divergent plate boundaries in the oceans

plate divergence in the ocean

Iceland provides us with a fabulous view of a mid-ocean ridge above sea level (Figure 1.1) 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. 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. Thats true - its all basalt and it even all has the same magnetic polarity. The rocks on both sides are extremely young. Whats 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. Click image to the left or use the URL below. URL:

instructional diagrams

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the seafloor is youngest in or near the rift valley.

-->  a) true

b) false

what happens at a mid-ocean ridge?

a) two plates slide past each other.

b) two plates push into each other.

-->  c) two plates move apart.

d) two plates collide.

the mid-ocean ridge can only be found below sea level.

a) true

-->  b) false

at mid-ocean ridges, a_ magma cools _ at the surface to form the rock ____ .

-->  a) basalt; quickly; basalt

b) gabbro; quickly; gabbro

c) gabbro; slowly; basalt

d) basalt; slowly; gabbro

the rocks on both sides of the mid-ocean ridge do not have the same magnetic polarity.

a) true

-->  b) false

at mid-ocean ridges, beneath the surface layer, a _ magma cools _ to form the rock ____.

a) basalt; quickly; basalt

b) gabbro; quickly; gabbro

c) gabbro; slowly; basalt

-->  d) basalt; slowly; gabbro

all of the rocks at divergent plate boundaries are sedimentary.

a) true

-->  b) false

if you walk across the mid-atlantic ridge in iceland you are traveling between

a) rocks of two different magnetic polarities.

b) rocks of two different ages.

-->  c) two lithospheric plates.

d) all of these.

assuming the leif the lucky bridge is infinitely strong, over a very long period of time, what will happen to it?

a) it will be compressed and fall down.

-->  b) it will split apart and fall down.

c) it will remain in place.

d) it will shear into two pieces.

diagram questions

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