Weathering is the disintegration and alteration of minerals and rocks near or at the earth’s surface. This shapes the earth's surface through such processes as decomposition, wearing down, and cracking from root growth or freezing and thawing. Each process has a distinct effect on rocks and minerals. The three forms of weathering are physical or mechanical, biological and chemical.
Cracking and Breaking
Mechanical weathering is the physical breakdown of rocks because of environmental factors such as heat, cold, water, and wind. One form of mechanical weathering is thawing or constant freezing of water. Water, in liquid form, penetrates the many fissures, joints, and holes within a rock. It starts freezing as temperatures drop to 32 degrees Fahrenheit and below. As the water freezes, it expands and becomes about 10 percent larger. This expansion pushes the cracks and holes in rocks outward. Even the hardest rocks such as granite, cannot match the extremely strong force. Salt wedging is the other form of mechanical weathering. Water that enters the cracks and holes on the rock surface contains salt. As it evaporates, it leaves the salt behind. With time, the salt deposits build up. They create a strong pressure that causes rocks to weaken and break. Mechanical weathering is highly common in cold climates.
Altering Minerals' Structure
Chemical weathering causes the decomposition, dissolving and loosening of rocks. Chemical reactions destroy the bonds that hold the rocks together. This causes them to break into small pieces. One effect of chemical weathering is hydrolysis. Through hydrolysis, water is incorporated into the chemical structure of a mineral, which turns the mineral into a new one. For example, hydrolysis changes feldspar into clay. Because water is a catalyst in chemical reactions, chemical weathering occurs mostly in areas with plenty of water and high temperatures. It tends to be common in hot and humid tropics.
Changing Chemical Composition
Biological weathering refers to the weakening and subsequent breakdown of rocks by microbes, animals, and plants. Growing plant roots exert pressure or stress on rocks. By altering the chemical composition of rocks, the microbial activity disintegrates rock minerals. Lichen is a perfect example of a microbial activity. Lichen is algae and fungi living together. Fungi release some chemicals that break up rock minerals. Algae consume the broken minerals released from the rock. As the process continues, gaps and holes continue to build up on the rock thus exposing the rock to weathering. Some of the effects of biological weathering are breaking of particles, movement of minerals, mixing of materials and production of carbon dioxide.
Rocks are symbols of durability and strength. Rocks are normally resistant to weathering. This resistance depends on the rock’s mineral porosity. and mineral composition. The physically soft minerals are easily broken apart and crushed. With the harder minerals, it is quite difficult. The entire weathering process is controlled by the arrangement of the mineral grains and size of a rock. Some of the rocks susceptible to weathering are limestone, and marble. Granite is a perfect example of a rock that is highly resistant to weathering.