• plain
    Plains near rivers are called flood plains.

    Photograph by Xi Luo, MyShot

    Plains on Other Planets
    Plains can be found on other planets. Mercury has large stretches of plains, and scientists have landed several probes on the boulder-covered plains of Mars.

    Thundering Hooves
    The Great Plains of North America once supported about 50 million bison, which are sometimes called buffalo. The bison roamed in vast herds, feeding on the prairie grasses. They were hunted to near-extinction in the 1800s.

    A plain is a broad area of relatively flat land. Plains are one of the major landforms, or types of land, on Earth. They cover more than one-third of the world’s land area. Plains exist on every continent.

    Grasslands

    Many plains, such as the Great Plains that stretch across much of central North America, are grasslands. A grassland is a region where grass is the main type of vegetation.

    In North America, temperate grasslands—those in places with warm summers and cold winters—are often called prairies. In areas with little rain and snow, short grasses grow. In areas that receive more rain and snow, tall grasses can grow 1.5 meters (5 feet) high. However, most tallgrass prairies have been plowed under and are now farmland or pasture.

    The Great Plains have supported a wide variety of cultures for thousands of years. The so-called “Plains Indians” are actually more than two dozen tribes. Plains Indian tribes include Blackfoot, native to the Canadian province of Alberta; Arapaho, whose center today is in the U.S. state of Wyoming; and Kickapoo, many of whom live today in the Mexican state of Coahuila.

    In Asia and eastern Europe, temperate grasslands are called steppes. Steppes usually do not receive enough rain for tall grasses and trees to grow.

    Tropical grasslands are called savannas. Savannas exist in places that are warm throughout the year. They often have scattered trees. Savannas such as the Serengeti plains stretch across much of central Africa. They are also found in Australia, South America, and southern North America.

    Not all plains are grasslands. Some, such as Mexico’s Tabasco Plain, are forested. Forested plains have different types of trees, shrubs, and other vegetation.

    Deserts can also be plains. Parts of the Sahara, a great desert in North Africa, are plains.

    In the Arctic, where the ground is frozen, plains are called tundra. Despite the cold, many plants survive here, including shrubs and moss.

    Plain Formation

    Plains form in many different ways. Some plains form as ice and water erodes, or wears away, the dirt and rock on higher land. Water and ice carry the bits of dirt, rock, and other material, called sediment, down hillsides to be deposited elsewhere. As layer upon layer of this sediment is laid down, plains form.

    Volcanic activity can also form plains. Lava plains form when lava pushes up from below ground and flows across the land. The earth in a lava plain is often much darker than the surrounding soil. The dark earth is a result of the lava, mostly a dark-colored mineral called basalt, broken down into tiny particles over millions of years.

    The movement of rivers sometimes forms plains. Many rivers run through valleys. As rivers move from side to side, they gradually erode the valley, creating broad plains.

    As a river floods, it overflows its bank. The flood carries mud, sand, and other sediment out over the land. After the water withdraws, the sediment remains. If a river floods repeatedly, over time this sediment will build up into a flood plain. Flood plains are often rich in nutrients and create fertile farmland. The flood plain surrounding Africa’s Nile River has helped Egyptian civilization thrive for thousands of years.

    Alluvial plains form at the base of mountains. Water carrying sediment flows downhill until it hits flat land. There, it spreads out, depositing the sediment in the shape of a fan. The Huang He River in China has created an alluvial plain that covers about 409,500 square kilometers (158,000 square miles). Because much of the sediment the Huang He carries is yellowish in color, it is also called the Yellow River.

    Many rivers deposit their sediment in the ocean. As the sediment builds up, it might eventually rise above sea level, forming a coastal plain. The Atlantic Coastal Plain stretches along much of the eastern coast of North America. These broad underwater plains slope gently down beneath the water.

    Abyssal plains are found at the bottom of the ocean. These plains are 5,000 to 7,000 meters (16,400 to 23,000 feet) below sea level, so scientists have a hard time studying them. But scientists say abyssal plains are among the flattest, smoothest places on Earth.

  • Term Part of Speech Definition Encyclopedic Entry
    abyssal plain Noun

    extensive, featureless region of the deep ocean floor.

    alluvial plain Noun

    flat or gently sloping surface created by sediments left by flowing water.

    Arapaho Noun

    people and culture native to the Midwest of the U.S.

    Arctic Noun

    region at Earth's extreme north, encompassed by the Arctic Circle.

    Encyclopedic Entry: Arctic
    bank Noun

    a slope of land adjoining a body of water, or a large elevated area of the sea floor.

    basalt Noun

    type of dark volcanic rock.

    bison Noun

    large mammal native to North America. Also called American buffalo.

