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 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