Plain

• 1. Introduction

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  Plains cover more than one-third of the world's land area, and they exist on every single continent on Earth. That makes them the most quietly dominant landform on the planet. Not mountains, not oceans. Flat land. But the word "plain" conceals an enormous variety. A polar tundra, a tropical savannah, a wind-scoured desert, and a deep-ocean basin can all qualify. What is a plain, exactly? How do they form? And why do so many of the world's great agricultural civilizations grow up on them? The answers stretch from the ocean floor to the surface of Mars.

• 2. What Makes a Plain

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  A plain is defined by what it lacks: rapid or steep changes in elevation. Beyond that simple criterion, the shapes plains take vary considerably. Some are lowlands nestled along river valleys or pressed against the feet of mountains. Others rise as coastal plains from sea level before running up against elevated terrain. Still others sit high as plateaus or uplands. In a valley, a plain finds itself enclosed on two sides. Elsewhere, it may be ringed by hills, mountains, or cliffs on all sides or only part of the way around. Where a region holds more than one plain, a pass, sometimes called a gap, can link them. Biomes on plains range from temperate and subtropical grassland, to semi-arid steppe, to tropical savannah, to polar tundra. Deserts count as plains in some cases as well.

• 3. How Plains Form

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  Flowing lava is one origin story for plains: sheets of molten rock spread and cool into broad, flat surfaces. Water tells a different story. Rivers deposit sediment across their flood plains and beds over long stretches of time, building what geologists call alluvial plains. A flood plain is the zone that floods fairly regularly today or in recent memory, while an alluvial plain is the wider record of where flooding has occurred, including areas that may only see inundation a few times in a century. Scroll plains form where a river meanders at a very low gradient, looping lazily across flat ground. Ice adds another mechanism entirely. Glaciers moving under gravity produce outwash plains, known in Icelandic-derived terminology as sandur (plural: sandar), which are built from melt-water sediments at a glacier's terminus. Those deposits are stratified, meaning layered and sorted, consisting mainly of gravel and sand. A different glacial type, the till plain, forms when a sheet of ice detaches from the main body of a glacier and melts where it sits, dropping all the unsorted material it carried. Lacustrine plains, by contrast, began as lake beds. Erosional plains arise when running water, rivers, wind, or glaciers wear down rugged terrain. A plain shaped by these agents into something almost flat is called a peneplain, while wind-driven erosion specifically produces a pediplain. Structural plains are the simplest category: undisturbed horizontal surfaces that sit in geologically depressed areas, making up some of the most extensive natural lowlands anywhere on Earth.

• 4. Plains and Agriculture

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  Where plains form from sediment laid down by water, the resulting soils can be both deep and fertile. That combination, paired with the flatness that enables mechanization of crop production, has made many plains central to how humanity feeds itself. Grassland plains serve a parallel purpose: they support livestock grazing. The Pampas of Argentina, Uruguay, and Brazil stretch across a vast region of South America. The Indo-Gangetic Plains span Bangladesh, India, Nepal, and Pakistan. The Great Plains run through both Canada and the United States. In each case, the geography of flatness and fertile soil has shaped patterns of settlement and food production for a long time.

• 5. Plains Across the Globe

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  The Atlantic coastal plain runs along the eastern United States. The Mississippi Alluvial Plain follows the great river south through Mississippi. In Europe, the North German Plain and the East European Plain form a vast connected lowland. The Pannonian Basin spreads across central Europe, and the Po Valley fills northern Italy. Asia holds the North China Plain and the Kanto Plain in Japan, the West Siberian Plain in Russia, and the Punjab Plains bridging Pakistan and India. In Australia, the Nullarbor Plain stretches across the south of the continent, and the Canterbury Plains spread across New Zealand's South Island. Salisbury Plain in England and the Hardangervidda in Norway represent European upland variants. The Altiplano, shared by Bolivia, Peru, and Chile, sits at high elevation in South America. The breadth of this list points toward something the geography makes plain: wherever water has flowed, sediment has settled, lava has spread, or glaciers have retreated, flat land has followed.

• 6. Beyond Earth

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  The Latin word for plain, planitia, pronounced pluh-NISH-ee-uh, has been adopted for naming flat regions on other planets and moons. Hellas Planitia on Mars is one such named feature, and Sedna Planitia on Venus is another. The abyssal plains of Earth's own deep ocean basins also qualify under the broad definition, being flat or very gently sloping areas far beneath the surface. Plains, it turns out, are not just a feature of continents; they are a feature of planetary surfaces wherever geology and time conspire to level the ground.