Stressing and Straining Rocks

Stressing and Straining Rocks

Stress - defined as force per unit area
Strain - a change in size, shape, or volume relative to initial conditions

Three types of stress

Compression: stresses are directed inward - produces thrust faults, reverse faults, or folding

Tension: stresses directed outward - produces normal faults

Shearing: stresses are lateral (horizontal) - produce strike-slip faults

Types of deformation

Elastic deformation: materials return to original state after deformation

Brittle deformation: materials are permanently deformed (ruptured) after reaching their yield points

Plastic deformation: materials are deformed and do not return to original shape, but have not ruptured

Orientation of the rocks

Strike - horizontal line along the rock.

Dip - angle that the rock is inclined relative to the horizontal.

Using strike and dip we can uniquely define any rock outcrop orientation that we find.

Factors affecting rock deformation

Intensity of applied stress

Lithostatic pressure: pressure due to overlying rock (confining vs. directed); increased pressure allows for plastic rather then brittle failure

Temperature: higher T allows increased deformation and a more plastic behavior

Time: stress rate affects strain. If material is subjected to a high stress rate then it experience brittle failure rather than plastic or elastic failure.

Deformed rocks in the field

Most apparent in sedimentary rocks
Importance of deformation
Determines past plate motions
Determines other past geologic events
Locates specific natural resources
Rock orientation: strike and dip

Folds

Synclines: rocks folded into troughs; youngest rocks in the center.
Anticlines: rocks folded into ridges; oldest rocks in the center.

Parts of a fold

Limbs - sides of the folds
Axial plane - imaginary plane dividing fold into two symmetrical halves.
Anticlines and synclines are structures in rocks, not surface landforms

Types of folds

Symmetrical or open folds - axial plane is vertical; halves are symmetric

Asymmetrical folds - axial plane is inclined; one limb dips more steeply than other limb

Recumbent folds - axial plane is close to horizontal

Overturned folds - axial plane has rotated more than 90 degrees

Plunging folds - folds are themselves tilted

Basin - synclinal in cross-section at any orientation

Dome - anticlinal in cross-section at any orientation (also known as pericline or quaquaversal fold)

Plate tectonics and folding

Faults

Fractures: any crack or break in the rock

Joints: fractures with no relative movement

Faults: fractures with relative movement

Fault blocks: the rocks on either side of a fault

Fault plane: plane on which displacement occurred

Evidence of faults

Visible displacement of rocks

Pulverized rock

Slickensides

Discontinuity of rock sequences

Types of faults

Strike-slip faults

Example: San Andreas Fault

Distinctive landforms (linear valleys, chains of lakes, sag ponds, topographic saddles)

Dip-slip faults

Hanging wall - block on which a miner would hang their lamp

Footwall - block on which a miner's foot rests

Normal faults

Graben - downdropped block between two normal faults that dip in opposite directions

Horst - upthrown block between two normal faults that dip in opposite directions

Reverse faults - hanging wall moves up; footwall moves down

Thrust faults - a reverse fault which has a very shallow dip (~ < 15°)

Plate tectonics and faulting

Normal faults: mid-ocean ridges and continental rifts

Reverse and thrust faults: convergent plate boundaries

Building mountains

Introduction

Mountain - isolated

Mountain range - a group, usually linear, of mountains

Mountain system - composed of several ranges

Types and process of mountain building (orogenesis)

Volcanic mountains

Fault-block Mountains

The Basin and Range Province

Origin

Fold-and-thrust mountains

Geographic examples

Upwarped mountains

Theories on origin

Adirondack Mountains

Mountain building on our planetary neighbors

Mountains of the Moon: caused by meteoroid impacts over last 4 Ga.

Mountains of Mars: caused by volcanism

Mountains of Venus: caused by Earth-like orogenic process