Chapter 9: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building
Study Plan
The Swiss Alps, in which this idyllic meadow lies nestled, rose when Italy collided with Europe, and they were carved by glaciers during the last ice age.
Credit: Stephen Marshak
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.
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 outlined for you. You will read about stress (compressional, tensional, and shear), strain, pressure, brittle and ductile deformation, joints, folds, and faults (normal, reverse, thrust, strike-slip, footwalls, and hanging walls). You are presented with details about these topics:
- orientation of these geologic structures (strike, dip, bearing, and plunge)
- joint sets, systematic and nonsystematic joints, and veins
- fault classification (dip-slip, right-lateral and left-lateral strike-slip, and oblique-slip faults)
- details of the fault zone (displacement or offset, fault scarps, fault breccia, fault gouge, slickensides, and slip lineations)
- types of folds and their component parts (hinge, limb, axial plane, anticline, syncline, monocline, plunging and non-plunging folds, domes, and basins)
- formation of folds (flex, flow, and buckle)
The very rocks making up an area may be changed by an orogeny. Tectonic foliation (layering due to alignment of deformed and/or reoriented grains) may occur in 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, buoyancy force, isostasy, isostatic equilibrium, and isostatic compensation.
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 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 (epeirogeny) are less-exuberant processes of land uplift than are orogenic events. And speaking of time, that's what the next chapter is all about: geologic time.
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