Mountain Building Vocabulary Orogenesis Orosmountain Genesisto come into

Mountain Building

Vocabulary • Orogenesis – 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 – 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 – 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 • 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