Sedimentary Rocks: Formation, Types, Examples

Sedimentary rocks are a type of rock that formed through the accumulation and lithification of sediments, which are particles of rock, minerals, or organic matter that are transported and deposited by wind, water, or ice. These particles, over time, undergo compaction and cementation, transforming into solid rock.

Sedimentary rocks make up the majority of the rock exposed at Earth's surface, covering about 75% of the Earth's crust. However, they are only about 8 percent of the volume of the entire crust. This is because sedimentary rocks are typically thinner than igneous and metamorphic rocks, which form deeper within the Earth's crust.

How Are Sedimentary Rocks Formed

The formation of sedimentary rocks involves a series of distinct processes, each contributing to their unique characteristics.

Weathering and Erosion

The initial step in sedimentary rock formation is the breakdown of pre-existing rocks and the creation of sediments. This process, known as weathering, can be caused by physical, chemical, or biological agents. Physical weathering involves the disintegration of rocks due to mechanical forces like temperature fluctuations, freezing-thawing cycles, and abrasion by wind and water. Chemical weathering, on the other hand, involves the dissolution of rocks by water and acids, altering their chemical composition. Biological weathering, mediated by organisms like plants and microorganisms, contributes to the decomposition of organic matter, creating sediments as a byproduct.

What are Sedimentary Rocks? - Types, Formation, Uses, and Examples
What are Sedimentary Rocks? - Types, Formation, Uses, and Examples

Transportation

Once sediments are formed, they are transported by various agents, primarily wind, water, and ice. Wind carries fine particles like silt and clay, while water moves heavier sediments like sand and gravel. Glaciers, with their immense ice masses, can transport a wide range of sediments, from boulders to fine clays. During transportation, sediments may undergo abrasion, sorting, and rounding, resulting in particles of similar size and shape.

Deposition

The final stage in sedimentary rock formation is the deposition of sediments at a particular location. Deposition occurs when the energy of the transporting agent decreases, causing the sediments to settle out of suspension. Calm water environments, like lakes and seas, promote the deposition of finer sediments, while more energetic environments, like rivers and coasts, favor the deposition of coarser sediments.

Lithification

After deposition, sediments undergo a process known as lithification, which transforms them into solid rock. Lithification involves several mechanisms, including compaction, cementation, and recrystallization. Compaction occurs as the weight of overlying sediments presses down on the deposited material, squeezing out water and reducing pore space. Cementation involves the precipitation of minerals between sediment grains, binding them together into a cohesive mass. Recrystallization involves the dissolution and rearrangement of existing minerals, creating a more tightly interlocking structure.

Types of Sedimentary Rocks

Sedimentary rocks can be classified into three main types based on their origin:

  • Clastic Sedimentary Rocks
  • Chemical Sedimentary Rocks
  • Bioclastic Sedimentary Rocks

Clastic Sedimentary Rocks

Clastic sedimentary rocks formed from the accumulation and lithification of clasts, which are fragments of pre-existing rocks or minerals. These clasts are transported by wind, water, or ice and eventually deposited in a variety of environments, such as rivers, lakes, and oceans. Over time, these clasts are compacted and cemented together, forming solid rock. Clastic sedimentary rocks are the most common type of sedimentary rock.

Clastic Sedimentary Rocks
Clastic Sedimentary Rocks Examples

Examples of clastic sedimentary rocks include sandstone, shale, and conglomerate.

Sandstone

Sandstone is a clastic sedimentary rock composed of sand-sized (0.0625 to 2 mm) grains, typically quartz or feldspar. It is formed by the lithification of sand, which is a loose sediment that is transported by wind, water, or ice. Sandstone is a common building material and is also used in a variety of other applications, such as glassmaking and electronics.

Shale

Shale is a clastic laminated sedimentary rock composed of clay-sized (less than 0.004 mm) particles. It is formed by the lithification of clay, which is a fine-grained sediment that is transported by water. Shale is a soft, flaky rock that is used in a variety of applications, such as making bricks, ceramics, and roofing tiles.

