mountain building1
TRANSCRIPT
MOUNTAIN BUILDING
Cotopaxi Volcano in Ecuador
Cotopaxi
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
