Plate Tectonics

Plate Tectonics

In 1977, after decades of tediously collecting and mapping ocean sonar data, scientists began to see a fairly accurate picture of the seafloor emerge. The Tharp-Heezen map illustrated the geological features that characterize the seafloor and became a crucial factor in the acceptance of the theories of plate tectonics and continental drift. Today, these theories serve as the foundation upon which we understand the geologic processes that shape Earth.


5 - 12


Earth Science, Geology, Oceanography

National Geographic Television and Film

In much the same way that geographic borders have separated, collided, and been redrawn throughout human history, tectonic plate boundaries have diverged, converged, and reshaped Earth throughout its geologic history. Today, science has shown that the surface of Earth is in a constant state of change. We are able to observe and measure mountains rising and eroding, oceans expanding and shrinking, volcanoes erupting and earthquakes striking.

Before the Tharp-Heezen map of the seafloor was published in 1977, scientists had little understanding of the geological features that characterized the seafloor, especially on a global scale. The data and observations represented by the Tharp-Heezen map became crucial factors in the acceptance of the theories of plate tectonics and continental drift. The theory of plate tectonics states that Earth’s solid outer crust, the lithosphere, is separated into plates that move over the asthenosphere, the molten upper portion of the mantle. Oceanic and continental plates come together, spread apart, and interact at boundaries all over the planet.

Each type of plate boundary generates distinct geologic processes and landforms. At divergent boundaries, plates separate, forming a narrow rift valley. Here, geysers spurt superheated water, and magma, or molten rock, rises from the mantle and solidifies into basalt, forming new crust. Thus, at divergent boundaries, oceanic crust is created. The mid-ocean ridge, Earth’s longest mountain range, is a 65,000 kilometers (40,390 miles) long and 1,500 kilometers (932 miles) wide divergent boundary. In Iceland, one of the most geologically active locations on Earth, the divergence of the North American and Eurasian plates along the Mid-Atlantic Ridge can be observed as the ridge rises above sea level.

At convergent boundaries, plates collide with one another. The collision buckles the edge of one or both plates, creating a mountain range or subducting one of the plates under the other, creating a deep seafloor trench. At convergent boundaries, continental crust is created and oceanic crust is destroyed as it subducts, melts, and becomes magma. Convergent plate movement also creates earthquakes and often forms chains of volcanoes. The highest mountain range above sea level, the Himalayas, was formed 55 million years ago when the Eurasian and Indo-Australian continental plates converged. The Mediterranean island of Cyprus formed at a convergent boundary between the African and Eurasian plates. Hardened mounds of lava, called pillow lavas, were once on the bottom of the ocean where this convergence occurred, but have been pushed up and are now visible at the surface.

Fast Fact

Scientists are able to calculate average rates of tectonic plate movement for a given time period. These rates of movement range widely. For example, the rate of spreading at the Mid-Atlantic Ridge near Iceland is relatively slow, about 2.5 centimeters (one inch) per year. This is similar to the rate at which fingernails grow. The fastest known rate of plate movement, 15 centimeters (six inches) per year, occurs on the East Pacific Rise in the South Pacific.

Fast Fact

The Mid-Atlantic Ridge runs down the center of the Atlantic Ocean. Along its crest, the ridge has a deep rift valley that, on average, is similar to the depth and width of the Grand Canyon: 1 to 3 kilometers (0.6 -1.8 miles) deep and 6.5 to 29 kilometers (four to 18 miles) wide.

Fast Fact

The highest mountain range above sea level, the Himalaya, was formed 55 million years ago when the Eurasian and Indo-Australian continental plates converged. Due to ongoing convergence, the Himalayas, including Mount Everest, continue to rise by approximately two centimeters (0.8 inch) each year.

Media Credits

The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.

Angela M. Cowan, Education Specialist and Curriculum Designer
Julie Brown, National Geographic Society
Jeannie Evers, Emdash Editing, Emdash Editing
World Ocean Floor Panorama, Bruce C. Heezen and Marie Tharp, 1977. Copyright by Marie Tharp 1977/2003. Reproduced by permission of Marie Tharp Maps, LLC 8 Edward Street, Sparkill, New York 10976
Production Assistant
Winn Brewer, National Geographic Education
Stock Footage Provider
Pillow lava sequence provided by Moonlight Productions, Dr. Lee Tepley, Moonlight Productions
Katy Andres
Julie Brown, National Geographic Society
Alison Michel, National Geographic Society
Last Updated

April 26, 2024

For information on user permissions, please read our Terms of Service. If you have questions about how to cite anything on our website in your project or classroom presentation, please contact your teacher. They will best know the preferred format. When you reach out to them, you will need the page title, URL, and the date you accessed the resource.


If a media asset is downloadable, a download button appears in the corner of the media viewer. If no button appears, you cannot download or save the media.


Text on this page is printable and can be used according to our Terms of Service.


Any interactives on this page can only be played while you are visiting our website. You cannot download interactives.

Related Resources

National Science Foundation