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Guide
to Reading 
This chapter, concerning mountains and the geologic
reasons they exist, offers a change of pace from the drama and danger
of earthquakes and volcanoes in the preceding two chapters. Mountains
are certainly not insignificant structures, geologically or aesthetically,
but their story is majestic rather than wildly exciting.
Before you begin, you’ll find it helpful to
review the following terms introduced in
| Chapter 8: |
|
| fault |
normal faults |
| stress |
reverse faults |
| footwall |
shear stress |
| strike-slip faults |
strain |
| hanging wall |
thrust faults |
The chapter begins by explaining that with a few
rare exceptions (some volcanic mountains that appeared almost overnight),
a mountain-building event, an orogeny, goes on for tens of millions
of years. An orogeny produces not just uplifted areas of land but
also highly deformed rock layers and unique mountain structures.
Many of the orogenic processes are reviewed for you. Once again you
read about stress (compressional, tensional, and shear), strain,
brittle and ductile deformation, joints, folds, and faults (normal,
reverse, thrust, strike-slip, footwalls, and hanging walls). You
are presented with more details about these topics than before:
- orientation of these geologic structures (strike,
dip, bearing, and plunge)
- fault classification (dip-slip, right-lateral,
and left-lateral strike-slip)
- details of the fault zone (fault breccia, fault
gouge, slickensides, and slip lineations
- types of folds (hinge, limb, axial plane, anticline,
syncline, monocline, plunging and nonplunging folds, domes, and
basins)
- formation of folds (buckle)
The very rocks making up an area may be changed by
an orogeny. Tectonic foliation may occur to existing rocks, or totally
new igneous, sedimentary, and metamorphic rocks may appear.
Once the background of processes and rock types has
been established, the author looks at the mountain itself. Why does
it stick up above the surrounding crustal surface? This brings up
a consideration of crustal roots, isostasy, isostatic equilibrium,
and isostatic compensation.
Even mountains don’t last forever. The chapter
continues with erosion issues including agents of erosion (water
and ice) and climate influences.
Why are mountains located where they are? Wouldn’t
you know it, plate tectonics again! There may be a new term or two
introduced here, like accreted terranes, fold-thrust belts, and fault-block
mountains, but the concepts are all old acquaintances (subduction,
convergent plate boundaries, and continental rifting).
The chapter draws to a close with a few new terms
for some continental areas (shields, cratons, and cratonic platforms)
and the information that dome and basin formation are less exuberant
processes of land uplift than are orogenic events.
By the time you’ve finished this chapter,
you very likely will have a different concept of “mountains”
than when you started. Your new understanding will be that all major
mountain ranges are the result of multiple and complex orogenies
over long geologic time. Majestic they may be, but they are neither
ageless nor unchanging. They are destined to wear down and possibly
be uplifted, time and time again.
And speaking of time, that’s what the next
chapter is all about, geologic time.
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Key Terms
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