LANDFORM

any conspicuous topographic feature on the Earth or a similar planetary body or satellite. Familiar examples are mountains (including volcanic cones), plateaus, and valleys. Comparable structures have been detected on Mars, Venus, the Moon, and certain satellites of Jupiter and Saturn. The term landform also can be applied to related features that occur on the floor of the Earth's ocean basins, as, for example, seamounts, mid-oceanic ridges, and submarine canyons. The distribution and structure of landforms reflect the geomorphic processes that created them. Most landforms occurring at the surface of the terrestrial land masses result from the interaction of two fundamental types of processes over geologic time. These are (1) vertical tectonic movements and extrusion of magma (molten rock material) and (2) denudational processes, which encompass the weathering and erosion of rocks and the accumulation of the resulting sedimentary debris. Relief features produced chiefly by uplift and subsidence of the Earth's crust or by upward magmatic movements can be classified as tectonic landforms. They include rift valleys, plateaus, mountains, and volcanic cones. Other topographic features, such as pediments, sand dunes, subterranean caves, fjords, and beaches, are formed by denudational processes. These features, categorized as structural landforms, are attributable to the erosional and depositional action of rivers, wind, groundwater solution, glaciers, sea waves, and other external agents. Although tectonic and denudational processes account for the origin of most landform types, a few have been produced by other means. Impact craters, for one, are formed by collisions with asteroids, comets, and meteorites. Biogenic landforms, for another, are producedas the term impliesby living organisms. They range from giant termite mounds and coral reefs to open-pit mines and dams created by humans.