6.9 Divergent Plate Boundaries

What can we see in Western North America?

Many locations in western North America are being rifted apart. One such location is the Rio Grande Rift in Texas. At this location, hot mantle rises up beneath the crust to cause the divergence.

Tectonic Features of Western North America

Western North America has all three of the different types of plate boundaries and the features that are seen at them. A divergent plate boundary runs through the Gulf of California in Mexico. The divergence is pulling Baja California and mainland Mexico apart. Just north of there, around the US-Mexico border, in California, there is a transform boundary. Plates on both sides of the plate boundary are sliding past each other. Still further north, in the Pacific Northwest, a subduction zone just offshore is part of a convergent plate boundary.

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).

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 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.

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?