Weathering of Rocks: Physical and Chemical

key Concepts

• Weathering
• Physical weathering
• Chemical weathering

Introduction: 

Earth is the place where we live, and it is the third planet from the sun. 70% of the Earth is covered by water. Earth is a unique planet. It has supported life since many years. It is round in shape, and is made up of various layers. Each layer of the Earth has different physical and chemical characteristics. Earth is also called a water planet or blue planet due to the presence of water on it. It is the only planet that has water on its surface. There are many features on the Earth’s surface. These features are called landforms. There are many factors such as wind, water, chemicals, and depositions that shape the land. 

 Weathering :

We know that the upper layer of the Earth is the crust, and forces inside the Earth build up the crust into various landforms such as plateaus, mountains, hills, etc. There are some processes like weathering that breaks down the crust. Weathering is a slow and natural process of breaking down rocks into smaller pieces. Even the large rocks can break apart due to weathering (Fig. no. 2). In future, the rock seen in the image can look very different or it may even break into pieces due to weathering.  

Factors affecting weathering :

There are various factors such as flowing water, living things, rainfall, waves, plants, chemicals that affect the weathering. Weathering can be caused by living or non-living things. There are two types of weathering.  

Physical weathering: 

It is a process in which rocks can change size and shape without changing their chemical composition. In this process, only physical changes occur, whereas chemical structure does not change. Physical weathering is also known as mechanical weathering. This type of weathering is caused by plants, animals, freezing water, or moving water. 

When water seeps into the cracks in rock and freezes, frost wedging takes place. Here the freezing water expands, and that forces the rocks to move apart. 

When the ice melts, water percolates deep into the cracks and finally, the rock breaks down into tiny pieces. These pieces are carried away by moving water. The water splashes as it moves along shores or rapids, and pieces of chunks of rocks carried by water collide with each other and breaks apart. 

 

Wind is also one of the factors that affect weathering. Over long periods of time, wind can wear away rocks and carry small pieces of the rocks to new places. This type of weathering can create wonderful landscapes, such as rocks that take the shape of mushrooms. 

 

Plants and animals cause biological weathering. In this type of weathering, plant roots can enter the rocks through the cracks present in the rock. As the roots of the plants grow, they force the cracks to widen up and eventually, this force leads to breakdown. In this way, the rock breaks apart. 

Weathering of rocks is also caused by animals. Burrowing animals such as ants, worms, moles, rabbits dig the soil and bring rock pieces to the upper surface of the land and expose them for weathering. 

Chemical weathering: 

There are certain forces that cause weathering that are chemical in nature. Chemical weathering is the breakdown of rock by changes in the chemical structure of the rock. The strong or powerful agents of chemical weathering are acids and oxygen. As we know, air consists of 21% oxygen and many types of rock contain iron. When iron-containing rock get exposed to air and water, a chemical reaction takes place where air and oxygen combine with iron and rust form. The rusty rock is not as hard as the original rock and hence can break apart very easily.  

When CO₂ (carbon dioxide) present in the air gets dissolved in rainwater, the chemical reaction takes place and carbonic acid forms. It is a weak acid that reacts with limestone. If water with carbonic acid gets percolated into the ground that consists of limestone, then the limestone dissolves in the water, and it gets carried away by the flow of water. Over a period of time, eventually, this process forms a cavern. 

Summary

• Earth is the place where we live. It is the third planet from the Sun.
• Crust is the thinnest outermost layer of the Earth.
• Weathering is a slow and natural process of breaking down rocks into smaller pieces.
• B There are two types of weathering – Physical weathering (mechanical weathering) and chemical weathering.
• Physical weathering is a process in which rocks can change size and shape without
• changing their chemical composition.
• Chemical weathering is the breakdown of rock by changes in the chemical structure of the rock.