Chapter 9: How Rock Bends, Buckles, and Breaks

How Is Rock Deformed?

Tectonics forces continuously squeeze, stretch, bend, and break rock in the lithosphere.

The source of energy is the Earth’s heat, which is transformed into to mechanical energy.

Stress

Uniform stress is a condition in which the stress is equal in all directions. In rocks it is also confining stress because any body of

rock in the lithosphere is confined by the rock around it.

Differential stress is stress that is not equal in all directions.

Differential Stress

The three kinds of differential stress are: Tensional stress, which stretches rocks. Compressional stress, which squeezes them. Shear stress, which causes slippage and translation.

Figure 9.1

Stages of Deformation

Strain describes the deformation of a rock. When a rock is subjected to increasing stress, it passes

through three stages of deformation in succession: Elastic deformation is a reversible change in the volume or shape of

a stressed rock.. Ductile deformation is an irreversible change in shape and/or

volume of a rock that has been stressed beyond the elastic limit. Fracture occurs in a solid when the limits of both elastic and ductile

deformation are exceeded.

Figure 9.2

Figure 9.3

Ductile Deformation Versus Fracture

A brittle substance tends to deform by fracture. A ductile substance deforms by a change of shape. The higher the temperature, the more ductile and

less brittle a solid becomes. Rocks are brittle at the Earth’s surface, but at depth,

where temperatures are high because of the geothermal gradient, rocks become ductile.

Figure 9.4

Confining Stress

Confining stress is a uniform squeezing of rock owing to the weight of all of the overlying strata.

High confining stress hinders the formation of fractures and so reduces brittle properties.

Reduction of brittleness by high confining stress is a second reason why solid rock can be bent and folded by ductile deformation.

Fracture

All the constituent atoms of a solid transmit stress applied to a solid.

If the stress exceeds the strength of the bond between atoms: Either the atoms move to another place in the crystal

lattice in order to relieve the stress, or; The bonds must break, and fracture occurs.

Strain Rate

The term used for time-dependent deformation of a rock is strain rate. Strain rate is the rate at which a rock is forced to change

its shape or volume.

Strain rates in the Earth are about 10-14 to 10-15/s. The lower the strain rate, the greater the tendency

for ductile deformation to occur.

Figure 9.6

Enhancing Ductility

High temperatures, high confining stress, and low strain rates (characteristic of the deeper crust and mantle): Reduce brittle properties. Enhance the ductile properties of rock.

Composition Affects Ductility (1)

The composition of a rock has pronounced effects on its properties. Quartz, garnet, and olivine are very brittle. Mica, clay, calcite,and gypsum are ductile.

The presence of water in a rock reduces brittleness and enhances ductile properties.

Water affects properties by weakening the chemical bonds in minerals and by forming films around minerals grains.

Composition Affects Ductility (2)

• Rocks that readily deform by ductile deformation are limestone, marble, shale, phyllite, and schist.

Rocks that tend to be brittle rather than ductile are sandstone and quartzite, granite, granodiorite, and gneiss.

Rock Strength (1)

Rock strength in the Earth does not change uniformly with depth.

There are two peaks in the plot of rock strength with depth. Strength is determined by composition, temperature, and pressure.

Rocks in the crust are quartz-rich, so the strength properties of quartz play an important role in the strength properties of the crust.

Rock strength increases down to a depth about 15 km. Above 15 km rocks are strong (they fracture and fail by brittle deformation).

Rock Strength (2)

Below 15 km, fractures become less common because quartz weakens and rocks become increasingly ductile.

Rocks in the mantle are olivine-rich. Olivine is stronger than quartz, and the brittle-ductile transition of olivine-rich rock is reached only at a depth about 40 km.

Figure 9.7

Rock Strength (3)

By about 1300oC, rock strength is very low. Brittle deformation is no longer possible. The

disappearance of all brittle deformation properties marks the lithosphere-asthenosphere boundary.

In the crust large movements happens so slowly (low strain rates) that they can be measured only over a hundred or more years.

Abrupt Movement

Abrupt movement results from the fracture of brittle rocks and movement along the fractures. Stress builds up slowly until friction between the two

sides of the fault is overcome, when abrupt slippage occurs.

– The largest abrupt vertical displacement ever observed occurred in 1899 at Yakutat Bay, Alaska, during an earthquake. A stretch of the Alaskan shore lifted as much as 15 m above the sea level.

– Abrupt movements in the lithosphere are commonly accompanied by earthquakes.

Gradual Movement

Gradual movement is the slow rising, sinking, or horizontal displacement of land masses. Tectonic movement is gradual. Movement along faults is usually, but not always, abrupt.

Figure 9.9

Evidence Of Former Deformation

Structural geology is the study of rock deformation. The law of original horizontality tells us that

sedimentary strata and lava flows were initially horizontal.

If such rocks are tilted, we can conclude that deformation has occurred.

