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

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• 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