• Continental drift describes one of the earliest ways geologists thought continents moved over time. Today, the theory of continental drift has been replaced by the science of plate tectonics
    The theory of continental drift is most associated with the scientist Alfred Wegener. In the early 20th century, Wegener published a paper explaining his theory that the continental landmasses were “drifting” across the Earth, sometimes plowing through oceans and into each other. He called this movement continental drift. 
    Wegener was convinced that all of Earth’s continents were once part of an enormous, single landmass called Pangaea
    Wegener, trained as an astronomer, used biology, botany, and geology describe Pangaea and continental drift. For example, fossils of the ancient reptile mesosaurus are only found in southern Africa and South America. Mesosaurus, a freshwater reptile only one meter (3.3 feet) long, could not have swum the Atlantic Ocean. The presence of mesosaurus suggests a single habitat with many lakes and rivers.
    Wegener also studied plant fossils from the frigid Arctic archipelago of Svalbard, Norway. These plants were not the hardy specimens adapted to survive in the Arctic climate. These fossils were of tropical plants, which are adapted to a much warmer, more humid environment. The presence of these fossils suggests Svalbard once had a tropical climate.
    Finally, Wegener studied the stratigraphy of different rocks and mountain ranges. The east coast of South America and the west coast of Africa seem to fit together like pieces of a jigsaw puzzle, and Wegener discovered their rock layers “fit” just as clearly. South America and Africa were not the only continents with similar geology. Wegener discovered that the Appalachian Mountains of the eastern United States, for instance, were geologically related to the Caledonian Mountains of Scotland. 
    Pangaea existed about 240 million years ago. By about 200 million years ago, this supercontinent began breaking up. Over millions of years, Pangaea separated into pieces that moved away from one another. These pieces slowly assumed their positions as the continent we recognize today.
    Today, scientists think that several supercontinents like Pangaea have formed and broken up over the course of the Earth’s lifespan. These include Pannotia, which formed about 600 million years ago, and Rodinia, which existed more than a billion years ago.
    Tectonic Activity
    Scientists did not accept Wegener’s theory of continental drift. One of the elements lacking in the theory was the mechanism for how it works—why did the continents drift and what patterns did they follow? Wegener suggested that perhaps the rotation of the Earth caused the continents to shift towards and apart from each other. (It doesn't.)
    Today, we know that the continents rest on massive slabs of rock called tectonic plates. The plates are always moving and interacting in a process called plate tectonics. 
    The continents are still moving today. Some of the most dynamic sites of tectonic activity are seafloor spreading zones and giant rift valleys. 
    In the process of seafloor spreading, molten rock rises from within the Earth and adds new seafloor (oceanic crust) to the edges of the old. Seafloor spreading is most dynamic along giant underwater mountain ranges known as mid-ocean ridges. As the seafloor grows wider, the continents on opposite sides of the ridge move away from each other. The North American and Eurasian tectonic plates, for example, are separated by the Mid-Atlantic Ridge. The two continents are moving away from each other at the rate of about 2.5 centimeters (1 inch) per year. 
    Rift valleys are sites where a continental landmass is ripping itself apart. Africa, for example, will eventually split along the Great Rift Valley system. What is now a single continent will emerge as two—one on the African plate and the other on the smaller Somali plate. The new Somali continent will be mostly oceanic, with the Horn of Africa and Madagascar its largest landmasses.
    The processes of seafloor spreading, rift valley formation, and subduction (where heavier tectonic plates sink beneath lighter ones) were not well-established until the 1960s. These processes were the main geologic forces behind what Wegener recognized as continental drift.
    continental drift
    The way some continents fit together like puzzle pieces inspired the theory of continental drift.
    Alfred Wegener’s original name for his proposed, ancient continent was “Urkontinent”—ur meaning “first or original,” and kontinent meaning “continent” in Wegener’s native language, German. A more popular name for this huge ancient landmass is Pangaea, which means “all lands” in Greek.

    Colliding Skyward
    The collision of the Indian subcontinent and Asian continent created the Himalayan mountain range, home to the world's highest mountain peaks, including 30 that exceed 7300 meters (24,000 feet). Because continental drift is still pushing India into Asia, the Himalayas are still growing.

  • Term Part of Speech Definition Encyclopedic Entry
    adapt Verb

    to adjust to new surroundings or a new situation.

    Alfred Wegener Noun

    (1880-1930) German meteorologist and geologist.

    ancient Adjective

    very old.

    archipelago Noun

    a group of closely scattered islands in a large body of water.

    Encyclopedic Entry: archipelago
    Arctic Noun

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

    Encyclopedic Entry: Arctic
    astronomer Noun

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

    biology Noun

    study of living things.

    botany Noun

    study of plants.

    climate Noun

    all weather conditions for a given location over a period of time.

    Encyclopedic Entry: climate
    coast Noun

    edge of land along the sea or other large body of water.

    Encyclopedic Entry: coast
    continent Noun

    one of the seven main land masses on Earth.

    Encyclopedic Entry: continent
    continental drift Noun

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

    Encyclopedic Entry: continental drift
    dynamic Adjective

    always changing or in motion.

    enormous Adjective

    very large.

    fossil Noun

    remnant, impression, or trace of an ancient organism.

    Encyclopedic Entry: fossil
    freshwater Adjective

    having to do with a habitat or ecosystem of a lake, river, or spring.

    frigid Adjective

    very cold.

    geologist Noun

    person who studies the physical formations of the Earth.

    geology Noun

    study of the physical history of the Earth, its composition, its structure, and the processes that form and change it.

    Great Rift Valley system Noun

    series of faults and other sites of tectonic activity stretching from southwestern Asia to the Horn of Africa.

    habitat Noun

    environment where an organism lives throughout the year or for shorter periods of time.

    Encyclopedic Entry: habitat
    hardy Adjective

    strong or able to withstand severe weather.

    humid Adjective

    air containing a large amount of water vapor.

    jigsaw puzzle Noun

    interlocking pieces that, when correctly put together, display a picture or design.

    landmass Noun

    large area of land.

    massive Adjective

    very large or heavy.

    mechanism Noun

    process or assembly that performs a function.

    mesosaurus Noun

    freshwater reptile that lived during the early Permian period, about 300 million years ago.

    Mid-Atlantic Ridge Noun

    underwater mountain range that runs from Iceland to Antarctica.

    mid-ocean ridge Noun

    underwater mountain range.

    molten Adjective

    solid material turned to liquid by heat.

    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.

    Pangaea Noun

    supercontinent of all the Earth's landmass that existed about 250 million years ago.

    plate tectonics Noun

    movement and interaction of the Earth's plates.

    reptile Noun

    animal that breathes air and usually has scales.

    rift valley Noun

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

    Encyclopedic Entry: rift valley
    rotation Noun

    object's complete turn around its own axis.

    Encyclopedic Entry: rotation
    seafloor spreading Noun

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

    Encyclopedic Entry: seafloor spreading
    slab Noun

    flat, thick piece of material such as earth or stone.

    specimen Noun

    individual organism that is a typical example of its classification.

    stratigraphy Noun

    study of rock layers and layering.

    subduction Noun

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

    supercontinent Noun

    ancient, giant landmass that split apart to form all the continents we know today.

    tectonic plate Noun

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

    tropical Adjective

    existing in the tropics, the latitudes between the Tropic of Cancer in the north and the Tropic of Capricorn in the south.

    tropical climate Noun

    climate group that experiences hot, wet summers.