Weathering is the process by which rocks, soils, and minerals break down through contact with the atmosphere, hydrosphere, and biosphere. This deterioration occurs directly in place (in situ), meaning the material is not immediately moved away. Weathering is the first step in the rock cycle, leading to subsequent erosion, transport, and the formation of sedimentary rocks and nutrient-rich soil.
Physical Processes of Rock Fragmentation
Mechanical weathering involves physical processes that fragment large rocks into smaller pieces without altering the chemical makeup of the minerals. This process relies on the application of physical force to overcome the rock’s internal strength. A primary mechanism is frost wedging, which occurs when water seeps into cracks and freezes. Since water expands in volume by about 9% when it turns to ice, it exerts force that pushes the rock apart.
Another form of mechanical breakdown is abrasion, where rock surfaces scrape, grind, or rub against one another, driven by agents like wind, water, or glaciers. Pressure release, known as exfoliation, occurs in large igneous rock bodies formed deep underground. As overlying material is removed, the reduced pressure allows the rock to expand, causing outer layers to fracture and peel off in sheets. Living organisms also contribute, as plant roots grow into fissures and expand, splitting the rock structure.
Chemical Transformation of Rock Material
Chemical weathering involves reactions that change the mineral composition of the rock, leading to decomposition and the formation of new compounds. Water accelerates this transformation and acts as the primary agent. Dissolution is one process where minerals dissolve completely in water. This is particularly effective when the water is slightly acidic due to absorbed atmospheric carbon dioxide, a reaction often called carbonation.
A common reaction affecting silicate minerals is hydrolysis, where water molecules react with the mineral structure. This converts minerals like feldspar into new, more stable materials, most notably clay minerals, while releasing soluble ions. Oxidation involves the reaction of minerals with oxygen dissolved in water or air, frequently seen when iron-bearing minerals are exposed. The iron forms iron oxides like rust, which weakens the rock structure and gives it a characteristic reddish-brown color.
How the Two Processes Interact and Differ
Physical and chemical weathering rarely occur in isolation; they work together to accelerate rock breakdown. The primary connection is that physical fragmentation significantly increases the rock’s surface area. Breaking a large rock into smaller pieces exposes a greater area to water and air, allowing chemical reactions like oxidation and hydrolysis to proceed much faster.
The difference lies in the end product of each process. Physical weathering disintegrates the parent rock into smaller fragments (sand, silt, and gravel) that retain the original mineral composition. Chemical weathering alters the rock’s internal structure, producing new substances, such as dissolved ions and newly formed stable minerals like clay. Physical processes reduce size using mechanical force, while chemical processes change composition using molecular reactions.