The Earth’s surface is constantly being reshaped by weathering and erosion. Weathering is the breakdown of rocks, soils, and minerals, occurring in situ—meaning the material remains in place. Erosion is the subsequent process involving the movement and transport of that weathered material by various mobile agents. These processes transform the solid geology of the lithosphere into the sediments that cover the globe.
Mechanical Forces Driving Physical Weathering
Physical weathering disintegrates rock into smaller pieces without altering its chemical composition. One of the most effective mechanical forces is frost wedging, which occurs when water seeps into rock fractures and then freezes. Because water expands by approximately 9% upon freezing, the growing ice crystal exerts immense pressure, gradually widening the crack until the rock fragments. This freeze-thaw cycle is a powerful agent of breakdown, particularly in temperate and alpine environments where temperatures frequently fluctuate around the freezing point.
Another significant physical process is thermal stress, which results from repeated expansion and contraction of rock material due to temperature changes. Although the rock mass is a poor conductor of heat, the outer layers heat up and cool down faster than the inner core, creating internal stresses. Over long periods, this differential expansion and contraction, especially in rocks with varying mineral compositions, leads to fatigue and the formation of fractures parallel to the rock surface.
A process known as exfoliation or unloading occurs when the pressure on deeply buried rock is released by the removal of overlying material through erosion. Intrusive igneous rocks, like granite, form under tremendous pressure deep within the crust. When they are exposed, the rock expands, causing concentric sheets to peel away from the main rock mass, similar to the layers of an onion. This pressure release creates massive dome-like structures and contributes large volumes of debris to the surface environment.
Abrasion occurs where one rock fragment physically grinds against another, often facilitated by a mobile agent like wind or water. The impact and scraping action of these particles smooths, polishes, and fragments the rock surfaces they encounter. For example, sand and dust carried by strong winds act as natural sandpaper, eroding exposed rock formations and creating distinctive features such as ventifacts.
Chemical Processes of Breakdown
Chemical weathering involves reactions that change the molecular structure and composition of the rock and mineral material. This alteration transforms primary minerals formed under specific conditions into secondary minerals stable at the Earth’s surface. A common mechanism is dissolution, where minerals are dissolved in water, often made slightly acidic by dissolved carbon dioxide. This reaction, known as carbonation, is particularly effective on carbonate rocks like limestone, dissolving the rock into soluble ions and creating extensive cave systems.
Oxidation is a chemical process where oxygen reacts with minerals, especially those containing iron, in the presence of water. The reaction converts ferrous iron (Fe²⁺) into ferric iron (Fe³⁺), forming iron oxides like hematite or goethite. This change weakens the mineral structure and gives the rock a characteristic rusty-red or yellowish color.
Hydrolysis is the reaction of water with silicate minerals, which are the most abundant minerals in the Earth’s crust. In this process, the hydrogen and hydroxide ions from the water interact with the mineral’s ions, breaking down the crystal lattice. For instance, the weathering of feldspar, a common mineral in granite, through hydrolysis yields new, more stable compounds, such as clay minerals and dissolved salts.
Primary Agents of Erosion and Transport
Once rock and mineral material is broken down by weathering, erosion agents transport the debris away from the original site. Water is the primary agent of transport. Flowing water in rivers and streams carries sediment as suspended load, bedload, and dissolved load, carving out valleys and moving material vast distances. Coastal wave action also causes erosion by pounding coastlines and transporting sand and rock fragments along the shore.
Wind is a powerful agent of erosion, particularly in arid and semi-arid regions where vegetation cover is sparse. This process is known as deflation, where the wind lifts and carries loose, fine-grained particles like silt and dust over enormous areas. Wind also contributes to abrasion by using the carried sand grains to blast and wear down stationary rock surfaces.
Glacial ice represents a formidable agent of erosion due to its immense mass and slow movement across the landscape. As glaciers flow downhill, they incorporate rock fragments into their base through a process called plucking. The embedded debris then scours and grinds the bedrock beneath the ice, an action called glacial abrasion, which creates distinctive landforms like U-shaped valleys and fjords.
The Influence of Gravity and Biology
Gravity drives all forms of erosion and transport by pulling material downward. Its direct influence is most evident in mass wasting, the collective term for the rapid or slow movement of soil and rock down a slope. This includes dramatic events like landslides and rockfalls, where gravity acts alone to pull material from higher to lower elevations. Slower forms of mass wasting, such as soil creep, represent the gradual, continuous downhill movement of material, constantly reshaping hill slopes.
Living organisms contribute to both weathering and erosion. In weathering, the growth of plant roots into rock crevices exerts significant outward pressure, a process called root wedging that physically splits the rock. Lichens and some bacteria also contribute to chemical weathering by secreting organic acids that chelate minerals from the rock. Burrowing animals like earthworms and rodents displace and loosen soil and rock particles, contributing to erosion. Human activities, such as farming and construction, represent an accelerated form of biological erosion that drastically increases the rate of sediment transport.