Plains are expansive geographical features covering more than one-third of the terrestrial surface, characterized by their notably flat or gently undulating topography. This lack of significant vertical variation, known as low relief, distinguishes them from more rugged landforms like mountains and plateaus. These vast, open areas have been profoundly important to human civilization, serving as primary global centers for agriculture, settlement, and large-scale infrastructure development due to their rich soil and accessible landscape.
Physical Characteristics of Plains
A plain is technically defined as any relatively level area of the Earth’s surface that exhibits gentle slopes and minimal local relief. The elevation of plains above sea level can vary considerably, ranging from coastal lowlands near sea level to high plains situated thousands of feet high, such as parts of the Great Plains in North America. The factor that unifies all plains is not their absolute height, but their internal flatness and the imperceptible slope across vast distances.
Geological Processes That Form Plains
The formation of plains is driven by large-scale geological mechanisms that either flatten rugged terrain or build up a level surface. These mechanisms fall into two main categories: destructive processes and constructive processes.
Destructive forces like weathering and erosion, carried out by water, wind, and ice, systematically wear down higher landforms. Over millions of years, these agents can reduce mountains and plateaus to near-level surfaces known as plains of denudation.
Constructive processes involve the deposition and accumulation of material, where sediments are transported and settle in low-lying areas. Rivers are potent constructive agents, depositing fine-grained sediments like silt and clay across their floodplains and deltas. Ancient ice sheets and strong winds also carry and deposit massive volumes of material, creating thick, level blankets of sediment.
Tectonic activity also plays a quieter role in plain formation through gentle, broad movements of the Earth’s crust. This activity, known as epeirogeny, involves the slow, regional uplift or subsidence of large blocks of the crust. When this occurs, it raises or stabilizes vast areas of horizontally layered rock, preserving the necessary flatness.
Classification Based on Formation Origin
Plains are classified primarily by the dominant process that created them, leading to three major types: structural, erosional, and depositional plains.
Structural plains are formed by the stable, horizontal layering of sedimentary rocks that have been subjected to gentle crustal warping or uplift without intense folding or faulting. These areas often represent the relatively undisturbed portions of continental shields, such as the extensive plains found in the central United States.
Erosional plains result from the long-term wearing down of an existing, higher landmass by agents of denudation. Examples include peneplains, which are landscapes reduced by running water to a gently undulating surface, and pediplains, which are formed in arid regions where wind action and slope retreat have worn down the landscape.
Depositional plains are created by the accumulation of sediments transported and deposited by natural agents. Alluvial plains, formed by rivers, are characterized by highly fertile silt and clay deposited during periodic flooding, such as the Indo-Gangetic Plain in South Asia. Glacial plains are formed by the movement of ice sheets, while Loess plains form from the deposition of fine, wind-blown silt that accumulates in deep, fertile layers.
Importance and Human Settlement
The level topography and fertile soils of plains have made them the most desirable areas for human settlement and economic activity. The flatness significantly reduces the cost and complexity of building infrastructure, facilitating the construction of extensive road networks, railways, and canals. This ease of transport and communication supports high population densities and the development of large urban centers.
Regions like the North American Great Plains and the Indo-Gangetic Plain are global breadbaskets that produce massive quantities of staple crops like wheat and rice. This agricultural productivity and ease of access have allowed civilizations to flourish, supporting global food security and economic activity.