Earthguide Online Classroom

Plate tectonics

Definition: What is plate tectonics

The concept that describes and explains how the major features of the Earth form and/or deform. The essential processes of plate tectonics are 1)seafloor spreading and 2) subduction. These two processes operate on basic units called plates. Key surface features such as mountain ranges form in particular tectonic settings when these processes act on plates directly or indirectly. Plates exist as a consequence of the specific conditions of temperature and pressure, and materials found on Earth. Plates move around, and change in size and number over geologic time.

Shadaed-relief map of the world. Lighter colors indicate higher elevation, darker colors indicate lower elevation. Color-scale not available.

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Image from computerized digital images and associated databases available from the National Geophysical Data Center (NGDC), National Oceanic and Atmospheric Administration, U.S. Department of Commerce. Data compiled from GLOBE elevations with bathymetry from Smith and Sandwell.

Key points

  1. Although other planets in our solar system are tectonically active, "plate tectonics" is unique to Earth. The more general term "tectonics" refers to processes operating within any rocky planet that result in the formation or deformation of significant surface features. Earth is the only planet in the solar system with tectonic activity dominated by structural units called plates.
  2. On Earth, the areas along plate boundaries are the most susceptible to deformation because these places are the most exposed to being bumped and scraped by neighboring plates, or being pulled apart from them.
  3. Distinctive features tend to form at each of the three major kinds of plate boundaries - convergent, divergent and transform types of plate boundaries.
  4. Tectonic activity is fueled by sources of energy within the Earth's interior. Sources of energy include heat trapped inside the Earth long ago and energy released by ongoing radioactive decay. Convective motion occurs in response to the density differential of material that exists between the surface and interior of the Earth. The physical and insulating qualities of the materials that make up Earth depend on the specific materials that make up the Earth.
  5. The key processes of plate tectonics are 1) seafloor spreading that takes place at midocean ridges, and 2) subduction into trenches. Our current understanding of plate tectonic processes explains the existence and location of oceans, continents, large mountain ranges, volcanoes, earthquakes, and other large structures such as folds and faults.
  6. What appears to be the movement of continents is in many cases the movement of entire plates including attached pieces of ocean. That motion is mostly accommodated by changes in the area of the seafloor, not changes in the area of land. Oceans change size when the rate of loss of seafloor by subduction is not equal to rate of growth by seafloor spreading. The Atlantic grows because the growth of seafloor by seafloor spreading isn't offset at all by loss of seafloor by subduction. The Pacific is shrinking as subduction zones roll back from advancing plates.
  7. Both the Pacific and Atlantic oceans are now making new seafloor by seafloor spreading at midocean ridges, but the Atlantic is growing while the Pacific is shrinking.

Points of difficulty

  1. Defining plate tectonics. Is it a force, scientific theory, mountain-building, structural feature of the Earth, concept, or other process as often implied?
  2. Constructing a permanent concept of plate tectonics that differs from continental drift. It is easier to envision continental drift where blocks of solid continental crust plow through solid oceanic crust.
  3. Making judgments about what constitutes a "significant" surface feature of the Earth".
  4. Envisioning three-dimensional processes.
  5. Understanding the difference between a map view and a cross-sectional view.
  6. Many familiar mountain ranges are difficult to place in a simple view of tectonics because they are complex and/or not well understood (Sierra Nevada and Rockies) or they exist as relicts in the absence of the tectonic activities that produced them (Appalachians).


  1. Prediction of geologic hazards such as earthquakes, volcanic eruptions and tsunami
  2. Locating geologic resources
  3. Reconstructing the history of the Earth

Engagement, etc.

  1. The ANZA Gap - Earthquake predictions for southern California
    Guess which part of Caifornia is due for action along the San Andreas?
  2. Cascadia - The confirmation of an event in the U.S., analogous to the Sumatra Earthquake and Tsunami, December 26, 2004
    Are we at risk of killer tsunami?

Questions for thought

  1. What is plate tectonics?
  2. What are the key landforms and structures at the Earth's surface that form as a result of tectonic activity?

    Landforms - "Any feature of Earth's surface having a distinct shape and origin. Landforms include major features (such as continents, ocean basins, plains, plateaus, and mountain ranges) and minor features (such as hills, valleys, slopes, drumlins, and dunes). Collectively, the landforms of Earth constitute the entire surface configuration of the planet."
    - Digital Atlas of Idaho - Glossary

    Structures - Rock features, at many different scales, that form when rock is subjected to stress greater than the strength of rock (the rock's ability to resist deformation). Structures include folds, faults, domes, basins, monoclines, and joints.
  3. What specific rock types and features can be found at convergent plate boundarires?
  4. What specific rock types and features can be found at divergent plate boundarires?
  5. What specific rock types and features can be found at transform plate boundarires?
  6. Do active tectonic processes tend to raise or lower land surfaces?
  7. Speculate on what the Earth's surface might look like once the Earth's interior cools and solidifies.
  8. Is continental drift a theory?
  9. How do we know that other planets in our solar system are tectonically active?
  10. Do any other planets have the same style of tectonic active as Earth, dominated by the presence of plates?
  11. If you took the materials that make up Earth and placed them somewhere else in the solar system, do you think plates would form??

Conceptual flow

  1. TBA

Other definitions for comparison

  1. "the theory that explains how large pieces of the lithosphere, called plates, move and change shape"
    - Earth Science, Holt
    Is the focus of plate tectonics on understanding the plates themselves, or the landforms, hazards and the specific history of change that we find at the surface?

  2. "Movements of the Earth's crustal plates, which result in changes in the position, size, and shape of continents and oceans."
    Galileo Journey to Jupiter, NASA
    Are plates mad up of "crust"?

  3. "... students should know about plate tectonics as a driving force that shapes Earth's surface."
    Usage of the term in Science Framework, Standard Set 3. Dynamic Earth Processes
    Is plate tectonics a "force"?

  4. "The theory that the Earth's lithosphere consists of large, rigid plates that move horizontally in response to the flow of the asthenosphere beneath them, and that interactions among the plates at their borders cause most major geologic activity, including the creation of oceans, continents, mountains, volcanoes, and earthquakes."
    Houghton Mifflin College Division Online Study Center
    Is the motion of the Earth's plates entirely horizontal?

To Try

  1. To compare several definitions of any word that interests you -
    Type "define" and "the word" in a Google search, i.e. "define plate tectonics". Choose on "Web definitions for Plate Tectonics" to see several different definitions for comparison.

Related links

    This Dynamic Earth: The Story of Plate Tectonics
    Equivalent map showing the topography of Mars.
    by Kious and Tilling, U.S. Geological Survey

© 2007-2008 Earthguide at Scripps Institution of Oceanography.
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