M ountains form some of the most majestic landscapes
M ountains form some of the most majestic landscapes
on Earth. People find peace in the high mountain air and quiet valleys. But mountains also project nature’s power. Storms swirl among the silent peaks that were lifted skyward millions of years ago by tectonic processes. Rocks have been folded as if squeezed by a giant’s hand. In the first portion of this chapter, we will study how rocks be- have when tectonic forces stress them. In the second por- tion, we will learn how tectonic forces raise mountains.
Tectonic forces contorted these once-horizontal sedimentary rocks near Carlin, Nevada, into tight folds. (David Matherly/Visuals Unlimited)
200 CHAPTER 12 G E O L O G I C S T RU C T U R E S , M O U N TA I N R A N G E S , A N D C O N T I N E N T S
Geologic Structures
Confining pressure Confining pressure is 䊳
ROCK DEFORMATION
12.1 equal from all sides, Original dimension
reduces volume
of rock
Recall from Chapter 10 that stress is a force exerted against an object. Tectonic forces exert different types of
(a)
stress on rocks in different geologic environments. The
Tectonic compression
first,called confining stress or confining pressure,oc- curs when rock or sediment is buried (Fig. 12–1a).
Tectonic compression deforms rock and
Confining pressure merely compresses rocks but does
shortens the
not distort them,because the compressive force acts
distance between
equally in all directions,like water pressure on a fish. As
two points
you learned in Chapter 7,burial pressure compacts sed- iment and is one step in the lithification of sedimentary rocks. Confining pressure also contributes to metamor-
(b)
phism during deep burial in sedimentary basins.
Extensional stress
In contrast, directed stress acts most strongly in one
Extensional stress
direction. Tectonic processes create three types of di-
fractures rock and
rected stress. Compression squeezes rocks together in
lengthens the distance between
one direction. It frequently acts horizontally,shortening
two points
the distance parallel to the squeezing direction (Fig. 12–1b). Compressive stress is common in convergent plate boundaries,where two plates converge and the rock crumples,just as car fenders crumple during a head-on collision. Extensional stress (often called tensional
(c)
stress) pulls rock apart and is the opposite of tectonic compression (Fig. 12–1c). Rocks at a divergent plate
Shear stress
boundary stretch and pull apart because they are subject
Acts in parallel
to extensional stress. Shear stress acts in parallel but
but opposite
opposite directions (Fig. 12–1d). Shearing deforms rock directions by causing one part of a rock mass to slide past the
other part,as in a transform fault or a transform plate boundary.
(d)
STRAIN Figure 12–1 (a) Confining pressure acts equally on all sides of a rock. Thus, the rock is compressed much as a balloon is
Strain is the deformation produced by stress. As ex- compressed if held under water. Rock volume decreases with- plained in Chapter 10,a rock responds to tectonic stress
out deformation. (b) Tectonic compression shortens the dis- by elastic deformation,plastic deformation,or brittle
tance parallel to the stress direction. Rocks fold or fracture to fracture. An elastically deformed rock springs back to its
accommodate the shortening. (c) Extensional stress lengthens original size and shape when the stress is removed.
the distance parallel to the stress direction. Rocks commonly During plastic deformation,a rock deforms like putty fracture to accommodate the stretching. (d) Shear stress de- forms the rock parallel to the stress direction.
and retains its new shape. In some cases a rock will de- form plastically and then fracture (Fig. 12–2).
Factors That Control Rock Behavior brittle fracture. In contrast,if you strike the gold
Several factors control whether a rock responds to stress nugget,it deforms in a plastic manner; it flattens by elastic or plastic deformation or fails by brittle fracture: and stays flat. If you hit the rubber ball,it deforms
1. The nature of the material. Think of a quartz crys- elastically and rebounds immediately,sending the tal,a gold nugget,and a rubber ball. If you strike
hammer flying back at you. Initially,all rocks react
Geologic Structures 201
A fold is a bend in rock. These are in quartzite Territories, Canada) folded plastically and then fractured.
Figure 12–2 This rock (in the Nahanni River, Northwest
Figure 12–3
in the Maria Mountains, California. (W. B. Hamilton, USGS)
surface,where temperature and pressure are low,
A geologic structure is any feature produced by rock different types of rocks behave differently with con-
deformation. Tectonic forces create three types of geo- tinuing stress. Granite and quartzite tend to behave
logic structures: folds,faults,and joints. in a brittle manner. Other rocks,such as shale,lime- stone,and marble,have greater tendencies to deform