Mountain Building

Vocabulary Orogenesis Lithosphere Accretion Oros—mountain Genesis—to come into being Lithosphere The rigid outer layer of Earth, including the crust and upper mantle Accretion the increase in size of a tectonic plate by addition of material along a convergent boundary

Crustal Uplift Evidence Isostasy Marine fossils often found in high elevations in mountains Terraces hundreds of meters above sea level Isostasy A floating crust in gravitational balance Example: blocks of wood floating in water Mountain belts stand higher above the surface of the Earth and have roots that extend deeper into the supporting material below. Crustal thicknesses for some mountain chains are twice as much as the average for the continental crust

Crustal Uplift Crust beneath the oceans is thinner than that beneath the continents Oceanic rocks are denser than continental rocks **adding weight to the crust makes it subside **when weight is removed, crustal uplifting occurs (ex.: cargo ship)

Crustal Uplift Isostatic Adjustment Ice Age glaciers added weight to the continents, making them downwarp by hundreds of meters When glaciers melted, uplift occurred Erosion of mountains causes uplift, also

Rock Deformation Elastic deformation Plastic deformation When stress is applied, rocks bend, but will snap back if the stress is relieved Plastic deformation When the elastic limit is surpassed, rocks deform plastically or break (earthquakes) They are permanently altered through folding and flowing

Rock Deformation Folds When flat-lying sedimentary and volcanic rocks are bent into a series of wavelike undulations Example: pushing on one edge of a carpet until it folds Anticline Upfolding or arching of rock layers Syncline Downfolds, or troughs

Rock Deformation Dome Basin When upwarping produces a circular or somewhat elongated structure Basin When downwarping produces a circular or somewhat elongated structure

Rock Deformation Faults and Joints Fractures in the Earth’s crust Dip-slip faults Vertical movement Hanging wall—rock that is higher than the fault surface Footwall—rock that is lower than the fault surface Normal—hanging wall moves downward relative to the footwall Reverse—hanging wall moves upward relative to the footwall Thrust faults—have a very low angle

Rock Deformation Strike-slip faults The dominant displacement is along the strike or trend, of the fault (horizontal) Transform faults—associated with plate boundaries Oblique-slip faults—both vertical and horizontal movement Tensional forces—pull the crust apart Graben—central block bounded by normal faults; drop as the plates separate Horsts—upfaulted structures that are adjacent to graben Compressional forces—sections of crust are displaced toward one another

Rock Deformation Joints Fractures along which no appreciable displacement has occurred Columnar joints form when igneous rocks cool and develop shrinkage fractures, producing elongated, pillarlike columns Sheeting produces a pattern of gently curved joints that develop more or less parallel to the surface of large exposed igneous bodies.

Mountain Types Fault-block mountains Tensional stresses elongate and fracture the crust into numerous blocks. Movement along the fractures tilt the blocks producing parallel mountain ranges.

Mountain Types Folded mountains (complex mountains) Upwarped mountains Caused by a broad arching of the crust or because of great vertical displacement along a high-angle fault Volcanic mountains

Mountain Building Convergent boundaries Volcanic arcs are forming in most modern-day subduction zones Aleutian-type subduction zones occur where two oceanic plates converge

Mountain Building Andean type subduction zones Passive continental margin—part of the same plate as the adjoining oceanic crust Becomes active—subduction zone forms and the deformation process begins The oceanic plate descends and becomes magma while there is an accumulation of sedimentary and metamorphic rocks along the subduction zone (accretionary wedge)

Mountain Building Continents converge Continental lithosphere is too buoyant to undergo subduction, a collision eventually results Example: India colliding with the Eurasian plate

Mountain Building Mountain Building and Continental Accretion Smaller crustal fragments collide and accrete to continental margins Example: mountainous regions rimming the Pacific As oceanic plates move, they carry with them embedded oceanic plateaus or microcontinents The upper portions of these thickened zones are peeled from the descending plate and thrust in relatively thin sheets onto the adjacent continental block. This increases the width of the continent Terrane—accreted crustal blocks