Erosion sculpts the Earth’s surface, constantly wearing down high points and filling in low ones. It is distinct from weathering, which is the breakdown of rock material in situ by chemical or physical means. Erosion involves the movement and transportation of those broken-down geological materials by natural agents like water, ice, wind, and gravity. The constant action of these forces creates a wide array of distinctive landforms.
Landforms Shaped by Flowing Water (Fluvial and Coastal Erosion)
Flowing water, both in rivers and along coastlines, is one of the most widespread agents of erosion, creating a suite of recognizable landforms. River erosion, known as fluvial action, is intense in the upper reaches of a stream where the gradient is steep and water velocity is high. Mechanisms include hydraulic action, the force of water dislodging material, and abrasion, where sediment grinds against the streambed and banks.
This downward cutting produces deep, narrow V-shaped valleys and features like gorges and canyons. Waterfalls form at nickpoints, breaks in the stream’s gradient where resistant rock lies above softer rock. The cascading water excavates a plunge pool at the base, causing the less resistant rock to collapse and allowing the waterfall to slowly retreat upstream.
In coastal environments, waves and currents shape the boundary between land and sea. Hydraulic action is intense, as breaking waves compress air in rock cracks, causing fragmentation. Abrasion by sand and pebbles cuts into headlands to form sea cliffs.
Differential erosion leads to sea caves where weaker rock is worn away. If a cave erodes through the headland, it becomes a sea arch. The collapse of the arch leaves an isolated pillar of rock standing offshore, known as a sea stack.
Landforms Shaped by Glacial Ice
Moving ice is a major erosional agent, capable of transforming landscapes into massive, smoothed forms. Glacial erosion occurs through two primary mechanisms: plucking and abrasion. Plucking, or quarrying, happens when meltwater seeps into rock cracks, freezes, and expands, tearing out large blocks of rock carried away by the glacier.
These embedded rock fragments act like sandpaper, scouring and grinding the bedrock below in a process called abrasion. This leaves behind polished surfaces and parallel scratches called glacial striations. When a glacier moves down a pre-existing V-shaped river valley, this combination of processes widens the valley floor and steepens the walls, creating a characteristic U-shaped valley or glacial trough. Tributary valleys cut less deeply are left high above the main trough, creating hanging valleys.
In mountainous regions, glaciers carve deep, armchair-shaped hollows at their source, known as cirques. Where three or more cirques erode back-to-back, they isolate a pyramid-shaped peak called a horn. Knife-edge ridges, or arêtes, form when two cirques erode toward each other on opposite sides of a mountain ridge. When U-shaped glacial valleys are flooded by the sea, they form long, deep, steep-sided inlets known as fjords.
Landforms Shaped by Wind (Aeolian Erosion)
Wind erosion is most effective in arid or semi-arid environments where there is sparse vegetation cover and an abundance of dry, loose sediment. The primary mechanisms are deflation and abrasion. Deflation is the lifting and removal of fine, loose material like silt and dust from the land surface.
The removal of this fine material can create shallow depressions known as deflation hollows or blowouts. Abrasion occurs when wind-carried sand grains impact and scour rock surfaces. This action is concentrated near the ground, where the wind carries the most sediment.
Abrasion shapes and polishes exposed rocks, creating multi-faceted stones called ventifacts. In areas with alternating layers of hard and soft rock, wind abrasion sculpts large, streamlined, elongated ridges known as yardangs. The wind’s role is often one of final sculpting and maintenance in arid zones.
Landforms Shaped by Gravity and Dissolution
The force of gravity acts directly in a suite of events collectively known as mass wasting. Mass wasting involves the downslope movement of soil, rock, and sediment under the direct influence of gravity. This process creates distinctive landforms such as landslides and slumps, which are rapid movements of material down a slope.
Over longer periods, gravity contributes to the formation of scree slopes, also known as talus, which are large accumulations of fragmented rock debris at the base of steep cliffs. Slower processes like soil creep contribute to the overall lowering of the landscape over vast timescales.
Another major erosional force is dissolution, which is particularly effective in areas with soluble bedrock, like limestone, creating karst landscapes. Rainwater absorbs carbon dioxide from the atmosphere and soil, forming a weak carbonic acid that reacts with the calcium carbonate in the rock. This chemical reaction slowly dissolves the rock, removing it entirely in solution.
The gradual enlargement of cracks and joints creates elaborate underground drainage systems, forming networks of caves and passages. When the roof of a cavern collapses, it creates a surface depression called a sinkhole.