Exercise 5Along diverging boundaries one plate moves directly away from another. Such
boundaries commonly are marked by mid-ocean ridges and a rift valley, and are sites
where new ocean crust and lithosphere are forming. Beneath rift valleys along divergent
boundaries hot asthenosphere has risen adiabatically i.e., without loss of heat) to
shallow levels. At such shallow levels it melts to form a magma chamber of crystal
mush, i.e., a chamber composed of crystal and silicate liquid (magma). Magma is fed
from the chamber through a series of dikes (transitory vertical cracks) to submarine
pillow lava erupting on the sea floor. Some of the magma in the dikes never makes it to
the sea floor and freezes in place, while some of the crystal mush and silicate liquid in the
magma chamber crystallizes to form the rock gabbro. Hence, oceanic crust
characteristically is made up of from seabed down to the Moho of marine sediments,
pillow lava, sheeted dikes, and then gabbro. When such a section is preserved in the
ancient rock record (1.e., in older rocks) it is called an ophiolite. In the immediate area of
the rift valley, oceanic crust directly overlies the asthenosphere, but further away from
the axis of the mid-ocean ridge a section of lithospheric mantle progressively enlarges
until the lithosphere reaches its normal -100 km thickness.
Question 9
On Figure 6, a cross section of a mid-ocean ridge, identify and label the layers
corresponding to marine sediments, pillow lava, sheeted dikes, and gabbro. Also label
the Moho, rift valley, and magma chamber containing crystal mush.
Two prominent mid-ocean ridge systems that you should be aware of are the Mid-
Atlantic Ridge, and the East Pacific Rise. The Mid-Atlantic Ridge separates North
America and South America from Africa and Europe while the East Pacific Rise extends
through the Gulf of California and forms the western boundary of the Nazca and Cocos
microplates.
Question 10
On Figure 4, which cross section (vertical slice of Earth’s interior) C-C’, A-A’, or
B-B’would look like Figure 6?
On Figure 4, locate and label the Mid-Atlantic Ridge and the East Pacific Rise.
Exercise 6
A fault is a surface across which Earth material has lost cohesion and across
which there has been perceptible displacement. Transform faults or conservative plate
boundaries are areas where lithospheric plates are sliding laterally past each other.
Ideally, in such a setting there can not be a component of convergence or divergence.
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Figure 6. Idealized cross section of a mid-oceanic ridge.
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juan de fuca plate
Eurasian plate
North American plate
B
philippine plate
Pacific plate
caribbean plate
arabian plate
cocos plate
African
South America plate
December 26, 2004
Nazca plate
Indian australian
scotia plate
Antarctic
– Convergent
Divergent
Transform
— Diffuse
SOURCE: http://wwwneic.cr.usgs.gov/neis/plate_tectonics/plates.html
Figure 4. Map showing the locations of different types of plate boundaries.
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Figure 6. Idealized cross section of a mid-oceanic ridge.
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