• A basin is a depression, or dip, in the Earth’s surface. Basins are shaped like bowls, with sides higher than the bottom. They can be oval or circular in shape, similar to a sink or tub you might have in your own bathroom. Some are filled with water. Others are empty.

    Basins are formed by forces above the ground (like erosion) or below the ground (like earthquakes). They can be created over thousands of years or almost overnight.

    The major types of basins are river drainage basins, structural basins, and ocean basins.

    River Drainage Basins

    A river drainage basin is an area drained by a river and all of its tributaries. A river basin is made up of many different watersheds.

    A watershed is a small version of a river basin. Every stream and tributary has its own watershed, which drains to a larger stream or wetland. These streams, ponds, wetlands, and lakes are part of a river basin. The Mississippi River basin in the U.S., for instance, is made up of six major watersheds: the Missouri, Upper Mississippi, Ohio, Tennessee, Lower Mississippi, and Arkansas-Red-White Rivers.

    Every river is part of a network of watersheds that make up a river system’s entire drainage basin. All the water in the drainage basin flows downhill toward bigger rivers. The Pease River, in northern Texas, is part of the Arkansas-Red-White watershed. It is a tributary of the Red River. The Red River is a major tributary of the Mississippi River, which flows into the Gulf of Mexico.

    The Amazon Basin, in northern South America, is the largest in the world. The Amazon River and all of its tributaries drain an area more than 7 million square kilometers (about 3 million square miles).

    Structural Basins

    Structural basins are formed by tectonic activity. Tectonic activity is the movement of large pieces of the Earth’s crust, called tectonic plates. Tectonic activity is responsible for such phenomena as earthquakes and volcanoes. The natural processes of weathering and erosion also contribute to forming structural basins.

    Structural basins form as tectonic plates shift. Rocks and other material on the floor of the basin are forced downward, while material on the sides of the basin are pushed up. This process happens over thousands of years. If a basin is shaped like a bowl, a structural basin is shaped like a series of smaller bowls, stacked inside each other. Structural basins are usually found in dry regions.

    Some structural basins are known as endorheic basins. Endorheic basins have internal drainage systems. This means they don’t have enough water to drain to a stream, lake, or ocean. The water that trickles into these types of basins evaporates or seeps into the ground.

    When enough water collects in an endorheic basin, it can form a very salty lake, such as the Dead Sea, between Israel and Jordan. While water evaporates into the atmosphere, minerals remain. The remaining water becomes even saltier. The Dead Sea is one of the saltiest natural body of water on Earth. Its shore, about 400 meters (1,300 feet) below sea level, is Earth’s lowest dry point.

    Death Valley, in the U.S. state of California, is another endorheic basin. At about 86 meters (282 feet) below sea level, it is the lowest place in North America. The water draining into Death Valley from its few streams does not exit the basin to a river or estuary. It evaporates or seeps into the ground.

    A lake basin is another type of structural basin. Lake basins often form in valleys blocked by rocks or other debris left by a landslide, lava flow, or glacier. The debris acts as a dam, trapping water and forming a lake. Hunza Lake in Pakistan was formed when an earthquake triggered a massive landslide in 2010. The debris dammed the Hunza River, in addition to killing 20 people and destroying the village of Attabad. The Hunza River continues to flow into the lake basin, and many geologists and villagers worry the basin won’t be strong enough to hold the water.

    Lake basins may also be carved out by glaciers—huge masses of ice—as they move down valleys or across the land. When the glaciers move, the basins they create remain. During the last ice age, glaciers carved the basins of the Finger Lakes, in the U.S. state of New York.

    Sedimentary basins are a type of structural basin that aren’t shaped like typical basins, sometimes forming long troughs. Sedimentary basins have been filled with layers of rock and organic material over millions of years. Material that fills up the basin is called sediment fill.

    Sedimentary basins are key sources of petroleum and other fossil fuels. Millions of years ago, tiny sea creatures called diatoms lived and died in ocean basins. Eventually, these ancient oceans dried up, leaving dry basins. The remains of the diatoms were at the bottom of these basins. The remains were crushed under billions of tons of sediment fill, over millions of years. In the right conditions, the pressure of the sediment fill turns the diatom remains into petroleum.

    The Niger Delta sedimentary basin, in the countries of Nigeria, Cameroon, and Equatorial Guinea, is one of the most productive petroleum fields in Africa. In North America, the Western Canadian Sedimentary Basin is one of the continent's largest suppliers of gas and coal.

    Ocean Basins

    Ocean basins are the largest depressions on Earth. Edges of the continents, called continental shelves, form the sides of ocean basins.

    There are five major ocean basins, coordinating with the major oceans of the world: the Pacific basin, the Atlantic basin, the Indian basin, the Arctic basin, and the Southern basin. Many smaller basins are often considered oceanic basins, such as the North Aleutian Basin, between the Pacific and Arctic Oceans.

    Tectonic activity constantly changes ocean basins. Seafloor spreading and subduction are the most important types of tectonic activity that shape ocean basins.

    Seafloor spreading happens along the boundaries of tectonic plates that are moving apart from each other. These areas are called mid-ocean ridges. New seafloor is created at the bottom, or rift, of a mid-ocean ridge. Ocean basins that have mid-ocean ridges are expanding. The Atlantic basin, for instance, is expanding because of seafloor spreading.

    Subduction happens along the boundaries of tectonic plates that are crashing into each other. In these subduction zones, the heavier plate moves underneath, or subducts, the lighter one. Ocean basins that experience subduction, such as the Pacific basin, are shrinking.

    Even though ocean basins make up more than 70 percent of the total land on Earth, scientists know relatively little about them. Some oceanographers (and some astronomers!) say that we know more about the surface of the moon than we do about the surface of the ocean floor.

