|
|
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 10:
- brittle deformation
- reverse faults
- compression (compressive stress)
- shear stress
- ductile deformation
- strain
- fault
- stress
- footwall
- strike-slip faults
- hanging wall
- tension (tensile stress)
- normal faults
- 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, pressure, 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)
- 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, slip lineations, mylonites, and shear zones)
- fault systems (detachment faults, grabens, half grabens, and horsts)
- types of folds and their component parts (drag fold, hinge, limb, axial plane, anticline, syncline, monocline, tight fold, open fold, 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, Archimedes’ principle, isostasy, isostatic equilibrium, and isostatic compensation.
Even mountains don’t last forever. The chapter continues with:
- erosion issues; agents of erosion (water and ice), and climate influences
- features created by erosion, like cuestas and hogbacks
- lateral spread, called orogenic collapse
- exposure of a mountain’s innards (exhumation)
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 accretionary orogens 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.
The last chapter topic is the life story of one particular mountain range, the Appalachian Mountains of North America. Read it carefully to make sure you understand the narrative, but don’t overwhelm yourself by trying to memorize this particular sequence of events. Do of course check with your instructor, but unless you live right in these mountains, it’s unlikely you’ll be expected to recite the history of the Appalachians. Instead, what you should understand is that there’s no such thing as a really simple explanation of the formation of any particular mountains. All major mountain ranges are the result of multiple orogenies over long geologic time spans.
And speaking of time, that’s what the next chapter is all about: geologic time. |
