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
Californias 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
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