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Transform faults are prominent in ocean basins where they appear to offset ridge axes.
Geologists used to think that transform faults like the one depicted in Figure 7 offset
ridge axes. However, as we will see shortly, this is not the case.
In Figure 7 hot asthenosphere rises beneath the ridge axis. Some of it melts and is
fed through dikes to pillow lava flows at the seabed. However some of the upwelling
asthenosphere cools and begins flow laterally away from the axis of the ridge. As it
does so it carries or drags the overlying lithosphere along with it. Hence, in Figure 7
plates I and II move away from plates III and IV, as new crust forms along the ridge axis
in a process called sea floor spreading.
Question 11
Note that line segment X-Y lies across the transform fault. As sea floor spreading
continues will it be offset?
How about line segment R-S?
Now note the position of line segment T-U. As sea floor spreading continues will
it be offset?
What is the significance of points A and B in Figure 7 and in general?
The San Andreas is a transform fault that connects the East Pacific Rise in the
Gulf of California to a ridge segment lying offshore of northern California. On Figure 4,
please locate and label this feature.
Exercise 7
The connection between diverging, transform, and converging plate margins is
depicted in Figure 8. Note that along converging plate margins, the over riding plate may
be either oceanic or continental.
The dominant but not only process that drives the motions of plates is convection
within the underlying mantle. This process begins with hot mantle upwelling along mid-
ocean ridges. The welled up mantle cools and spreads laterally. At convergent margins
it has cooled enough to begin sinking. As it sinks, it heats back up and eventually flows
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transform
fault
mid-ocean
ridge
oceanic
crust
B
mantle lithosphere
U
mantle lithosphere
upwelling
asthenosphere
Figure 7. Block diagram of a transform fault offsetting a mid-ocean ridge.
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