Landforms are the natural physical features on the Earth’s surface. These features, such as mountains, valleys, plains, and plateaus, are constantly being sculpted by various geological processes. They range in size and complexity from vast ocean basins to smaller hills and canyons.
Shaping by Earth’s Internal Forces
Forces originating from within the Earth, known as endogenic processes, are key drivers of landform creation. These internal forces involve the movement and deformation of the Earth’s crust, often over vast geological timescales.
The movement of Earth’s tectonic plates is a primary mechanism for forming major landforms. At convergent boundaries, where plates collide, different outcomes shape the landscape. When two continental plates meet, they can buckle and fold, creating mountain ranges like the Himalayas. If an oceanic plate collides with a continental plate, the denser oceanic plate typically slides beneath the continental plate in a process called subduction, leading to the formation of volcanic arcs and deep ocean trenches.
Divergent boundaries, where plates pull apart, result in new crust forming as magma rises from the mantle. This process creates mid-ocean ridges, which are underwater mountain chains, and rift valleys on continents, such as the East African Rift Valley. At transform boundaries, plates slide horizontally past each other, causing significant stress and fracturing of the crust, often resulting in fault lines and associated uplifted blocks.
Volcanic activity, often linked to plate tectonics, also plays a significant role in landform development. Extrusive igneous landforms are formed when magma reaches the surface and cools as lava. Common volcanic landforms include shield volcanoes, which have gentle slopes from fluid lava flows, and stratovolcanoes (composite volcanoes), which are steep-sided cones built from layers of viscous lava and ash. Lava plateaus, such as the Columbia Plateau, form when fluid lava flow over extensive areas and solidify. Volcanic islands, like the Hawaiian Islands, are created when volcanic eruptions build up new land from beneath the water or on existing land.
Stress within the Earth’s crust can lead to the breaking or bending of rock layers, influencing landform shapes. Faulting occurs when rocks fracture and move along a break, which can uplift or drop blocks of land, creating fault-block mountains such as the Sierra Nevada mountains. Folding happens when rock layers bend without breaking due to compressional forces, forming features like anticlines (upward folds) and synclines (downward folds) that contribute to the topography of fold mountain ranges.
Shaping by Earth’s External Forces
External forces, or exogenic processes, continually reshape landforms through the breakdown, movement, and deposition of material on the Earth’s surface.
Weathering is the initial process where rocks and soil break down into smaller pieces. Physical weathering, like frost wedging, occurs when water freezes and expands in rock cracks, causing them to widen. Abrasion, another form of physical weathering, involves the grinding away of rock surfaces by friction or impact. Chemical weathering, such as dissolution or oxidation, alters the rock’s chemical composition, making it softer and more susceptible to further breakdown. Biological weathering involves the action of living organisms, such as plant roots growing into rock crevices, which can expand and break the rock apart.
Water is a significant agent of erosion and deposition, shaping diverse landforms. Rivers, through fluvial processes, carve V-shaped valleys and canyons in their upper courses due to downward erosion. As rivers mature, they develop meandering paths, eroding the outer banks and depositing sediment on the inner banks, eventually forming oxbow lakes when meander loops are cut off. In their lower courses, rivers deposit large amounts of sediment, creating floodplains and deltas where they meet larger bodies of water. Along coastlines, ocean waves and currents sculpt features like cliffs, sea arches, and beaches through constant erosion and deposition of sand.
Wind, through aeolian processes, is effective in shaping arid and coastal environments. Wind erosion can create streamlined ridges called yardangs or remove fine particles, leaving behind desert pavement. Wind also transports sediment, leading to the depositional landforms like sand dunes depending on wind direction and sediment supply. Loess deposits are thick layers of fine, wind-blown silt, often found downwind of glacial areas or deserts.
Ice, in the form of glaciers, modifies landscapes. Glacial erosion, through processes like abrasion and plucking, carves out features such as U-shaped valleys, cirques (bowl-shaped depressions), and sharp ridges called arêtes. As glaciers melt and retreat, they deposit unsorted sediment known as glacial till, forming depositional landforms like moraines (ridges of debris). Other glacial depositional features include drumlins, which are elongated hills of till, and eskers, which are ridges formed by sediment deposited by meltwater rivers flowing beneath the ice.
Gravity also directly influences landform modification through mass movement, which involves the downslope motion of soil, rock, and other material. Rapid events like landslides and rockfalls occur when unstable slopes give way. These processes reshape slopes and can significantly alter existing landforms, moving large volumes of material over short or long distances.