Sedimentary Rocks: Formation, Properties, and Uses

Introduction:

Loose pieces of rock, minerals, and organic pieces created from natural processes such as weathering, erosion, etc. cause the breakdown of rocks. These pieces are called sediments. Most sedimentary rocks are made up of a combination of various types of sediments. The properties of sedimentary rocks are affected by the source of the sediment, the movement of sediment, and the condition under which sediments are deposited.

The rocks shown in the picture are formed by sediments. Sediments are mainly tiny pieces of rocks and minerals. Some sediments are pieces of plants, bones, shells, and other animal materials. Sedimentary rocks are formed by the pressing of sediment in layers.

Fig. No.1: Formation of sedimentary rocks 

Formation of Sedimentary Rocks

Once the sediments are formed, they are carried away by the wind, water, or ice to a new place. The composition of sediments is determined by their source. As the sediments move from one place to another their physical and chemical properties change. In this process, the sediments get deposited somewhere.

Compaction and cementation convert loose sediments into sedimentary rock. Compaction is the process in which sediments squeeze and the pore space between the sediments decreases by the weight and pressure of other layers of sediments. Cementation is the method in which sediments are attached together by minerals that are deposited by water.

Fig. No. 2: Formation of sedimentary rocks

Fig. No. 3: Cementation and compaction

Properties of Common Sedimentary Rocks:

The properties of sedimentary rocks depending on the type of sediment used in their formation. Some sedimentary rocks are hard whereas some are soft. Examples: Limestone, shale, and sandstone.

Properties of Limestone:

• Limestone is a sedimentary rock, generally white in color, and forms from the remains of once-living things at the bottom of the oceans.
• Limestone generally contains shells or bones of animals or remains of plants
• The remains of plants or animals from a long time ago are called fossils.

Fig. No. 4: Fossil rock and limestone

Properties of Sandstone:

• Sandstone is formed when sand is glued (cemented) together.
• Sandstone is mostly made up of quartz.
• Sandstone when formed in water may have a layer that looks like ripples or waves.

Fig. No. 5: Sandstones

Properties of Conglomerate:

Conglomerate rocks form from larger rocks, made up of round pebbles, stones, or even boulders that are carried by fast-moving water. The word Conglomerate originates from a Latin word that means ‘lumped together.’  Rocks mix with sand and are bound together by natural cement. Conglomerates are rough and chunky because they are made up of a collection of other rocks.   

Fig. No.6: Conglomerate rocks   

Uses of Sedimentary Rocks:

• Sedimentary rocks are very useful. Chalk is made up of limestone.
• Shale rock is used for making pottery and bricks.
• A combination of shale with limestone is used to make cement.
• Bituminous coal or soft coal is formed over millions of years from dead plants buried in ancient swamps and forests. When coal is burnt, we get energy stored in ancient plants.
• Fossil rocks give us information about the structure of living things in the past.

Fig. No.7: Chalk – limestone, Pottery – shale, Bituminous – Heat energy

Summary

• Sediments are mainly tiny pieces of rocks and minerals.
• Sedimentary rocks are formed by the pressing of sediment in layers.
• Compaction and cementation convert loose sediments into sedimentary rock.
• Limestone generally contains shells or bones of animals or remains of plants.
• Sandstone is formed when sand is glued (cemented) together.
• Shale rock is used for making pottery and bricks.
• Fossil rocks give us information about the structure of living things in the past.