    It is very difficult to get information about landforms of the ocean basin, such as trenches and mid-ocean ridges. These landforms are thousands of feet below the surface of the water. Few instruments can endure the intense pressure, cold, and dark at the bottom of ocean basins. Occasionally, researchers themselves explore ocean basins in special submarines called submersibles.

    A lake collects water at the bottom of this basin.

    What's your basin?
    Everyone lives in a watershed or river basin, even if they don't live near water. What is the name of the watershed or river basin you live in?

    Pacific Ocean Basin: Ring Of Fire
    The Pacific Ocean basin is the largest in the world. It is more than 155 million square kilometers (59 million square miles)all of the continents could fit into it. It is also the oldest basin; researchers say its rocks are 200 million years old.

    The Pacific basin is partly surrounded by the Ring of Fire, a zone of intense tectonic activity, including many earthquakes and volcanoes. The Ring of Fire touches Alaska, the Americas, New Zealand, and eastern Asia.

  • Term Part of Speech Definition Encyclopedic Entry
    astronomer Noun

    person who studies space and the universe beyond Earth's atmosphere.

    atmosphere Noun

    layers of gases surrounding a planet or other celestial body.

    Encyclopedic Entry: atmosphere
    basin Noun

    a dip or depression in the surface of the land or ocean floor.

    Encyclopedic Entry: basin
    continent Noun

    one of the seven main land masses on Earth.

    Encyclopedic Entry: continent
    continental shelf Noun

    part of a continent that extends underwater to the deep-ocean floor.

    Encyclopedic Entry: continental shelf
    coral Noun

    tiny ocean animal, some of which secrete calcium carbonate to form reefs.

    crust Noun

    rocky outermost layer of Earth or other planet.

    Encyclopedic Entry: crust
    dam Noun

    structure built across a river or other waterway to control the flow of water.

    debris Noun

    remains of something broken or destroyed; waste, or garbage.

    diatom Noun

    type of algae, most of which are only one cell.

    drainage basin Noun

    an entire river system or an area drained by a river and its tributaries. Also called a watershed.

    earthquake Noun

    the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity.

    endorheic basin Noun

    watershed that empties into an internal body of water, not the ocean.

    erosion Noun

    act in which earth is worn away, often by water, wind, or ice.

    Encyclopedic Entry: erosion
    estuary Noun

    mouth of a river where the river's current meets the sea's tide.

    Encyclopedic Entry: estuary
    evaporate Verb

    to change from a liquid to a gas or vapor.

    fossil fuel Noun

    coal, oil, or natural gas. Fossil fuels formed from the remains of ancient plants and animals.

    gas Noun

    state of matter with no fixed shape that will fill any container uniformly. Gas molecules are in constant, random motion.

    geologist Noun

    person who studies the physical formations of the Earth.

    glacier Noun

    mass of ice that moves slowly over land.

    Encyclopedic Entry: glacier
    ice age Noun

    long period of cold climate where glaciers cover large parts of the Earth. The last ice age peaked about 20,000 years ago. Also called glacial age.

    internal drainage system Noun

    process where streams do not drain to a larger watershed.

    lake basin Noun

    dip or depression in the surface of the Earth that used to be the site of a lake.

    landslide Noun

    the fall of rocks, soil, and other materials from a mountain, hill, or slope.

    Encyclopedic Entry: landslide
    lava Noun

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

    mid-ocean ridge Noun

    underwater mountain range.

    mineral Noun

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

    moraine Noun

    material, such as earth, sand, and gravel, transported by a glacier.

    Encyclopedic Entry: moraine
    ocean basin Noun

    depression in the Earth's surface located entirely beneath the ocean.

    oceanographer Noun

    person who studies the ocean.

    organic Adjective

    composed of living or once-living material.

    petroleum Noun

    fossil fuel formed from the remains of ancient organisms. Also called crude oil.

    petroleum field Noun

    geographic area whose rock strata contain oil or gas.

    phenomena Plural Noun

    (singular: phenomenon) any observable occurrence or feature.

    remains Noun

    materials left from a dead or absent organism.

    rift Noun

    break in the Earth's crust created by it spreading or splitting apart.

    seafloor spreading Noun

    rift in underwater mountain range where new oceanic crust is formed.

    Encyclopedic Entry: seafloor spreading
    sedimentary basin Noun

    depression in the Earth's surface that has slowly been filled with layers of sand, rock, and other debris (sediment).

    sediment fill Noun

    rock, debris, and other material that fills a sedimentary basin.

    seep Verb

    to slowly flow through a border.

    structural basin Noun

    depression in the Earth's surface caused by tectonic activity.

    subduct Verb

    to pull downward or beneath something.

    subduction Noun

    process of one tectonic plate melting, sliding, or falling beneath another.

    submarine Noun

    vehicle that can travel underwater.

    submersible Noun

    small submarine used for research and exploration.

    tectonic activity Noun

    movement of tectonic plates resulting in geologic activity such as volcanic eruptions and earthquakes.

    tectonic plate Noun

    massive slab of solid rock made up of Earth's lithosphere (crust and upper mantle). Also called lithospheric plate.

    trench Noun

    long, deep depression, either natural or man-made.

    tributary Noun

    stream that feeds, or flows, into a larger stream.

    Encyclopedic Entry: tributary
    valley Noun

    depression in the Earth between hills.

    volcano Noun

    an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.

    Encyclopedic Entry: Plate Tectonics and Volcanic Activity
    watershed Noun

    entire river system or an area drained by a river and its tributaries.

    Encyclopedic Entry: watershed
    weathering Noun

    the breaking down or dissolving of the Earth's surface rocks and minerals.

    Encyclopedic Entry: weathering
    wetland Noun

    area of land covered by shallow water or saturated by water.

    Encyclopedic Entry: wetland