The Earth’s surface is a dynamic mosaic of natural features, known as landforms, which shape the planet’s topography. Landforms range from towering mountains to expansive, flat plains. They are the result of continuous interaction between destructive and constructive geological processes acting over vast timescales. Understanding the variety of landforms requires looking at both internal forces beneath the crust and external interactions that constantly sculpt our world.
What Defines a Landform
A landform is classified based on its physical attributes, including elevation, slope, orientation, and composition. Geomorphologists, the scientists who study these features, recognize that landforms exist across a spectrum of sizes, from micro-scale elements like sand ripples to macro-scale features like entire mountain ranges. The shape and size of any landform depend on the two fundamental categories of forces that create them.
These formative forces are categorized as either endogenous or exogenous. Endogenous forces originate from within the Earth, driven by internal heat, and are responsible for large-scale structural changes like crustal movement and volcanism. Exogenous forces originate on or above the Earth’s surface and include agents such as water, wind, ice, and the chemical breakdown of rock materials. Landforms are constantly being created, modified, or destroyed by these opposing sets of processes.
Landforms Formed by Internal Earth Processes
Internal Earth processes, often driven by the movement of tectonic plates, are responsible for the largest and most foundational landforms on the planet. These endogenous forces, including folding, faulting, and volcanism, create immense vertical irregularities in the crust. These features are the structural backbone upon which surface processes later carve smaller details.
Mountains are the most dramatic result of these internal pressures, forming primarily through three mechanisms. Fold mountains, such as the Himalayas and the Alps, are created when two continental tectonic plates collide, causing the crustal rock layers to buckle upwards under intense compression. Fault-block mountains, like the Sierra Nevada, form when tensional forces cause the crust to break into large blocks that move along fractures called faults. Volcanic mountains, including Mount Fuji, are built by the accumulation of lava, ash, and other material extruded from the Earth’s interior.
Plateaus are another significant landform created by internal forces, characterized by a large, flat terrain raised steeply above the surrounding area. The Tibetan Plateau is an example of an intermontane plateau, formed by the immense compressional forces of crustal shortening when the Indian and Eurasian plates collided. Volcanic plateaus, such as the Columbia Plateau in the northwestern United States, form from vast, successive flows of highly fluid basaltic lava that spread out from fissures over long periods.
Plains are expansive, generally flat landforms that can result from various processes, such as tectonic uplift of the seafloor or the filling of vast basins with sediment. Rift valleys represent a distinct type of landform created by tensional forces pulling the crust apart. This process causes a central block of the crust to sink down between two parallel faults, creating a trough-like depression, as seen in the East African Rift.
Landforms Formed by External Surface Processes
The landforms created by external, or exogenous, processes involve the continuous wearing down (erosion) and rebuilding (deposition) of the Earth’s surface by natural agents. These processes, including the action of water, wind, ice, and chemical weathering, constantly modify the larger structures created by internal forces. The resulting features are highly diverse, reflecting the specific dominant agent in a given region.
Fluvial landforms are those shaped by the action of running water, primarily rivers and streams. Erosional features include steep-sided V-shaped valleys and deep canyons, such as the Grand Canyon, formed by the relentless downward cutting of a river’s channel. Depositional fluvial landforms appear when the water’s velocity slows and it drops its sediment load, creating features like wide, flat floodplains adjacent to a river channel. Deltas are formed at a river’s mouth, where the water meets a larger body of water and deposits sediment into a fan-like shape.
Aeolian landforms are those shaped by the wind, which is a powerful agent of erosion and deposition in arid and semi-arid regions. Wind erosion can remove softer material, leaving behind isolated, flat-topped hills called mesas and smaller versions known as buttes. The most recognizable aeolian depositional features are sand dunes, which are mounds of loose sand sculpted and shifted by the wind.
Glacial landforms are created by the movement of large masses of ice. Glacial erosion carves out distinctive U-shaped valleys, which contrast sharply with the V-shape of river valleys. Other erosional features include bowl-shaped depressions high on mountainsides called cirques. When glaciers melt, they leave behind depositional features like moraines, which are ridges of unsorted rock and sediment called till.
Karst landforms are produced by chemical weathering, specifically the dissolution of soluble bedrock, most commonly limestone, by groundwater. The most characteristic features are sinkholes, which are surface depressions that form when the underlying rock is dissolved or when a cave roof collapses. Underground, this chemical action creates extensive cave systems, often decorated with stalactites and stalagmites formed by the slow deposition of mineral-rich water.