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# 5.9: Divergent Plate Boundaries

Difficulty Level: At Grade Created by: CK-12
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Practice Divergent Plate Boundaries

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#### What can we see in Western North America?

When we got off the Atlantis in Iceland a new batch of scientists got on for a different scientific investigation. We're now going to fly to western North America to see a different set of plate tectonic features. Western North America has all three of the different types of plate boundaries and the features that are seen at them.

### Tectonic Features of Western North America

We're on a new trip now. We will start in Mexico, in the region surrounding the Gulf of California, where a divergent plate boundary is rifting Baja California and mainland Mexico apart. Then we will move up into California, where plates on both sides of a transform boundary are sliding past each other. Finally we’ll end up off of the Pacific Northwest, where a divergent plate boundary is very near a subduction zone just offshore.

This map shows the three major plate boundaries in or near California.

In the Figure above a red bar where seafloor spreading is taking place. A long black line is a transform fault and a black line with hatch marks is a trench where subduction is taking place. Notice how one type of plate boundary transitions into another.

### Plate Divergence on Land

A divergent plate boundary on land rips apart continents (Figure below).

When plate divergence occurs on land, the continental crust rifts, or splits. This effectively creates a new ocean basin as the pieces of the continent move apart.

In continental rifting, magma rises beneath the continent, causing it to become thinner, break, and ultimately split apart. New ocean crust erupts in the void, ultimately creating an ocean between continents. On either side of the ocean are now two different lithospheric plates. This is how continents split apart.

These features are well displayed in the East African Rift, where rifting has begun, and in the Red Sea, where water is filling up the basin created by seafloor spreading. The Atlantic Ocean is the final stage, where rifting is now separating two plates of oceanic crust.

#### Baja California

Baja California is rifting apart from mainland Mexico, as seen in this satellite image.

Baja California is a state in Mexico just south of California. In the Figure above, Baja California is the long, skinny land mass on the left. You can see that the Pacific Ocean is growing in between Baja California and mainland Mexico. This body of water is called the Gulf of California or, more romantically, the Sea of Cortez. Baja is on the Pacific Plate and the rest of Mexico is on the North American Plate. Extension is causing the two plates to move apart and will eventually break Baja and the westernmost part of California off of North America. The Gulf of California will expand into a larger sea.

Volcanism in Baja California is evidence of rifting.

Rifting has caused volcanic activity on the Baja California peninsula as seen in the Figure above.

Can you relate what is happening at this plate boundary to what happened when Pangaea broke apart?

### Summary

• Where continental rifting takes place, continents are split apart and an ocean may grow or be created between the two new plates.
• Baja California is rifting apart from mainland Mexico.
• Continental rifting can create major ocean basins, like the Atlantic.

### Review

1. How is a divergent plate boundary on land different from one in the ocean?
2. What is happening to the Baja California peninsula?
3. How did continental rifting play into the breakup of Pangaea?

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### Vocabulary Language: English Spanish

TermDefinition
continental rifting A divergent plate boundary that breaks up a continent.

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