement themselves together through compression and become hardened to form sedimentary rocks. Sedimentary rocks are usually formed within bodies of water. Sedimentary rocks are stratified (or layered) rocks, the plane between two strata is known as a bedding plane.

Types of Sedimentary Rocks:

There are three categories of Sedimentary Rocks. Sedimentary rocks are categorized based on how they are formed, these categories are;

1. Mechanically Formed Sedimentary Rocks:

This type of sedimentary rock is formed from pre-existing rocks by mechanical weathering. The pre-existing rock particles are pressed together becoming hardened and solid by the weight of the overlying deposits. This process of hardening and solidification is known as compaction or cementation. Examples of mechanically formed sedimentary rocks are sandstone, breccia, shale, clay and conglomeratesA conglomerate is a type of multi-industry company that consists of several different and unrelated business entities that operate in various industries under one corporate group. More.

Mechanically formed Sedimentary Rocks may give rise to Conglomerates. Conglomerates are formed from the cementing of rounded cobble and pebble-sized rock fragments. In conglomerate sedimentary rocks, the rock particles are transported by river movement or ocean wave action. 

2. Organically Formed Sedimentary Rocks:

This type of sedimentary rock is formed from the remains of living organisms. These living organisms could be either plants or animals. Sedimentary rocks formed organically from plant matter (from swamps and forests) are called carbonaceous rocks. Examples of carbonaceous rocks are coal, peat, lignite and petroleum. Sedimentary rocks formed organically from animal remains in water bodies (e.g. corals and shellfish) are called calcareous rocks. Examples of calcareous rocks are limestones, dolostones and chalk.

3. Chemically Formed Sedimentary Rocks:

This type of sedimentary rock is formed through the chemical precipitationPrecipitation is any liquid or frozen water that forms in the atmosphere and falls back to the Earth. It comes in many forms, like rain, sleet, and snow. More of pre-existing rock materials. These precipitated particles or sediments harden over time to form sedimentary rocks chemically. Examples of chemically formed sedimentary rocks are potash, gypsum, borax, and rock salt.