Laboratory Manual for Introductory Geology 4e

FIGURE 7.3 Types of stress.
(a) Compressional stress. (b) Tensional stress. (c) Shear stress.
7.3.3 The Effect of Stress
The pressure rocks experience during burial is equal from all directions. During
plate-tectonic activity, however, the forces acting on rocks are greater in some directions
than in others, subjecting the rocks to stress. Depending on how the forces
are applied, the stress may be compressional (the rock is squeezed from two sides as
between the jaws of a vise), tensional (the rock is pulled apart), or shear (the rock is
stressed as if caught between the blades of a pair of scissors) (FIG. 7.3). Rocks at low
temperatures and pressures, close to the surface, respond to stress differently from
those at high temperatures and pressures, deep in the crust and mantle. At shallow
depths, rocks break, causing the faults that we will examine in Chapter 10. Under
stress at depth, rocks flow much like Silly Putty in what is called plastic deformation.
Stress can therefore change the shapes of grains in a protolith, like the flattened
metaconglomerate clasts in Figure 7.1a.iii. It can also change protolith grains from
random orientations to preferred orientations; thus, the flattened metaconglomerate
clasts in Figure 7.1a.iii are all flattened in the same direction. Preferred orientation
most commonly develops when new metamorphic minerals crystallize under stress,
producing foliation, in which platy minerals like mica flakes are aligned parallel to
one another, as in the muscovite in Figure 7.1c.i, or lineation, in which rod-shaped
crystals lie parallel to one another, as in the amphibolite in Figure 7.1c.ii.
7.3.4 The Effect of Hydrothermal Fluids
In many subsurface environments, rocks contain hot liquids, gases, and, at the highest
temperatures, fluids that have properties of both gases and liquids. These fluids
are composed mainly of H 2
O and CO 2
and are collectively called hydrothermal fluids
(from hydro, meaning water, and thermal, meaning hot). Some of these fluids are
given off from magma when it solidifies, others form when H 2
O or CO 2
is released
from protolith minerals as they react to form metamorphic minerals, and still others
are produced when groundwater percolates deep into the crust.
Hydrothermal fluids act as a catalyst, speeding up metamorphic reactions by
greatly increasing the diffusion of ions. They can carry large amounts of ions and
may change the basic chemistry of a protolith by either dissolving elements or
depositing them as they pass through the rock. The process of change in protolith
chemistry by interaction with hydrothermal fluids is called metasomatism.
7.4 Studying Metamorphic Rocks
The first step in studying a rock is to determine whether it is igneous, sedimentary,
or metamorphic. Each class of rock has specific properties by which it can be recognized,
but doing so may be more challenging with some metamorphic rocks because
they may preserve some of the original igneous or sedimentary characteristics of
their protoliths. Once you know a rock is metamorphic, a few simple observations
180 CHAPTER 7 INTERPRETING METAMORPHIC ROCKS