What Breaks Down Rocks to Form Soil?

Chemical Weathering

• Chemical weathering occurs when minerals within a rock change from one form to another or completely degrade. These processes often occur when rocks that form deep within the Earth at high pressures and temperatures become exposed at the land surface. The different conditions at the land surface, such as lower temperatures and pressures and increased water and oxygen, will cause some minerals to be less stable and begin to degrade.

Processes of Chemical Weathering

• The dominant factors in chemical weathering are the presence of water and oxygen in the environment. Oxygen encourages the oxidation process, which alters a mineral to an oxide mineral. Water assists chemical weathering processes such as hydrolysis, leaching, hydration and dissolution. Hydrolysis causes minerals to change into less stable forms while hydration causes water to combine with existing minerals to form a new type. Water will also cause some elements to leach out of minerals, altering their form, or it may completely dissolve the mineral. A final chemical weathering process is the dehydration of minerals that removes water from one mineral and forms a new type.

Physical Weathering

• Physical weathering is the process of breaking down a rock and its minerals by mechanical or physical processes. As outside processes act upon a rock and break it into smaller and smaller pieces, it creates more surface area that is exposed on the land surface that increases its likelihood of becoming subject to additional weathering processes. Most physical weathering is caused by the weather and animal and vegetation activities.

Processes of Physical Weathering

• In dry climates, warm weather causes water within a rock to dehydrate and form salts. As the salt crystals grow in size during the dehydration process, they become a wedge within the rock, causing cracks that eventually break the rock apart. The same process may occur when water seeps into cracks within a rock and precipitates minerals such as calcite or quartz. Temperature swings from cold to hot will cause rocks to expand and contract, forming cracks that will break the rock. Similarly, a rock that freezes will crack apart from the formation of ice in the crevices. Plant growth on and through a rock will cause the rock to weaken and break. Some plants will produce microorganisms that feed on nutrients within the rock, which also promotes weathering. Finally, physical movement of the rocks by humans, animals or through landslides and similar processes will break down the rocks.