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Chapter 9 Mountain Building

 

 

Crustal uplift

Evidence for crustal uplift

Marine fossils at high elevations in mountains

Wave-cut platforms high above sea level

 

Reasons for crustal uplift

Not so easy to determine

Floating crust in gravitational balance

Called isostasy

When weight is removed from the crust, crustal uplifting occurs

Called isostatic adjustment

Considerable vertical crustal movement can occur

 

Rock deformation

Rocks subjected to stresses will deform by either

Folding, or

Fracturing

 

Types of rock deformation

Elastic deformation - where the rock returns to nearly its original size and shape when the stress is removed

Plastic deformation

One elastic limit is surpassed, rocks either deform plastically or fracture

Size and shape of rock are permanently altered through folding or flowing

 

Folds

Rocks bent into a series of waves

Most folds result from compressional stresses which shorten and thicken the crust

Two common types of folds

Anticline - upfolded, or arched, rock layers

Syncline - downfold, or trough, rock laters

 

Anticlines and synclines can be

Symmetrical

Asymmetrical

Overturned

Where folds die out they are said to be plunging

 

Other types of folds

Dome

Circular, or slightly elongated

Upwarped displacement of rocks

Oldest rocks in core

Basin

Circular, or slightly elongated

Downwarped displacement of rocks

Youngest rocks in core

 

Faults

Faults are fractures (breaks) in rocks along which appreciable displacement has occured

 

Types of faults

Dip-slip fault

Movement along the inclination (dip) of fault plane

Parts of a dip-slip fault

Hanging wall - the rock above the fault surface

Foot-wall - the rock below the fault surface

 

Types of dip-slip faults

Normal fault

Tensional force - pulls rock apart

Hanging wall moves down

Reverse fault

Compressional force - squeezes rock

Hanging wall move up

Thrust fault

Strong compressional force

Hanging wall moves up

Low-angle reverse fault

More horizontal

 

Types of faults continued

Strike-slip fault

Movement along trend, or strike

Transform fault

Large strike-slip fault

Often associated with plate boundaries

 

Pulling of crust apart (tension) can form

Graben

Down-dropped block

Bounded by normal faults

Can produce and elongated valley (e.g., Great Rift Valley, Africa)

Horst

Flanking a graben

Relatively uplifted block

 

Joints

Fractures along which no appreciable displacement has occurred-has no movement

Most are associated with stresses during mountain building

Other types

Columnar joints caused by igneous rock shrinking and producing pillarlike columns

Sheeting caused by curved joints in large igneous bodies like batholiths

 

Mountain Types

Mountains are classified according to their most dominant characteristics

Four main categories

Fault-block mountains

Produced by tensional stresses that stretch the crust

Mountains bounded at least on one side by normal faults of high-to-moderate angle

e.g.,Basin and range Province, Teton Range, and Sierra Nevada

 

Mountain types continued

Folded mountains

Largest - most complex mountains

e.g., Alps, Urals, Himalayas, Appalachians, and northern Rockies

Upwarped mountains

Broad arching of crust

e.g., Black Hills, Adirondacks

Volcanic mountains

 

Mountain building

Refers to processes that uplift mountains

Major mountain systems are related to plate tectonics where most mountains form along convergent plates

Mountains are associated with

Convergent boundaries

Oceanic-continental crust convergence

Andean-type mountains

Subduction zone forms

Defromation of continental margin

Volcanic arc forms

Accretionary wedge

Examples

Sierra Nevada Range

California’s Coast Ranges

 

Where continental crusts converge

E.g., India and Eurasian plate collision

Himalaya Mountains

Tibetan Highlands

Other examples

Alps

Appalachians

 

Continental accretion

Third mechanism of mountain building

Small crustal fragments collide with and accrete to continental margins

Accreted crustal blocks are called terranes

Occurred along the Pacific Coast

Back To:  348 Earth Science