11.1 | Crustal Deformation

Folding & faulting indicate...

compressional forces

Tectonic forces cause rocks to ____, _____, and _____

move, tilt, change shape

Deformation

changes/processes of folding, faulting, shearing, compression, or extension of rock

Where does deformation mainly occur?

along plate boundaries

Tectonic/Geologic Structures

features that reflect a rock’s tectonic history

What are the different types of tectonic structures?

folds, faults, joints/cracks, & foliation

Stress

force per unit area acting on a solid surface (s = f/a)

Confining Pressure

force/stress applied uniformly in all directions, reduces rock volume

Differential Stress

force stronger in 1 direction & unequal in all directions

Most deformation is caused by…

differential stress

What are the 3 types of differential stress?

compressional, tension, shear

Rock Strength

resistance to being deformed permanently by stress

What does rock strength depend on?

temperature, confining pressure, duration of applied stress

Compressional Stress

differential stress that shortens rock bodies, associated with convergence, produces mountains

How does compressional stress affect the crust vertically & horizontally?

shorten horizontally, thicken vertically

Tensional Stress

differential stress pulling rock body apart, associated with divergent plate boundaries

How does tensional stress affect the crust vertically & horizontally?

stretch/lengthen horizontally, thin vertically

What forces are present in the Basin & Range province of North America?

tensional

Shear Forces

differential stress causing 2 adjacent rocks to slide past 1 another, associated with transform plate boundaries

Strain

irreversible change in shape/size of rock caused by differential stress

How can you determine the type of stress that deformed a rock?

measuring strain

How does solid rock change its shape?

small-scale shearing

What are the two ways rock can undergo small scale shearing?

slippage along parallel surfaces of weakness, within defect bonds in a mineral grain (break & reform)

Recrystallization

atom movement from high stress to low stress position within mineral grain

What are the 3 types of deformation?

elastic, brittle, ductile

Elastic Deformation

rock returns to original size & shape when stress removed, temporary deformation

How does elastic deformation occur?

chemical bonds in mineral act like a spring, stretch not break, snap bask when stress is removed

Elastic deformation causes events known as…

earthquakes

Brittle Deformation

rocks break into smaller pieces, sudden fracturing causes material to lose strength

How does brittle deformation occur?

stress breaks chemical bonds holding material together

Ductile Deformation

rock changes shape without breaking under stress

How does ductile deformation occur?

slippage along surfaces of weakness, gradual reshaping of mineral grains

Solid State Flow

rock flows very slowly, changes shape without fractures or melting

Where does ductile deformation occur?

high temps & confining pressures

Rock deformation at shallow depths is ____, while rock deformation at greater depths is ____

brittle, ductile

What factors influence how a rock deforms?

temperature, confining pressure, rock type, time

Describe the role of temperature in deformation:

higher temps, ductile

cooler temps, brittle fracture

Describe the role of confining pressure in deformation:

higher pressure induces ductile deformation by preventing fracture

______ pressure and _____ temperature create ductile deformation

higher, higher

Describe how rock type influences deformation:

strong brittle rocks like granite, basalt, and quartz sandstone brittley deform

weak rocks like clay, rock salt, shale, limestone, and schist ductiley deform

Rock salt has intergrown crystals. Why does it deform ductiley instead of brittley?

mineral grains continually recrystallize, allowing salt dome formations

Describe how time influences deformation:

stress applied slowly overtime allows ductile deformation

stress applied too quickly causes elastic & brittle deformation