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MOUNTAIN BUILDING
Deformation – refers of all changes in the original
shape or size of a rock body.
• Brittle deformation – at the earth’s surface, low
temperatures and low pressures, solid rock
fractures
• Ductile deformation – deep with in the Earth,
high temperatures and high pressures, rock is
deformed without breaking
• The mineral composition and texture also affects
how it will deform.
• Small stresses applied over time will cause the
rock to bend.
• Stress – force per unit area acting on a
solid
• Strain – the change in shape or volume
• Tensional stress – causes a material to be
stretched
• Compressional stress – causes a material
to shorten
• Shear stress – causes a material to be
distorted
Folds – during mountain building flat-lying
sedimentary and igneous rock are bent
into a series of ripples
• Anticlines – arching of rock layers
• Synclines – downfolds or troughs
• Monoclines – large step-like folds
Faults – fractures in the crust along which movement has
taken place
• Normal fault – when the hanging wall block moves down
relative to the footwall block, caused by tensional forces
• Reverse fault – the hanging wall block moves up relative
to the footwall, caused by compressional forces
• Thrust faults – reverse faults with dips less than 45o
• Strike-slip faults – the movement is horizontal and
parallel, caused by shear stress, San Andreas fault
• Joints – most common rock structure, fractures along
which no appreciable movement has occurred
• Mountains – classified by the dominant
processes that deformed them
• Folded Mountains – formed by folding,
compressional stress is the major force
that formed them; examples –
Appalachians, Alps, northern Rocky
Mountains
Fault-block Mountains – mountains that form
as large blocks of crust are uplifted and
tilted along normal faults; examples –
Tetons Range, Sierra Nevadas
• Horst and Grabens – formed from tensional
forces, horsts are uplifted structures and grabens
is where the blocks dropped down; example –
the Basin and Range region of Nevada,
California and Utah
• Domes – formed by
upwarping and
exposing older
igneous and
metamorphic rock;
example – Back Hills
of South Dakota
• Basins –
downwarping
structures having a
circular shape
Mountain Building – Orogenesis
• Mountain Building at Convergent
Boundaries – colliding plates provide the
compressional forces that deform rock
• Oceanic-Oceanic Convergence – forms
volcanic island arcs, Aleutian Islands of
Alaska
• Ocean- Continental Convergence – ocean crust
subducts the continental crust, the continental
crust is deformed, creates volcanic arcs on
continent
• Accretionary wedge – accumulation of different
sedimentary and metamorphic rocks
• Continent-Continent Convergence – form folded
mountains; examples – Himalayas, Ural
mountains
• Mountain Building at Divergent Boundaries –
fault-block mountains
• Non-Boundary Mountains – Hawaiian Islands
are volcanic islands formed by a hot spot
• Continental Accretion – smaller crustal
fragments collide and merge with continental
margins; example – many of mountains rimming
the Pacific Canada and Alaska
• Terranes – any crustal fragment that has a
geologic history distinct from adjoining terranes
• Isostacy – a floating crust in gravitational
balance. As mountains erode, the crust
rises in response to the reduced load.
Erosion and uplift continue until the
mountains reach normal crustal thickness
• The weight of the ice sheet during the
Pleistocene depressed the Earth’s crust
hundreds of meters. Since the ice age,
uplift has occurred