Shaping the Earth’s Crust

Already talked about plate tectonics

Large-scale movement of Earth’s lithosphere

Structural geology: study of crustal deformation & mountain building

I. Deformation

I. Force, Stress, and Strain

A. Stress: the force applied per unit area

Stress = σ = F/A

(Force = Mass x Acceleration)

Use example of an axe, sharp side vs dull side and hockey

B. Strain: the distortion or deformation.

The change in size or shape of a rock

The deformation that occurs due to stress

II. Types of Differential Stress

Stress applied unequally in different directions

1. Compressional2. Tensional3. Shear

II. Types of Differential Stress

Stress applied unequally in different directions

A. Compression: stress that shortens a rock body, pushes it together

Associated with plate collisions, convergence

Causes rocks to crumple, thicken vertically and shorten laterally

II. Types of Differential Stress

Stress applied unequally in different directions

B. Tension: stress that stretches or extends rocks

Associated with divergent plate boundaries

Causes rocks to thin vertically and lengthen laterally

II. Types of Differential Stress

Stress applied unequally in different directions

C. Shear: stress that pushes rocks past each other in parallel but opposite directions

Associated with transform plate margins

Slices rocks into parallel blocks, breaks and displaces preexisting rocks & structures

(deck of cards example)

III. Types of Deformation

When a rock is subjected to stress, they can strain in different ways

A. Elastic deformation = recoverable

When minor stress is applied, a rock may strain slightly, but then return to its original shape after the stress is removed

Rubber band strains, but returns to its original shape, rocks usually won’t take as much stress as a rubber band

III. Types of Deformation

B. Brittle deformation

Rocks crack or rupture

Faults

The actual atomic bonds within the rock are broken along the zone of max stress = where the crack appears

Occurs under conditions of low temp and pressure

Also occurs when stress is applied quickly, a quick swing of a hammer

III. Types of Deformation

C. Plastic (ductile) deformation

Folds An irreversible change in shape or

volume that occurs without the rock breaking

Atoms rearrange themselves ona microscopic scale, from areas of

maximum stress to areas of lower stress (i.e. toothpaste oozes away from pressure)

Occurs under conditions of higher temp and pressure

Sometimes a rock begins experiences ductile deformation but if the stress increases it may fail by brittle deformation

Draw folds caused by horizontal compression

III. Types of DeformationC. Plastic (ductile) deformation

Folds

An irreversible change in shape or volume that occurs without the rock breaking

Plastic (ductile): low rates of strain, exceed yield point

Brittle: high rates of strain, rocks break or fracture, exceed yield point

Silly Putty Example: Elastic, roll into ball and bounce

Stephen Marshak

IV. Factors affecting rock deformation

A. Heat (blacksmith heats metal to work with it, so its ductile)

1. low temperature brittle

2. high temperature ductile/plastic

B. Pressure1. low pressure brittle2. high pressure ductile

IV. Factors affecting rock deformation

C. Time dependent (just as with silly putty)

1. less time brittle2. more time (same stress)

ductile

D. Rock Type or composition

1. Some mineral bonds are simply stronger than others (all other factors being equal)

Salt is weak ductilebasalt is strong brittle

2. The presence of water helps rocks be more ductile