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5.8: Divergent Plate Boundaries in the Oceans

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How could you walk between two plates?

On a bridge! Let’s get off the Atlantis in Iceland. It's good to feel solid ground beneath our feet again! While in Iceland we'll take a walk on Leif the Lucky Bridge. Why did we sail across the ocean for this? Iceland is one place where a mid-ocean ridge is found above sea level.

Plate Divergence in the Ocean

Iceland provides us with a fabulous view of a mid-ocean ridge above sea level ( Figure below ) As you can see, where plates diverge at a mid-ocean ridge is a rift valley that marks the boundary between the two plates. Basalt lava erupts into that rift valley and forms new seafloor. Seafloor on one side of the rift is part of one plate and seafloor on the other side is part of another plate.

Iceland is the one location where the ridge is located on land: the Mid-Atlantic Ridge separates the North American and Eurasian plates

Leif the Lucky Bridge straddles the divergent plate boundary. Look back at the photo at the top. You may think that the rock on the left side of the valley looks pretty much like the rock on the right side. That’s true – it’s all basalt and it even all has the same magnetic polarity. The rocks on both sides are extremely young. What’s different is that the rock one side of the bridge is the youngest rock of the North American Plate while the rock on the other side is the youngest rock on the Eurasian plate.

This is a block diagram of a divergent plate boundary. Remember that most of these are on the seafloor and only in Iceland do we get such a good view of a divergent plate boundary in the ocean.

Convection Cells at Divergent Plate Boundaries

Remember that the mid-ocean ridge is where hot mantle material upwells in a convection cell. The upwelling mantle melts due to pressure release to form lava. Lava flows at the surface cool rapidly to become basalt, but deeper in the crust, magma cools more slowly to form gabbro. The entire ridge system is made up of igneous rock that is either extrusive or intrusive. The seafloor is also igneous rock with some sediment that has fallen onto it.

Earthquakes are common at mid-ocean ridges since the movement of magma and oceanic crust results in crustal shaking.

USGS animation of divergent plate boundary at mid-ocean ridge: http://earthquake.usgs.gov/learn/animations/animation.php?flash_title=Divergent+Boundary&flash_file=divergent&flash_width=500&flash_height=200 .

Divergent plate boundary animation: http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/11/AOTM_09_01_Divergent_480.mov .

Summary

  • Oceanic plates diverge at mid-ocean ridges. New seafloor is created in the rift valley between the two plates.
  • Lava cools to form basalt at the top of the seafloor. Deeper in the crust the magma cools more slowly to form gabbro.
  • Iceland is a location where we can see a mid-ocean ridge above sea level.

Making Connections

Practice

Use this resource to answer the questions that follow.

1. What causes divergence?

2. How is new crust created?

3. What erupts on the ocean floor?

4. How fast does divergence occur?

5. What is formed at a divergent boundary?

Review

1. What is the direction of plate motion at a divergent plate boundary?

2. Describe the relationship between the convection cell and volcanism at the mid-ocean ridge.

3. Why is the Leif the Lucky bridge so interesting?

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Difficulty Level:

At Grade

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Date Created:

Feb 24, 2012

Last Modified:

Aug 25, 2014
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