When plate tectonics causes part of the Earth's crust to compress, as in a zone of collision, or to extend, as in a zone of rifting, faults must form to help accomplish this task. While the deeper layers of rock, due to extreme heat and pressure, can fairly easily deform to accommodate the stretching of rifting or the "squishing together" of collision, the brittle rock of the uppermost crust must break and "slide" along faults to achieve this kind of motion.
The nature of the faults and the type of motion are thus connected. Some faults allow lengthening of the crust, and others allow shortening. On the animation page of this activity, animations of the two senses of dip slip are shown above a bar that represents the length of the original cross-section before faulting occurs (read the warning below first!). Watch these animations and notice the changes in length that take place as faulting progresses.
[Warning: The animations are shown from an angle, but the bar represents the length of only the front face of these blocks, so do not confuse the apparent length of the block (front face + angled side) with the actual length.]
Imagine drilling straight down through each of these
cross sections, very near the surface trace of each fault, and that each
animated block represents the Earth's crust in a particular area. Assuming
the crust is exactly the same thickness everywhere (except where
it has been faulted), and that you can drill deep enough to reach
the bottom of the crust, which section could you drill all the way
through more easily? If you need help here, take another look at the
animation page, this time watching
how crustal thickness near the fault changes.
Would changing the dip of these faults make any
difference in how effectively they lengthen or shorten the crust?
If so, what is the relation between the angle of dip and the
efficiency of the lengthening or shortening? If you change the
angle to increase the change in length, what happens to the
amount of change in crustal thickness for an equivalent amount of
slip along the fault? And, given such a change in dip angle, what
happens to the size of the area
over which the crustal thickness is altered?
Based on your answers above, if you were looking at
dip-slip faults in an area experiencing strong tectonic compression
or rifting, would you expect them to be high-angle (dip steep to vertical)
or low-angle (dip of 45 degrees or less) faults?
We have stressed before that tectonics, volcanism, and
earthquakes are all related. This activity actually demonstrates a
direct link among all three. If volcanism can be thought of simply
as material breaking through the crust from the mantle beneath,
then which of the tectonic environments represented by the animations
seems to make it easier
for volcanism to take place, and why? Look back at the second question
(and the animations page) if
you need a hint.
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