We All Have Boundaries

When two plates are sliding by each other, we call that a transform boundary. Can you think of an example of a well-known transform boundary? You probably have heard of 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 video example of a sliding boundary or you can make a model! Here are some associated resources for the model. This is what the completed model should look like.

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.

Try It!

Make the model! Print the following parts of the model:

• Part 1
• Part 2
• Part 3

This is what the completed model should look like.

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? The answer is density!

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 more dense than the other, it sinks beneath and a subduction boundary can form. Oceanic crust is usually more dense 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.

Try It!

You can make a model! You will need both the Top and bottom parts of the model. If you need help, this is what the completed model should look like.