Dip and Strike

The dip is the angle in degrees between a horizontal plane and the inclined plane, measured down from horizontal.

The strike is the compass direction of the horizontal line formed by the intersection of a horizontal plane and an inclined plane.

Figure 9.10

Figure 9.11

Deformation By Fracture

Rock in the crust tends to be brittle and to be cut by innumerable fractures called either joints or faults.

Most faults are inclined. To describe the inclination, geologists have adopted two

old mining terms:– The hanging-wall block is the block of rock above an inclined

fault.– The block of rock below an inclined fault is the footwall block.

These terms, of course, do not apply to vertical faults.

Figure 9.12

Classification of Faults (1)

Faults are classified according to: The dip of the fault. The direction of relative movement.

Normal faults are caused by tensional stresses that tend to pull the crust apart, as well as by stresses created by a push from below that tend to stretch the crust. The hanging-wall block moves down relative to the footwall block.

Figure 9.13

Figure 9.13B

Classification of Faults (2)

A down-dropped block is a graben, or a rift, if it is bounded by two normal faults.

It is a half-graben if subsidence occurs along a single fault. An upthrust block is a horst.

The world’s most famous system of grabens and half-grabens is the African Rift Valley of East Africa.

The north-south valley of the Rio Grande in New Mexico is a graben.

The valley in which the Rhine River flows through western Europe follows a series of grabens.

Figure 9.14

Classification of Faults (3)

Reverse faults arise from compressional stresses. Movement on a reverse fault is such that a hanging-wall block moves up relative to a footwall block. Reverse fault movement shortens and thickens the crust.

Classification of Faults (4)

Thrust faults are low-angle reverse faults with dip less than 15o. Such faults are common in great mountain chains.

Strike-slip faults are those in which the principal movement is horizontal and therefore parallel to the strike of the fault. Strike-slip faults arise from shear stresses. The San Andreas is a right-lateral strike-slip fault.

– Apparently, movement (more than 600 km) has been occurring along it for at least 65 million years.

Figure 9.17

Figure 9.18

Classification of Faults (5)

Where one plate margin terminates another commences, their junction point is called a transform. J. T. Wilson proposed that the special class of strike-slip

faults that forms plate boundaries be called transform-faults.

Figure 9.19

Evidence of Movement Along Faults

Movement of one mass of rock past another can cause the fault’s surfaces to be smoothed, striated, and grooved. Striated or highly polished surfaces on hard rocks,

abraded by movement along a fault, are called slickensides.

In many instances, fault movement crushes rock adjacent to the fault into a mass of irregular pieces, forming fault breccia.

Deformation by Bending

The bending of rock is referred to as folding. Monocline: the simplest fold. The layers of rock are

tilted in one direction. Anticline: an upfold in the form of an arch. Syncline: a downfold with a trough-like form. Anticlines and synclines are usually paired.

Figure 9.21

Box 9.1

The Structure of Folds (1)

The sides of a fold are the limbs. The median line between the limbs is the axis of the fold. A fold with an inclined axis is said to be a plunging fold. The angle between a fold axis and the horizontal is the

plunge of a fold. An imaginary plane that divides a fold as symmetrically

as possible is the axial plane.

Figure 9.22 C,D,E

Figure 9.22 A, B

The Structure of Folds (2)

An open fold is one in which the two limbs dip gently and equally away from the axis.

When stress is very intense, the fold closes up and the limbs become parallel to each other. Such a fold is said to be isoclinal.

The Structure of Folds (3)

Eventually, an overturned fold may become recumbent, meaning the two limbs are horizontal. Common in mountainous regions,such as the Alps and

the Himalaya, that were produced by continental collisions.

Anticlines do not necessarily make ridges, nor synclines valleys.

Figure 9.23

Figure 9.24

Figure 9.25

Figure 9.26

Examples of Faults (1)

In the Valley and Ridge province of Pennsylvania, a series of plunging anticlines and synclines were created during the Paleozoic Era by a continental collision of North America, Africa, and Europe. Now the folded rocks determine the pattern of the topography

because soft, easily eroded strata (shales) underlie the valleys, while resistant strata (sandstones) form the ridges.

The San Andreas Fault in California is a strike-slip fault.

Examples of Faults (2)

The Alpine Fault is part of the boundary between the Pacific plate and the Australian-Indian plate, and slices through the south island of New Zealand.

The North Anatolian Fault, also with right-lateral motion, slices through Turkey in an east-west direction, and is the cause of many dangerous earthquakes.

The Great Glen Fault of Scotland was active during the Paleozoic Era. Loch Ness lies in the valley that marks its trace.

Tectonism And its Effect On Climate

Temperature decreases with altitude. The Sierra Nevada influences the local climate.

It imposes a topographic barrier to flow that forces the winds upward, causing wind, rain, and snow on the western slopes.