• seafloor spreading
    The newest crust created by seafloor spreading is usually rocky and dark—magma cooled by cold seawater.

    Photograph by Emory Kristof

    Drifting Apart
    Seafloor spreading in the East Pacific Rise is pulling the continents of Australia, South America, and Antarctica apart. The East Pacific Rise is one of the most active sites of seafloor spreading, spreading more than 14 centimeters (5.6 inches) every year.

    Seafloor spreading is a process of plate tectonics. New oceanic crust is created as large slabs of the Earth's crust split apart from each other and magma wells up to fill the gap.

    The large slabs of rock that make up the Earth’s crust are called tectonic plates. As they slowly move away from each other beneath the ocean floor, hot magma from the Earth’s mantle bubbles to the surface.

    This magma is then cooled by seawater. The new rock forms a new part of the Earth’s crust. Seafloor spreading occurs along mid-ocean ridges—large mountain ranges rising from the ocean floor.

    The newest oceanic crust is located near the center of the ridge, the actual site of seafloor spreading. The Mid-Atlantic Ridge, which separates the North American plate from the Eurasian plate, and the South American plate from the African plate, is the site of new oceanic crust in the middle of the Atlantic Ocean.

    Over time, new oceanic crust pushes older crust farther away. New bodies of water and even continents can be created through seafloor spreading. The Red Sea, for example, was created through seafloor spreading, as the African plate and the Arabian plate tear away from each other. Today, the northern Sinai Peninsula connects the Middle East (Asia) with North Africa. Eventually, geologists predict, seafloor spreading will expand the Red Sea so that it will completely separate the two continents.

    Rift valleys, which exist on continental crust as well as oceanic crust, can be created by seafloor spreading. Two of the largest rift valleys in the world, the Mid-Atlantic Ridge and the East Pacific Rise, are products of seafloor spreading.

    Seafloor spreading disproves an early part of the theory of continental drift. Continental drift was one of the first theories that the Earth's crust was dynamic and always in motion. Supporters of continental drift originally theorized that the continents moved (drifted) through unmoving oceans. Seafloor spreading proves that the ocean floor itself is the site of tectonic activity.

    Subduction is the opposite of seafloor spreading. Subduction happens where tectonic plates crash into each other instead of spreading apart. In subduction zones, the edge of the heavier plate subducts, or slides, beneath the lighter one. It then melts back into the Earth's mantle.

    Seafloor spreading creates new crust. Subduction destroys old crust. The two forces roughly balance each other, so the shape and diameter of the Earth remains constant.

  • Term Part of Speech Definition Encyclopedic Entry
    continent Noun

    one of the seven main land masses on Earth.

    Encyclopedic Entry: continent
    continental crust Noun

    thick layer of Earth that sits beneath continents.

    continental drift Noun

    the movement of continents resulting from the motion of tectonic plates.

    Encyclopedic Entry: continental drift
    crust Noun

    rocky outermost layer of Earth or other planet.

    Encyclopedic Entry: crust
    diameter Noun

    width of a circle.

    disprove Verb

    to prove wrong.

    dynamic Adjective

    always changing or in motion.

    East Pacific Rise Noun

    mid-ocean ridge where several tectonic plates are moving apart from one another.

    geologist Noun

    person who studies the physical formations of the Earth.

    magma Noun

    molten, or partially melted, rock beneath the Earth's surface.

    Encyclopedic Entry: magma
    mantle Noun

    middle layer of the Earth, made of mostly solid rock.

    Encyclopedic Entry: mantle
    Mid-Atlantic Ridge Noun

    underwater mountain range that runs from Iceland to Antarctica.

    mid-ocean ridge Noun

    underwater mountain range.

    mountain range Noun

    series or chain of mountains that are close together.

    oceanic crust Noun

    thin layer of the Earth that sits beneath ocean basins.

    plate tectonics Noun

    movement and interaction of the Earth's plates.

    predict Verb

    to know the outcome of a situation in advance.

    rift valley Noun

    depression in the ground caused by the Earth's crust spreading apart.

    Encyclopedic Entry: rift valley
    seafloor spreading Noun

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

    Encyclopedic Entry: seafloor spreading
    seawater Noun

    salty water from an ocean or sea.

    subduct Verb

    to pull downward or beneath something.

    subduction Noun

    process of one tectonic plate melting or going beneath another.

    subduction zone Noun

    area where one tectonic plate slides under another.

    tectonic activity Noun

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

    tectonic plate Noun

    large, moveable segment of the Earth's crust.

    theorize Verb

    to formulate and propose a group of ideas to explain a scientific question.

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