    Blackfoot Noun

    people and culture native to the northern U.S. and southern Canada.

    broad Adjective

    wide or expansive.

    civilization Noun

    complex way of life that developed as humans began to develop urban settlements.

    Encyclopedic Entry: civilization
    coastal plain Noun

    low, flat land lying next to the ocean.

    Encyclopedic Entry: coastal plain
    continent Noun

    one of the seven main land masses on Earth.

    Encyclopedic Entry: continent
    deposit Verb

    to place or deliver an item in a different area than it originated.

    desert Noun

    area of land that receives no more than 25 centimeters (10 inches) of precipitation a year.

    Encyclopedic Entry: desert
    dozen Noun

    a group of 12.

    erode Verb

    to wear away.

    extinction Noun

    process of complete disappearance of a species from Earth.

    farmland Noun

    area used for agriculture.

    fertile Adjective

    able to produce crops or sustain agriculture.

    flood plain Noun

    flat area alongside a stream or river that is subject to flooding.

    Encyclopedic Entry: flood plain
    forest Verb

    to cover with trees and other vegetation.

    grass Noun

    type of plant with narrow leaves.

    grassland Noun

    ecosystem with large, flat areas of grasses.

    Great Plains Noun

    grassland region of North America, between the Rocky Mountains and the Mississippi River.

    herd Noun

    group of animals.

    Kickapoo Noun

    people and culture native to the southern U.S. and northern Mexico.

    landform Noun

    specific natural feature on the Earth's surface.

    Encyclopedic Entry: landform
    lava Noun

    molten rock, or magma, that erupts from volcanoes or fissures in the Earth's surface.

    lava plain Noun

    large, flat piece of land created by lava spreading out evenly across a region.

    mercury Noun

    chemical element with the symbol Hg.

    mineral Noun

    inorganic material that has a characteristic chemical composition and specific crystal structure.

    moss Noun

    tiny plant usually found in moist, shady areas.

    mountain Noun

    landmass that forms as tectonic plates interact with each other.

    mud Noun

    wet soil.

    nutrient Noun

    substance an organism needs for energy, growth, and life.

    Encyclopedic Entry: nutrient
    pasture Noun

    type of agricultural land used for grazing livestock.

    plain Noun

    flat, smooth area at a low elevation.

    Encyclopedic Entry: plain
    Plains Indian Noun

    one of many people and cultures native to the Great Plains in North America.

    planet Noun

    large, spherical celestial body that regularly rotates around a star.

    Encyclopedic Entry: planet
    plow noun, verb

    tool used for cutting, lifting, and turning the soil in preparation for planting.

    prairie Noun

    large grassland; usually associated with the Mississippi River Valley in the United States.

    Encyclopedic Entry: prairie
    probe Noun

    spacecraft designed to study part of the solar system and send information back to Earth.

    province Noun

    division of a country larger than a town or county.

    Encyclopedic Entry: province
    region Noun

    any area on the Earth with one or more common characteristics. Regions are the basic units of geography.

    Encyclopedic Entry: region
    relatively Adverb

    in comparison to something else.

    rock Noun

    natural substance composed of solid mineral matter.

    Sahara Desert Noun

    world's largest desert, in north Africa.

    sand Noun

    small, loose grains of disintegrated rocks.

    savanna Noun

    type of tropical grassland with scattered trees.

    sea level Noun

    base level for measuring elevations. Sea level is determined by measurements taken over a 19-year cycle.

    Encyclopedic Entry: sea level
    sediment Noun

    solid material transported and deposited by water, ice, and wind.

    Encyclopedic Entry: sediment
    Serengeti plains Noun

    grassland of the Serengeti ecosystem of Kenya and Tanzania.

    snow Noun

    precipitation made of ice crystals.

    soil Noun

    top layer of the Earth's surface where plants can grow.

    steppe Noun

    dry, flat grassland with no trees and a cool climate.

    Encyclopedic Entry: steppe
    tallgrass prairie Noun

    plain where grasses grow up to 2 meters (6 feet) tall.

    temperate grassland Noun

    flat, grassy area where there are seasonal differences in temperature and precipitation.

    thrive Verb

    to develop and be successful.

    thundering Adjective

    very loud.

    tundra Noun

    cold, treeless region in Arctic and Antarctic climates.

    valley Noun

    depression in the Earth between hills.

    vast Adjective

    huge and spread out.

    vegetation Noun

    all the plant life of a specific place.

    volcanic Adjective

    having to do with volcanoes.

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