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 the Earth throughout its geologic history. Today, science has shown that the surface of the 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 the 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 super-heated 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, the 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.

  1. Given the limited technology and data set available to Marie Tharp and Bruce Heezen in the 1950s-1970s, what could be potential limitations of the Tharp-Heezen map? 


    • Answer

      Potential limitations or inaccuracies in the 1977 Tharp-Heezen map could include the following: general locations of geologic features were accurate but specific landscapes were not and could have been misrepresented through artistic interpretation; the map was vertically exaggerated to make features visible; gentle slopes may have been overlooked or underestimated; inaccuracy due to merging maps and data of different scales and precision, often collected by different organizations.

  2. At which type of tectonic boundary is oceanic crust created? At which type of tectonic boundary is oceanic crust destroyed? Provide at least one example where each type of boundary is found on the Earth. 


    • Answer

      Oceanic crust is created at divergent boundaries, such as the mid-ocean ridge. Oceanic crust is destroyed at convergent boundaries where subduction results in a trench, such as the Mariana Trench or Cayman Trough.]

  3. If an oceanic and continental plate converged, which one (if any) would subduct?


    • Answer

      If an oceanic and continental plate converged, the denser oceanic plate would subduct under the continental plate.

  • 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 (1 inch) per year. This is similar to the rate at which fingernails grow. The fastest known rate of plate movement, 15 centimeters (6 inches) per year, occurs on the East Pacific Rise in the South Pacific.


  • 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 (4-18 miles) wide. 


  • The highest mountain range above sea level, the Himalayas, 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 2 centimeters (≈1 inch) each year.



layer in Earth's mantle between the lithosphere (above) and the upper mantle (below).

continental plate

tectonic plate found beneath continents.

convergent plate boundary

area where two or more tectonic plates bump into each other. Also called a collision zone.

divergent boundary

area where two or more tectonic plates are moving away from each other. Also called an extensional boundary.


intensely hot region deep within the Earth that rises to just underneath the surface. Some hot spots produce volcanoes.


outer, solid portion of the Earth. Also called the geosphere.


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

mid-ocean ridge

underwater mountain range.


having to do with the ocean.


movement and interaction of the Earth's plates.


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


method of determining the presence and location of an object using sound waves (echolocation).


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


This material is based in part upon work supported by the National Science Foundation under Grant No. DRL-1114251. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation.