16 Applications: Plate Tectonics

Elizabeth Johnson

Learning Objectives

Students will be able to:

  • Describe the changes in the elevation and slope of the surface of Earth across different types of plate boundaries and continental or oceanic crust.
  • Determine the type of plate boundary based upon a cross-section of topography and bathymetry.


Plate Boundaries

The tectonic plates are pieces of the lithosphere that move relative to each other over the surface of Earth.  The lithosphere includes continental crust, oceanic crust, and the uppermost part of the mantle:

Figure 20. The oceanic crust, continental crust, and lithospheric mantle make up the tectonic plates. Note the elevation of the continental crust is higher than oceanic crust. Steven Earle. CC BY.

There are three main types of boundaries between plates: divergent, convergent, and transform boundaries.

Divergent Boundaries

Divergent plate boundaries are where two plates move apart from each other.  Divergent boundaries can occur in the ocean, at mid-ocean ridges, or on the continents where they form continental rift zones.  Continental rifts form valleys as the plates thin and split at the boundary.  Mid-ocean ridges are volcanoes, and the hot new oceanic crust formed from magma at the plate boundary is expanded from the heat.  Divergent boundaries in the oceans look like enormous symmetric mountain ranges.

Figure 21. Two types of divergent plate boundaries: continental rifts and mid-ocean ridges. Image modified from Hannes Grobe, Alfred Wegener Institute for Polar and Marine Research, vectorised by User:Lichtspielderivative work: Lichtspiel (Ocean-birth_hg.png) [CC BY-SA 2.5], via Wikimedia Commons.
Figure 22. Gulf of Aden, an example of a rift. Plate boundary marked in red. Images created using GeoMapApp (http://www.geomapapp.org/).
Figure 23. The Mid-Atlantic Ridge, an example of a mid-ocean ridge. Plate boundary marked in red. Images created using GeoMapApp (http://www.geomapapp.org/).

COnvergent Boundaries

Convergent plate boundaries are where two plates are moving towards each other.  When one plate is pushed under another at a convergent boundary, subduction occurs.  The subducted plate pulls down the ocean floor where it is sinking into the Earth, forming a trench or linear depression on the ocean floor.  The difference in elevation between continental and oceanic crust can clearly be seen in ocean-continent subduction zones.  Where two continents collide, large mountain belts form around the plate boundary.

Figure 24. Types of convergent plate boundaries, Images compiled from https://opentextbc.ca/physicalgeology2ed/chapter/10-4-plate-plate-motions-and-plate-boundary-processes/, Steven Earle. CC BY.
Figure 25. The Alaskan arc, an example of continent-ocean subduction. Plate boundary marked in red. Images created using GeoMapApp (http://www.geomapapp.org/).
Figure 26. India and the Himalayan mountains, an example of continent-continent collision. Plate boundary marked in red. Images created using GeoMapApp (http://www.geomapapp.org/).

Transform Boundaries

Transform boundaries are where two plates move alongside each other.  Because the plates are moving alongside each other, theoretically there should be no change in topography from one plat to another, because the plates are neither pulling apart nor colliding at this type of boundary.  In reality, though, transform boundaries are usually not perfect, meaning they are a combination of transform and convergent boundaries, or of transform and divergent boundaries.  So, there can be some changes in topography on either side of a transform boundary, although it is not as large as changes in topography across other types of boundaries.

Figure 27. Transform boundaries. The San Andreas Fault extends from the north end of the East Pacific Rise in the Gulf of California to the southern end of the Juan de Fuca Ridge. All of the red lines on this map are transform faults. From https://opentextbc.ca/physicalgeology2ed/chapter/10-4-plate-plate-motions-and-plate-boundary-processes/, Steven Earle, CC-BY.
Figure 28. California near San Francisco, an example of a transform boundary. Plate boundary marked in red. Images created using GeoMapApp (http://www.geomapapp.org/).

Which Type of Boundary Is It?

Figure 29. Map and topographic profile of the western coast of South America. Images created using GeoMapApp (http://www.geomapapp.org/).


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