Sedimentary rocks are formed when the fragments of igneous rocks are broken down because of weathering and are carried away by the agents of gradation such as water, glaciers, and ocean waves. Deposition of these fragments of rocks results in the formation of sedimentary rocks. Lithification is a process where loose sediments are turned into rock.
Characteristics of sedimentary rocks
- Sedimentary rocks are also known as secondary rocks as they are formed from the sediments of other rocks denuded and deposited by the agents of gradation.
- They are largely found on the Earth’s surface, covering 75% area of the Earth.
- These rocks are generally not crystalline. They are soft and have many layers as they are formed because of the deposition of sediments.
- These rocks may have remains of plants and animals in between various layers.
- Sedimentary rocks may be subdivided on the basis of the nature of sediment, origin and composition.
Classification of sedimentary rocks
On the basis of processes
Evaporation: When the water evaporates, it leaves behind deposits of one or more chemicals. These chemicals are compacted and cemented to form rocks.
Compaction: When soft and loose sediments are squeezed by overlying layers of rocks and rocks lying beneath them, they become hardened. For example, sandstone is formed after compacting of sand.
Cementation: Binding together compacted sediments by natural materials such as silica and iron is called cementation.
On the basis of the formation
Mechanically formed rocks: When old rocks are denuded and broken into fragments by running water or moving glaciers, they are deposited on new lands resulting in the formation of rocks.
Chemically formed rocks: They are formed when chemical sedimentation takes place generally because of the evaporation of water-containing salts in the solution.
Organically formed rocks: These rocks are formed when the remains of plants and animals get compressed forming a rock. For example, limestone and lignite.
On the basis of agents of formation
Riverine rocks: These rocks are formed by the deposition of alluvial soil by streams of water.
Lacustrine rocks: These are found on the bed of a lake after long periods of deposition.
Glacial rocks: These are formed because of the deposition of debris by glaciers.
Aeolian rocks: The cementation of sand particles brought by winds result in the formation of Aeolian rocks.
Marine rocks: These rocks are formed on the ocean floor. There are two types of marine rocks. Calcareous marine rocks are formed by the deposition of shells and skeletons of sea organisms.
Carbonaceous rocks: These rocks are formed when sea plants and animals are buried under the sea for a long period of time.