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Deformation of the Crust How Rocks Deform • Deformation = bending, tilting, or breaking Earth’s crust. Isostasy Deformation can be due to two opposing forces: • gravity, or weight, of the lithosphere pressing down on the asthenosphere. • And the buoyant force of the asthenosphere pressing up on the lithosphere. • When these two forces are in balance = isostasy • As the earth changes, isostatic adjustments occur until isostasy (balance) is reached again. • Isostatic adjustments cause rock to deform. Isostastic Adjustments • As the Lithosphere thickens. • Becomes heavier • Sinks deeper into asthenosphere • Mountain building, glaciation, and deposition of sediments by rivers adds weight = Subsidence (sinking). • As the Lithosphere becomes thinner. • Becomes lighter • Rises higher in the asthenosphere • Erosion off mountains and glacial retreat can cause the crust to become lighter = Uplift Sinking, uplift, sinking, uplift, etc. Stress • The amount of force that is exerted on rock. • Occurs when crust is squeezed, stretched, and twisted when the lithosphere moves. • Compression – – – – – Squeezes and shortens Reduces the amount of space a rock occupies Reduces the volume of rock Pushes rock higher up, uplift Near convergent boundaries • Tension – Stretches and pulls rock – Rock becomes thinner – Occurs near divergent boundaries • Shear – Distorts rock by pushing parts of the rock in opposite directions. – Rocks bend, twist, or break as they slide past each other. – Common at transform boundaries Strain • Any change in the shape or volume of rock that results from stress. • If stress is applied slowly, the deformed rock may regain its original shape when the stress is removed. • Some stress leads to permanent deformation of the rock. • Type of strain depends on composition of rock, temperature, and pressure. – Brittle strain appears as cracks or fractures. • Occurs mostly at the surface, lower temperature/pressure • Also occurs when stress is applied more quickly. – Ductile materials bend or deform without breaking. • Occur at higher temperature/pressure Ductile: bend without breaking Folding • A form of ductile strain • A fold is a bend in a rock layer. • Occurs when rock is compressed and squeezed. • Can also occur from shear stress. • MONOCLINE fold – Both limbs are horizontal – Form when one side moves up or down • ANTICLINE fold – Oldest layers are in the center, turns downwards • SYNCLINE fold – Youngest layers are in the center, turns upwards Faults • Stress (brittle strain) may cause rocks to break. • If no movement occurs along the break = fracture. • If movement occurs along the break = fault • Normal fault – Hanging wall (which is above fault) moves down compared to footwall (below fault). – Occur at divergent boundaries – Great Rift Valley, Africa • Reverse fault – – – – Hanging wall moves up compared to footwall Occur at convergent boundaries (compression) Thrust fault (type of reverse fault) – hanging wall pushed up over the footwall Rockies and Alps • Strike-slip fault – Rocks slide horizontally to each other – Due to shear stress at transform boundaries – San Andreas fault Normal Fault (tensional stress) Reverse Fault or Thrust Fault (compressional stress) Strike-Slip Fault (shear stress) How Mountains form (orogeny) • A mountain is the most extreme type of deformation. – Mt. Everest… 8 km and still rising – Part of the Great Himalaya range – Mountain ranges: Great Smokey, Blue Ridge, Cumberland, Green, Appalachian. – Mountain belts: Circum-Pacific, Eurasian-Melanesian. Plate Tectonics and Mountains • Collisions: continental and oceanic crust • Melting may also form volcanic mountains • Cascade range, N. America • Andes, S. America Cascade Mountains Mt. Hood Mt. Jefferson Three Sisters Plate Tectonics and Mountains • Collisions: oceanic and oceanic crust • Melting may form an arc of volcanic mountains. – Mariana islands Aleutian Islands Plate Tectonics and Mountains • Collisions: continental and continental crust • Forms uplift mountains – Himalayas Himalayas Folded Mountains • Occur when two continents collide • Form high mountains – Alps, Himalayas, Appalachians, Urals. Plateau • Occur when large, flat, areas of rock are slowly uplifted and remain flat. • Located near mountain ranges. – Tibetan plateau (Himalaya) – Colorado plateau (Rockies) • Can also form when layers of molten rock accumulate. • Or when large areas of rock are eroded. Fault-Block Mountains • Occur where parts of Earth’s crust have been stretched and broken into large blocks. • Some blocks tilt or drop relative to other blocks. – Sierra Nevada Range, CA Grabens • Also forms long narrow valleys • Form when steep faults break the crust into blocks and one block slips downward relative to the surrounding blocks. • Occur with Fault-Block Mountains. – Basin and Range Province, Western U.S. The Grand Tetons (Fault-Block Mountains) Dome Mountains Volcanic Mountains • Occur when magma rises through • the crust and pushes up the rock • layers above the magma. – Black Hills, S. Dakota – Adirondack, NY • • Occur when magma erupts onto Earth’s surface. Common along convergent boundaries Cascades (Washington, Oregon, N. CA) Mid-Ocean Ridges form volcanic islands – Azores, N. Atlantic Ocean • Some also form at hot spots (volcanically active areas that do not lie near tectonic plate boundaries). • Hawaiian Islands