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We All Have Boundaries

When two plates meet, there are basically three ways they can move relative to each other: They can slide past each other, they can move away from each other, or they can move towards each other. It can get much more complex than that, but let's keep it simple for now.
When two plates are sliding by each other, we call it a transform boundary. An example of a well-known transform boundary is the San Andreas Fault. This fault, along with other transform boundaries, causes many earthquakes. The Los Angeles side is sliding north relative to the San Francisco side. At the current speed, the two cities will be next to each other in only 10 million years! There is a videoexample of a sliding boundary, or you can make a model!


There is another type of boundary where the plates are moving away from each other. These are called divergent boundaries. These types of boundaries are very common underneath oceans. New crust is constantly being created in the openings at the boundary where the plates are separating. Iceland has several active volcanoes and is growing all the time: It is part of a divergent boundary that has grown above the surface of the ocean. There is a video example of a divergent boundary.


When two plates are moving toward each other, there are two possible outcomes. Both plates can collide and rise at an uplift boundary and create mountains; or one plate will sink below the other plate and cause a subduction type boundary.

Why is there a difference? Why don't all colliding plates rise to make mountains? What is different about each plate to affect what happens? Do you have a guess?


When the two plates have the same density (usually continental crust), they both uplift and mountains can be formed. The Himalayan range in Tibet, including Mount Everest, was formed from two colliding plates. Mount Everest is the highest mountain in the world, and it is still growing! The peak gets about one cm taller each year - that is one foot every 30 years. Here is a short video showing an example of an uplift boundary.

If one plate is denser than the other, it sinks beneath, and a subduction boundary can form. Oceanic crust is usually denser than continental crust, so these boundaries are common on the edges of continents. A lot can happen when subduction occurs. Earthquakes are very common, a trench is usually formed off shore, and volcanoes can form inland from the boundary. The coast of Oregon and Washington is a subduction zone, and all three of the above can be found there. Here is a short video example of subduction.

Are there other types of plate movement and boundaries. Of course there are. If you would like to explore those, here are some resources to make 7 different fault type models (normal, reverse, strike-slip, oblique, etc.).

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Updated October 24, 2008 by: Glen Westbroek

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