Conglomerate

Conglomerate is a clastic sedimentary rock composed of rounded gravel-sized (2 millimetres (0.08 inch) in diameter) clasts. It is formed by the lithification of gravel, which is a coarse-grained sediment that is transported by water. Conglomerate is a durable rock that is used in a variety of applications, such as construction, landscaping, and road building.

Chemical Sedimentary Rocks

Chemical sedimentary rocks are formed from the precipitation of minerals from solution. 

Examples of chemical sedimentary rocks include limestone, dolostone, and gypsum.

Limestone

Limestone is a chemical sedimentary rock composed of calcium carbonate(CaCO₃). Limestone is formed by the precipitation of calcium carbonate from seawater or from freshwater lakes and ponds. Limestone is a common building material and is also used in a variety of other applications, such as making cement, fertilizer, and paper.

Dolostone

Dolostone is a chemical sedimentary rock composed of calcium magnesium carbonate CaMg(CO₃)₂.  Dolostone is similar in appearance to limestone, but it is harder and denser. Dolostone is formed by the alteration of limestone or by the precipitation of calcium magnesium carbonate from seawater. Dolostone is a common building material and is also used in a variety of other applications, such as making fertilizer and glass.

Gypsum

Gypsum is a chemical sedimentary rock composed of calcium sulfate CaSO₄·2H₂O. Gypsum is a soft, white rock that is easily carved and molded. Gypsum is formed evaporates, leaving behind calcium sulfate minerals. Gypsum rock can be found in a variety of environments, including deserts, caves, and salt domes. Gypsum is used in a variety of applications, such as making plaster, fertilizer, and wallboard.

 

Chemical and Bioclastic Sedimentary Rocks
Chemical and Bioclastic Sedimentary Rocks Examples


Bioclastic Sedimentary Rocks

Bioclastic sedimentary rocks are a type of sedimentary rock formed from the accumulation and lithification of the remains of dead organisms. These organisms can be plants, animals, or microorganisms. Bioclastic sedimentary rocks are often composed of a variety of different types of fossils, including shells, bones, and teeth.

Examples of bioclastic sedimentary rocks include chalk, limestone, and coquina.

Chalk

Chalk is a bioclastic sedimentary rock composed of microscopic skeletons of marine organisms, such as plankton and algae. It is formed by the accumulation of these skeletons on the seafloor. Chalk is used in a variety of applications, such as making blackboard chalk, cosmetics, and paint.

Limestone

Limestone can also be a bioclastic sedimentary rock if it is formed from the accumulation of the shells or skeletons of marine organisms. Bioclastic limestone is a common building material and is also used in a variety of other applications, such as making cement, fertilizer, and paper.

Coquina

Coquina is a bioclastic sedimentary rock composed of fragments of shells or skeletons of marine organisms. It is formed by the accumulation of these fragments on beaches or in shallow water environments. Coquina is a common building material and is also used in a variety of other applications, such as making jewelry and decorative items.

Diatomite

Diatomite is a bioclastic sedimentary rock composed primarily of the microscopic skeletons of diatoms, which are single-celled algae. It is formed from the accumulation of diatoms in freshwater or marine environments.

Characteristics Distinguish Sedimentary Rocks

Stratified: Sedimentary rocks typically exhibit a layered or stratified structure, meaning they comprise thin layers of different types of sediment.

 

Sandstone, shale and limestone layers
Sandstone, shale and limestone layers
Photo: Gailhampshire

Fossiliferous: Sedimentary rocks often contain fossils, the preserved remains of organisms that lived when the sediments were deposited, providing valuable clues about past environments. 

Variable Composition and Texture: Sedimentary rocks exhibit a wide range of compositions, textures, and colors, reflecting the diverse origins and depositional environments of their constituent particles.

Sedimentary Rocks Importance

Sedimentary rocks are important because they provide a record of Earth's history. Fossils, which are the preserved remains of organisms that lived in the past, are often found in sedimentary rocks. These fossils can be used to date the rocks and to learn about the environment in which they were formed.

Sedimentary rocks are also important economically. Many resources, such as coal, oil, and natural gas, are found in sedimentary rocks. These resources are essential for our economy and our way of life.


See also: The Differences Between Sedimentary Rocks and Igneous Rocks 

What Gemstones Can Be Found in Sedimentary Rocks

Next Post Previous Post