Weathering is the process that causes the breakdown of rocks and minerals located at or near the Earth’s surface. This natural process is fundamental to the geological cycle and plays a significant role in shaping the landscape. It is also the initial step in creating the loose material that eventually forms the parent material for soil development.
This disintegration occurs when rocks are exposed to atmospheric conditions, water, and biological activity. Understanding how these forces act upon solid rock explains the source of the sediment that is later transported and deposited by agents of erosion.
The Mechanics of Physical Weathering
Physical weathering, also known as mechanical weathering, involves the disintegration of rock into smaller fragments without causing any change in the chemical composition of the rock material. The resulting smaller pieces possess the exact same mineralogy as the original rock mass. This mechanical breakdown is driven by physical stresses that exceed the rock’s internal strength.
One common process is frost wedging, which occurs when water seeps into pre-existing cracks and then freezes. Water expands by about nine percent when it turns to ice, generating immense pressure that can widen and extend the rock fractures. This repeated freeze-thaw cycle is highly effective in cold climates and at high elevations, often leading to the accumulation of rock fragments known as talus slopes.
Another mechanism is thermal stress weathering, which involves the expansion and contraction of rock caused by frequent temperature fluctuations, such as the large daily temperature swings common in desert environments. Different minerals within a rock expand and contract at slightly different rates, creating internal stresses that eventually cause the rock to fracture.
Exfoliation, or pressure release, is also a significant process, where large masses of rock formed deep underground, such as granite, are exposed at the surface as overlying material is removed. The decrease in confining pressure causes the rock to expand and fracture parallel to the surface, often peeling off in curved sheets.
The Chemistry of Chemical Weathering
Chemical weathering represents a distinct process where the rock’s internal structure is transformed through chemical reactions, altering the original material into new mineral compounds. This process changes the chemical makeup of the rock, often producing minerals that are more stable under surface conditions. Water, oxygen, and carbon dioxide are the primary agents driving these reactions.
One prevalent form is hydrolysis, where water molecules react with minerals, particularly silicates like feldspar, to form new substances such as clay minerals. This process is responsible for the breakdown of many igneous rocks and weakens the overall rock structure.
Oxidation is another widespread reaction, where oxygen dissolved in water or air reacts with metal ions within the rock, most commonly iron. The reaction converts ferrous iron (Fe²⁺) into ferric iron (Fe³⁺), producing iron oxides that give the rock a characteristic reddish or rusty appearance and reduce its structural integrity.
Dissolution is the process where minerals dissolve completely in water, often enhanced by the presence of weak acids. Atmospheric carbon dioxide dissolves in rainwater to form a weak carbonic acid, which then reacts with soluble minerals like calcite found in limestone. This acid reaction transforms the solid rock into dissolved ions, leading to the formation of geological features such as caves and karst landscapes.
Abrasion: A Process of Mechanical Breakdown
Abrasion is classified as a form of physical weathering because it accomplishes the mechanical breakdown of rock without altering its chemical composition. It involves the direct grinding and wearing away of rock surfaces through friction or impact. This process is driven by the movement of sediment particles, which act as natural tools.
These sediment particles are carried by dynamic agents like running water in streams, wind across arid landscapes, and ice within glaciers. As these moving fragments collide with stationary rock or rub against each other, they chip away material, reducing the size of the particles and smoothing their edges. Cobbles found in riverbeds, for example, are often rounded and polished due to sustained abrasion.
The intensity of abrasion depends on the hardness, concentration, and velocity of the moving particles. Its effect is solely mechanical, reducing the rock mass to smaller pieces and creating distinct surface features like striations on bedrock beneath a glacier or pitted surfaces on rocks exposed to wind-blown sand. This reduction confirms abrasion as a purely mechanical force.