A plain is defined as a large area of land characterized by low elevation and a generally flat surface, with minimal local changes in height. These landforms are among the most widespread on Earth, covering more than one-third of the world’s terrestrial surface. Plains exist on every continent and are formed by a complex interplay of geological forces that either build up the land surface or wear down existing high ground.
Defining Characteristics and Categories of Plains
Plains are distinguished from other landforms, such as mountains and plateaus, primarily by their low relief and low gradient. Relief refers to the difference in elevation between the highest and lowest points within a given area. For a plain, this difference is relatively small, resulting in a surface that is nearly level or gently rolling.
Geologists classify plains based on the dominant process responsible for their creation, separating them into three fundamental types: structural, erosional, and depositional. Structural plains result from large-scale tectonic activity, where underlying rock layers are moved or exposed. Erosional plains are formed by the long-term wearing away of higher land by natural agents like wind and water. Depositional plains are created by the accumulation of material, or sediment, transported from other locations.
Plains Formed by Sedimentary Deposition
Depositional plains are built up by the continuous settling of material transported by water, ice, or wind. These plains are characterized by deep, fertile soils and are often associated with major river systems.
Alluvial plains are formed by the shifting and flooding of rivers over geological time. During flood events, the river overflows its banks and deposits a layer of fine sediment, known as alluvium, onto the adjacent floodplains. This repetitive deposition slowly builds up the plain, as the river channel constantly migrates and leaves behind a vast, low-gradient surface.
Deltaic plains form where a river carrying a substantial sediment load enters a larger, slower-moving body of water, such as an ocean or a large lake. The sudden decrease in water velocity causes the river to drop its sediment, creating a fan-shaped or lobate landform that builds outward into the standing water. The delta surface is a dynamic environment, divided into upper and lower plains based on the influence of marine processes like tides and waves.
Glacial action also forms extensive depositional plains, broadly categorized into till plains and outwash plains. A till plain is created when a massive sheet of ice melts and deposits its load of unsorted material, or till, directly onto the ground. This material contains a mixture of clay, sand, gravel, and boulders.
An outwash plain, conversely, is formed by the meltwater streams flowing away from a glacier’s snout. These streams sort the debris, depositing stratified layers of sand and gravel, with the coarser material settling closer to the ice margin. This sorting mechanism results in a flatter, more uniform surface compared to the gently undulating topography of a till plain.
Coastal plains represent another depositional type, formed either by the emergence of a former continental shelf or by the accumulation of marine and terrestrial sediments along a coastline. Fluctuations in global sea level cause the shoreline to shift, exposing the flat, sediment-covered sea floor. Alternatively, continuous deposition of sediment from rivers and waves can gradually push the coastline seaward, creating a broad, low-lying coastal expanse.
Plains Created by Erosion and Structural Forces
The processes of wearing down existing terrain and shaping underlying rock structures also create expansive plains. These mechanisms contrast with deposition by focusing on the removal or rearrangement of material.
Erosional plains result from the long-term denudation of high-relief areas like mountains and plateaus. Over millions of years, the relentless action of weathering and running water reduces the landscape to a surface of low relief, known as a peneplain. This represents a near-final stage of fluvial erosion where streams lack the energy to carve deep valleys, leaving behind only isolated, residual hills called monadnocks.
A different type of erosional plain, the pediplain, is common in arid and semi-arid environments. The formation involves the parallel retreat of mountain fronts, where intense, infrequent rainfall washes debris from the slopes. This process creates gently sloping, bedrock surfaces called pediments at the base of the receding mountains. As multiple pediments expand and coalesce, they form the vast, nearly flat pediplain.
Structural plains are shaped by tectonic forces that affect the Earth’s crust rather than by surface agents of erosion or deposition. The Great Plains of North America, for instance, are structural plains formed by the gentle uplift and tilting of flat-lying sedimentary rock layers.
Other structural plains form through the downward movement of the crust, a process called subsidence. Rift valleys, such as the East African Rift System, are tectonic troughs created by tensional forces pulling the crust apart along normal faults. The floor of this subsiding block, though initially rugged, is subsequently flattened by the infilling of sediments from surrounding highlands, producing a low-relief plain bounded by steep fault-block scarps.
The Role of Geological Time in Plain Formation
The formation of an erosional plain, or peneplain, through the wearing down of a mountain range necessitates a long period of tectonic stability. Without this stability, the landscape would be constantly rejuvenated by uplift, restarting the cycle of erosion before the plain could fully form.
Long-term erosion rates in stable continental interiors are remarkably slow, averaging only 4 to 200 meters per million years. This slow, steady removal of material allows the reduction of high-relief areas to a nearly flat surface over hundreds of millions of years.
In contrast, the process of deposition on alluvial plains can occur at much faster, episodic rates. During periods of active accumulation, such as major flood cycles, sedimentation rates can spike to between 1.8 and 23 millimeters per year. However, when averaged over long periods, the net accumulation rate is significantly lower.