What is weathering

This material may include particles of parent rock, clay minerals, metal oxides and organic matter. This loose material is collectively called regolith , whereas the term soil is reserved for the topmost layer which contains organic matter.

The A-horizon is the topmost layer and is usually a meter or two thick. The upper portion of the A-horizon is often rich in organic matter, called humus , and may also contain inorganic material like insoluble clays and quartz. The A-horizon may take thousands of years to develop depending on the climate and acitivity of plants and animals. This is the layer that supports crops and other types of vegetation. These precipitated minerals often accumulate in small pods, lenses and coatings.

Organic matter is sparse in the B-horizon. The lowest layer constitutes the C-horizon and is comprised of cracked and variably weathered bedrock mixed with clays. Soils can vary significantly in color and composition. The particular type of soil that is produced in a region depends on the available materials, climate and also time. The high temperatures, heavy rainfall and humidity of tropical regions have driven chemical weathering to the extreme.

As a result, feldspars and other silicates have been completely altered while silica and calcite is extensively leached from the soil. The upper zone of laterite consists of insoluble precipitated iron and other oxides along with some quartz. At best, only a very thin layer of organic matter resides at the top of the soil to support the jungle vegetation. When the jungle vegetation is cleared, the humus oxidizes quickly and soon disappears. For this reason, laterite can only be farmed extensively for a few years after clearing and afterwards must be abandoned.

As a result, very little chemical weathering occurs to alter the original mineralogy. These joints are several inches apart near the surface but increase in distance to several feet apart with depth. One after another these layers are spalled off resulting in rounded or dome-shaped rock forms. Most people believe exfoliation is caused by instability as a result of drastically reduced pressure at the earth's surface allowing the rock to expand.

Exfoliation domes are best developed in granitic rock. Yosemite National Park has exceptional examples of exfoliation domes. Idaho has good examples in the Quiet City of Rocks near Oakley as well as in many parts of the granitic Idaho Batholith. In fact, these characteristic rounded forms make rock exposure of the granitic Idaho Batholith easy to identify. Weathering is breaking down rocks, soil, and minerals as well as wood and artificial materials by contacting the atmosphere, water, and biological organisms of the Earth.

Weathering takes place in situ, i. It should therefore not be confused with erosion involving the movement of rocks and minerals by agents such as water, ice, snow, wind, waves and gravity, and then transported and deposited elsewhere. There are two important weathering process classifications—physical and chemical weathering; each involves a biological component at times. Mechanical or physical weathering involves rock and soil breakdown by direct contact with atmospheric conditions such as heat, water, ice and pressure.

The second classification, chemical weathering, involves the direct effect in the breakdown of rocks, soils and minerals of atmospheric chemicals or biologically produced chemicals also known as biological weathering. While physical weathering is emphasized in very cold or very dry environments, where the climate is wet and hot, chemical reactions are most intense.

Both types of weathering, however, take place together, and each tends to speed up the other. Once a rock is broken down, the bits of rock and mineral are carried away by a process called erosion. No rock on Earth is hard enough to resist weathering and erosion forces.

Weathering is often divided into mechanical weathering and weathering processes. Biological weathering may be part of both processes, in which living or once — living organisms contribute to weathering. Physical weathering, also known as mechanical weathering or disaggregation, is the process class that causes rocks to disintegrate without chemical change.

Abrasion the process by which clasts and other particles are reduced in size is the primary process in physical weathering. Due to temperature, pressure, frost etc. For instance, cracks exploited by physical weathering will increase the surface area that is exposed to chemical action, thereby increasing the rate of disintegration.

Caused mostly by the burning of fossil fuels, acid rain is a form of precipitation with high levels of sulfuric acid, which can cause erosion in the materials in which it comes in contact.

An example of physical weathering is wind blowing across the desert playas. This process causes rocks to form a specific pyramid-like shape and they are called ventifacts. Select from these resources to teach about the process of weathering in your classroom. Sedimentary rocks are one of three main types of rocks, along with igneous and metamorphic. Metamorphic rocks start as one type of rock and—with pressure, heat, and time—gradually change into a new type of rock. Join our community of educators and receive the latest information on National Geographic's resources for you and your students.

Skip to content. Twitter Facebook Pinterest Google Classroom. Encyclopedic Entry Vocabulary. Weathering describes the breaking down or dissolving of rock s and mineral s on the surface of the Earth.

Water, ice, acids, salts, plants, animals, and changes in temperature are all agents of weathering. Once a rock has been broken down, a process called erosion transports the bits of rock and mineral away. No rock on Earth is hard enough to resist the forces of weathering and erosion. Together, these processes carved landmark s such as the Grand Canyon, in the U.

This massive canyon is kilometers miles long, as much as 29 kilometers 18 miles wide, and 1, meters 1 mile deep. Weathering and erosion constantly change the rocky landscape of Earth. Weathering wears away exposed surfaces over time. The length of exposure often contributes to how vulnerable a rock is to weathering. Rocks, such as lava s, that are quickly buried beneath other rocks are less vulnerable to weathering and erosion than rocks that are exposed to agents such as wind and water.

As it smoothes rough, sharp rock surfaces, weathering is often the first step in the production of soil s. Tiny bits of weathered minerals mix with plants, animal remains, fungi, bacteria, and other organisms.

A single type of weathered rock often produces infertile soil, while weathered materials from a collection of rocks is richer in mineral diversity and contributes to more fertile soil. Soils types associated with a mixture of weathered rock include glacial till , loess , and alluvial sediment s. Weathering is often divided into the processes of mechanical weathering and chemical weathering.

Biological weathering , in which living or once-living organisms contribute to weathering, can be a part of both processes. Mechanical weathering , also called physical weathering and disaggregation, causes rocks to crumble. Water, in either liquid or solid form, is often a key agent of mechanical weathering. For instance, liquid water can seep into cracks and crevice s in rock. If temperatures drop low enough, the water will freeze. When water freezes, it expand s.

The ice then works as a wedge. It slowly widens the cracks and splits the rock. When ice melts, liquid water performs the act of erosion by carrying away the tiny rock fragments lost in the split.

This specific process the freeze-thaw cycle is called frost weathering or cryofracturing. Temperature changes can also contribute to mechanical weathering in a process called thermal stress. Changes in temperature cause rock to expand with heat and contract with cold. As this happens over and over again, the structure of the rock weakens. Over time, it crumbles. Rocky desert landscapes are particularly vulnerable to thermal stress.

The outer layer of desert rocks undergo repeated stress as the temperature changes from day to night.