The surface of Earth is a dynamic system constantly remodeled by natural forces. This ongoing transformation is primarily driven by two interconnected geological processes: weathering and erosion. Weathering involves the mechanical and chemical breakdown of rock material in situ, meaning the rock is destroyed where it stands. Erosion then takes the resulting broken-down material, known as sediment, and transports it across the landscape to a new location. These processes shape the planet’s features from the smallest grain of sand to the largest geological formations.
The Process of Weathering: Breaking Down Rock
Weathering initiates landform change by weakening the structural integrity of the parent rock. Physical weathering breaks rock into smaller fragments without altering its chemical composition. A common example is frost wedging, where water seeps into rock fractures, freezes, and expands, exerting force that widens the cracks over time. Another form occurs deep underground when overlying material is removed; the resulting pressure release causes igneous rocks, like granite, to expand and fracture into curved sheets, creating exfoliation domes.
Chemical weathering alters the internal structure of minerals by reacting with water or atmospheric gases. Hydrolysis occurs when water molecules react with minerals like feldspar, changing them into softer clay minerals. Oxidation is effective on iron-bearing minerals, where iron reacts with oxygen and water to form iron oxides, which weakens the rock structure. Dissolution is effective on carbonate rocks like limestone, as rainwater absorbs atmospheric carbon dioxide to form a weak carbonic acid that dissolves the rock material.
Biological factors also contribute to the breakdown of rock. The physical action of plant roots growing into fissures can exert pressure strong enough to split large blocks of rock. Microscopic organisms, such as bacteria and fungi, along with lichens, contribute to chemical weathering by producing weak organic acids. These acids react with the rock surface, dissolving minerals and accelerating decomposition.
The Agents of Erosion: Transporting Material
Once rock material has been broken down by weathering, erosion utilizes several agents to move the sediment. Water is the most widespread agent, acting through fluvial processes in rivers and streams. Rivers carry sediment ranging from fine silt to large boulders rolled along the bed. Coastal erosion demonstrates water’s power as waves undercut sea cliffs and transport beach sand along the shoreline.
Wind acts as an erosional agent primarily through aeolian processes, most noticeable in arid environments. Deflation occurs when wind lifts and removes fine particles from the surface. Abrasion involves wind-blown sand particles impacting rock surfaces, effectively sandblasting them. Although wind is less effective than water or ice at moving large particles, its ability to redistribute fine sediment shapes entire desert regions.
Ice, in the form of glaciers, is a powerful agent of erosion, capable of moving material over long distances. Glacial plucking occurs as meltwater seeps into cracks, freezes onto the bedrock, and pulls out large chunks of rock as the glacier advances. The base of the glacier is armed with embedded rock fragments, which scrape and polish the underlying surface in a process known as abrasion, grinding down mountainsides and deepening valleys.
Gravity acts as the ultimate driving force for all erosional processes, particularly in mass wasting events where materials move downslope. Mass wasting includes rapid, catastrophic movements, such as landslides and rockfalls, where gravity overcomes the shear strength of the slope material. It also encompasses the slow, continuous movement of material, such as soil creep, which is a downhill migration of soil and loose rock. These agents mobilize the weathered debris and prepare the landscape for deposition.
Features Shaped Primarily by Weathering
Some of Earth’s distinctive features are the direct result of weathering processes acting on the bedrock, often leaving the material in place. The formation of soil profiles is the most widespread result of combined chemical and biological weathering acting on the parent rock. This process breaks down rock into mineral particles and mixes them with organic matter, creating the layers necessary to support terrestrial ecosystems.
Large, dome-shaped landforms are created by the process of pressure release, a form of physical weathering. As overlying rock is removed by erosion, the underlying rock expands. This expansion causes concentric layers, called sheeting joints, to peel off the surface. These massive, rounded structures, such as Stone Mountain in Georgia, are formed because the rock is fractured in situ by the release of internal stress.
Karst topography is shaped almost entirely by the chemical weathering process of dissolution. In regions with thick layers of limestone, rainwater containing carbonic acid slowly dissolves the rock, creating networks of underground caves. Surface features like sinkholes and disappearing streams are the visible evidence of this subsurface chemical action.
Features Shaped by Erosion and Deposition
The material transported by erosion eventually settles and accumulates, creating major depositional landforms. River erosion carves out V-shaped valleys and canyons, such as the Grand Canyon, where the downcutting action of the water removes material faster than the surrounding slopes can weather. Glacial erosion creates characteristic U-shaped valleys due to the immense scouring power of the ice.
When rivers emerge from a steep mountain range onto a flat plain, they rapidly lose energy and deposit their sediment load in fan-shaped accumulations called alluvial fans. When a river meets a large body of standing water, such as an ocean or lake, the decrease in velocity forces the suspended sediment to drop out, forming fertile deltas. These depositional features represent the constructive phase of the cycle, building new land from the materials eroded upstream.
Glacial deposition leaves behind mounds of unsorted sediment, known as till, which form ridges called moraines at the edges and ends of the ice mass. These landforms remain long after the ice has melted, marking the furthest extent of the glacier’s advance. In arid regions, wind-blown sand is deposited when an obstacle or a decrease in wind speed causes the particles to settle, resulting in the formation of sand dunes.