Page 426 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

390 part III The Earth–Atmosphere Interface
Stress
Resulting strain
Surface expressions
Thinning crust
(a) Tension
(b) Compression
Stretching
Shortening
Normal fault
Folding
Bending horizontally
Reverse fault
(c) Shear
Shearing, twisting laterally
▲Figure 13.7 three kinds of stress and strain and the resulting surface expressions on Earth’s crust.
that affects an object, measured as force per unit area; note that these units are the same as for pressure (defined in Chapter 3). Three types of stress are important for crustal deformation: tension, which causes stretching; compression, which causes shortening; and shear, which causes twisting or tearing as objects slide parallel to one another (Figure 13.7).
Although stress is an important force in shaping Earth’s crust, the landforms we see result from strain, which is how rocks respond to stress. Strain is, by definition, a dimensionless measure of the amount of de- formation undergone by an object. Strain is the stretching, shortening, and twisting that results from stress and is expressed in rocks by folding (bending) or faulting (breaking). Whether a rock bends or breaks depends on several factors, including its composition and the amount of pressure it is undergoing. Figure 13.7 illustrates each type of stress and the resulting strain and surface expres- sions that develop.
Folding and Broad Warping
When rock strata that are layered horizontally are sub- jected to compressional forces, they become deformed
(Figure 13.8). Folding occurs when rocks are deformed as a result of compressional stress and shortening. We can visualize this process by stacking sections of thick fab- ric on a table and slowly pushing on opposite ends of the stack. The cloth layers will bend and rumple into folds similar to those shown in the landscape of Figure 13.8a, with some folds forming arches (upward folds) and some folds forming troughs (downward folds).
An arch-shaped upward fold is an anticline; the rock strata slope downward away from an imaginary centre axis that divides the fold into two parts. A trough-shaped downward fold is a syncline; the strata slope upward away from the centre axis. The erosion of a syncline may form a synclinal ridge, produced when the different rock strata offer different degrees of resistance to weathering processes (Figure 13.8b).
The hinge is the horizontal line that defines the part of the fold with the sharpest curvature. If the hinge is not horizontal, meaning that it is not “level” (parallel with Earth’s surface), the fold is plunging, or dipped down (inclined) at an angle. If the axial plane of the fold, an imaginary surface that parallels the hinge but descends downward through each layer, is inclined from vertical,
Strike-slip fault