DEFINITION OF DEFORMATION OF ROCKS

Deformation of rocks is measured

by three components: strain, rotation, and translation. Strain

measures the change in shape and size of a rock, rotation

measures the change in orientation of a reference frame in the

rock, and translation measures how far the reference frame

has moved between the initial and final states of deformation.

The movement of the lithospheric plates causes rocks to

deform, forming mountain belts and great fault systems like

the San Andreas. To describe how rocks are deformed, we

use the terms stress and strain. Stress is a measure of force

per unit area, and it is a property that has directions of maximum,

minimum, and intermediate values. Strain is a term

used to describe the changes in the shape and size of an

object, and it is a result of stress.

There are three basic ways in which a solid can deform.

The first is known as elastic deformation, which is a

reversible deformation, like a stretching rubber band or the

rocks next to a fault that bend and then suddenly snap back

in place during an earthquake. Most rocks can only undergo

a small amount of elastic deformation before they suffer permanent,

nonreversible, nonelastic strain. Elastic deformation

obeys Hooke’s Law, which simply states that for elastic deformation,

a plot of stress v. strain yields a straight line. In other

words, strain is linearly proportional to the applied stress. So,

for elastic deformation, the stressed solid returns to its original

size and shape after the stress is removed.

Solids may deform through fracturing and grinding processes

during brittle failure, or by flowing during ductile

deformation processes. Fractures form when solids are

strained beyond the elastic limit and the rock breaks, and

they are permanent, or irreversible, strains. Ductile deformation

is also irreversible, but the rock changes shape by flowing,

much like toothpaste being squeezed out of a tube.

When compressed, rocks first experience elastic deformation,

then as stress is increased they hit the yield point, at

which point ductile flow begins, and eventually the rock may

rupture. Many variables determine why some rocks deform

by brittle failure and others by ductile deformation. These

variables include temperature, pressure, time, strain rate, and

composition. The higher the temperature of the rock during

deformation, the weaker and less brittle the rock will be.

High temperature therefore favors ductile deformation mechanisms.

High pressures increase the strength of the rock, leading

to a loss of brittleness. High pressures therefore hinder

fracture formation. Time is also a very important factor

determining which type of deformation mechanism may

operate. Fast deformation favors the formation of brittle

structures, whereas slow deformation favors ductile deformation

mechanisms. Strain rate is a measure of how much

deformation (strain) occurs over a given time interval. Finally,

the composition of the rock is also important in determining

what type of deformation will occur. Some minerals (like

quartz) are relatively strong, whereas others (such as calcite)

are weak. Strong minerals or rocks may deform by brittle

mechanisms under the same (pressure, temperature) conditions

that weak minerals or rocks deform by ductile flow.

Water reduces the strength of virtually all minerals and rocks,

therefore, the presence of even a small amount of water can

significantly affect the type of deformation that occurs.

The bending or warping of rocks is referred to as folding.

Monoclines, folds in which both sides are horizontal, often

form over deeper faults. Anticlines are upward-pointing arches

that have the oldest rocks in the center, and synclines are

downward-pointing arches, with the oldest rocks on the outside

edges of the structure. There are many other geometric

varieties of folds, but most are variations of these basic types.

The fold hinge is the region of maximum curvature on the

fold, whereas the limbs are the regions between the fold

hinges. Folds may be further classified based on how tight the

hinges are, which can be measured by the angle between individual

fold limbs. Gentle folds have interlimb angles between

180° and 120°, open folds have interlimb angles between

120° and 70°, close folds between 70° and 30°, tight folds

have interlimb angles of less than 30°, and isoclinal folds

have interlimb angles of 0°. Folds may be symmetrical, with

similar lengths of both fold limbs, or asymmetrical in which

one limb is shorter than the other limb. Fold geometry may

also be described by using the orientation of an imaginary

surface (known as the axial surface), which divides the fold

limbs into two symmetric parts, and the orientation of the

fold hinge. Folds with vertical axial surfaces and subhorizontal

hinges are known as upright gently plunging folds, whereas

folds with horizontal hinges and axial surface are said to

be recumbent.

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