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 Normal fault Reverse fault Strike-slip fault 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 Grabens 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 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 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 Occur when magma rises through the crust and pushes up the rock layers above the magma. Black Hills, S. Dakota Adirondack, NY