The landscapes of the world are constantly changing. Rain, sun, wind and frost are breaking down even the hardest rocks into smaller pieces before they are carried away. This process is known as weathering. Weathering is the action of the elements of weather and climate, plants and animals on the land surfaces to break them down physically, chemically and biologically. It is the disintegration and decay of rocks in situ. It can be a very long slow process, taking hundreds of years. However, weathering can also be rapid, such as the damage to pavements or roads after a cold, frosty winter.
Weathering can be defined as the breaking huge rocks into small pieces like pebbles, sand grains etc. Or simply, Weathering is the breakdown of rocks by water, frost and temperature change. The effects of plants and animals can also break rocks down. In the absence of weathering process, the agents of water, the force of gravity, ice and wind would have nothing to move. Rocks starts to break down the moment they are exposed to the weather at surface of Earth.
Weathering is the first step in the formation of soil. Weathering takes place in four ways: physical weathering, chemical weathering, mechanical weathering and biological weathering. The three principle types of weathering are physical,chemical and mechanical weathering, which are described below.
Physical weathering is when rocks are broken into smaller fragments without any change in their chemical make up. The main causes of physical weathering are sudden changes in temperature and extreme cold or heat. Changes in temperature can cause the freeze-thaw action, which is when water collects in cracks in rocks during the day and the night time temperature drops enough to make the water turn to ice. Most substances expand when they are heated and contract when cooled. However, when water freezes and becomes ice, it actually expands. The expanding ice increases the size of the crack, forcing the rock apart and weathering its structure.
Chemical weathering occurs when chemicals dissolve in water and then attack and break down rock surfaces. Chemical weathering occurs faster in places with warm temperatures and abundant water, such as in humid tropical environments. During chemical weathering the mineral composition of rocks changes due to chemical reaction with air or water. It results in decomposition of the rocks. Some of the dissolved products of chemical weathering are carried away by water seeping through soil and rocks, a process called leaching. The water eventually may carry these materials to rivers and these to the sea. This is the source of the salinity of the oceans. The examples of chemical weathering are oxidation, decomposition of calcium carbonate etc.
The following are the different processes of chemical weathering.
Oxidation: Oxygen combines with minerals, particularly iron. In the rocks containing iron, oxygen combines with iron from iron oxide. The new minerals formed by oxidation become vulnerable to attack by other methods of weathering. The rusting of the iron compounds in the rocks when they react with oxygen in the air gives the affected rocks a reddish brown appearance. The rust expands and helps to break up iron bearing rocks.
Carbonation and dissolving by solution: When rain water falls, carbon dioxide in the air reacts with it to form a weak acid. This acid reacts with several minerals in the rocks.
Rocks are also broken down by mechanical force. This process is called mechanical weathering. Due to the frequent changes in temperature, rocks begins to expand and contract. This action results the break down of rocks. It is also caused by plant roots. They are growing through the cracks in the rocks and break the rocks apart. Mechanical and chemical weathering work together to break down rocks. Often, mechanical cracks and water seeps in to weather the rock chemically.