Plains cover over a third of Earth's land, and they even exist on Mars
Flat land sounds like the least interesting landscape imaginable, yet plains blanket more than a third of the world's continents and feed much of humanity. They are built by rivers, glaciers, lava and wind, and they turn up far beyond Earth, from Hellas Planitia on Mars to Sedna Planitia on Venus.
A plain is simply ground with few sharp or steep changes in height. It can sit low in a valley, spread out at the foot of mountains, run along a coast or even lie high up as a plateau. Plains appear on every continent and support many biomes: temperate grassland, semi-arid steppe, tropical savannah, polar tundra, and in a few cases desert. Some are boxed in by hills, mountains or cliffs, and neighbouring plains may be linked by a pass or gap.
Most plains are made by deposition. Rivers build alluvial plains by spreading sediment over long periods; a flood plain is the part flooded regularly now, while an alluvial plain also includes land that flooded in the past or only a few times a century. Glaciers leave two very different kinds. Outwash plains, called sandar, are neat layers of sand and gravel dropped by meltwater, whereas till plains form when a chunk of ice breaks away and melts where it stands, dumping unsorted debris of every size. Old lake beds, sheets of lava and the flat floors of the deep ocean, the abyssal plains, complete the set.
Other plains are carved rather than built. Running water, wind and ice can wear rugged country down until it is nearly flat; geographers call the results peneplains, meaning almost plains, and pediplains when wind did the work. Structural plains are broad, largely undisturbed horizontal surfaces that form some of the most extensive lowlands anywhere.
Their value to people is practical. Where soils arrived as sediment they can be deep and fertile, and level ground makes it easy to use machines for planting and harvest. Grassy plains also offer excellent grazing. The Indo-Gangetic Plains, stretching across Pakistan, India, Nepal and Bangladesh, are one of the best-known examples.
Source: Plain
