Deformation of the Crust

Chapter 11, Section 1 - How Rock Deforms                                                                                                                                           

Deformation - The bending, tilting and breaking of Earth's crust.  The change in the shape of rock in response to stress.

Isostasy - The balance between gravity and buoyancy on the Earth's lithosphere, as it floats on the asthenosphere.

                  • Changes in the lithosphere's weight can cause it to rise or sink.
                  • This causes the crust to bend, and rock to deform.
  • As mountains are eroded, the area may rise => uplift.
  • As sediments are deposited at the mouth of a river, the area may sink => subsidence.  (Gulf of Mexico at mouth of Mississippi River)
  • Glaciers...
    • As glaciers grow, the land they are on sinks.  Sea level drops, so the ocean floor rises.
    • As glacier melt, the lay rises.  Sea level rises, so the ocean floor sinks.
Stress - The amount of force per unit area that acts on a rock.
  • Compression - Stress that squeezes a body of rock.
        • Causes the rock to bulge up and down.
        • Occurs at convergent plate boundaries.
  • Tension - Stress that stretches and pulls a body of rock apart.
      • Causes the rock to become thinner.
      • Occurs at divergent plate boundaries.
  • Shear Stress - Stress that distorts a body of rock by pushing parts in opposite directions.
        • Causes rock to bend, twist, or break apart.
        • Occurs at transform plate boundaries.
Strain - Any change in a rock's shape or volume caused by stress.
  • If stress is applied slowly, the rock may regain its original shape once the stress is removed.
  • There are limits to how much stress a rock can endure before becoming permanently deformed.
  • Permanent Strains
    • Brittle strain results in cracks or fractures.
    • Ductile strain is change in volume or shape without breaking.
  • Strain is affected by...
    • type of rock
    • temperature & pressure (cooler and lower pressure tends to favor brittle strain)
    • amount of stress (higher favors brittle)
    • rate at which stress is applied (faster favors brittle)
Fold - A form of ductile strain in which rock layers bend; usually the result of a compression.   (A form of ductile strain.)
  • Limbs - The sides of a fold.
  • Hinge - The bend where the limbs meet.
  • Axial plane - Divides the fold in half if it is symmetrical.  (Most are not.)
  • Overturned fold seems to be laying in its side.
  • Types of Folds
    • Anticline - Oldest rock in center of fold.  Usually arch-shaped.
    • SynclineYoungest rock in center of fold.  Usually bowl-shaped.
    • Monocline - Both limbs are nearly horizontal.
  • Folds can be small (handheld rock sample) or huge (parts of landscape that run for miles).
  • Large anticlines and synclines can form ridges and valleys.

Fracture - A break in a body of rock, along which there is no movement of surrounding rock.
Fault - A break in a body of rock, along which one block slides relative to another.  (A form of brittle strain.)
  • Fault Plane - The plane/surface along which the motion occurs.
  • Hanging Wall - The rock above the fault plane (in a non-vertical fault).
  • Foot Wall - The rock below the fault plane.
  • Types of Faults
    • Normal Fault - Hanging wall moves downward.  Common at divergent boundaries.  (Great Rift Valley of East Africa)
    • Reverse Fault - Hanging wall moves upward.  Common in mountain ranges formed by compression.  (Rockies, Alps)
      • Thrust Fault - Low-angle (almost horizontal) reverse fault.
    • Strike-Slip Fault - Rock moves horizontally on either side of fault plane, due to shear stress.  (San Andreas Fault)

Chapter 11, Section 2 - How Mountains Form                                                                                                                                        

Mountain Range - A series of mountains that are closely related in orientation, age and mode of formation.
Mountain System - A group of adjacent mountain ranges.  (Appalachian Mountain System = Great Smoky, Blue Ridge, Cumberland, Green and White Mountain Ranges)
Mountain Belt - Made up of groups of mountain belts.
  • Circum-Pacific Mountain Belt
  • Eurasian-Melanesian Mountain Belt

Plate Tectonics & Mountain Formation
  • Continental & Oceanic Crust Collisions
    • Subduction of oceanic crust deforms rock on continental crust & uplifts mountains.
    • Subduction causes partial melting to produce magma, which may lead to volcanic mountains.
      • Examples: Cascade Range, Andes Mountains
    • Terranes may be scraped off the oceanic crust and added to continents, forming new mountains.
  • Oceanic & Oceanic Crust Collisions
    • The denser plate subducts under the other, causing partial melting.  This leads to volcanic mountains.
      • Example: Mariana Islands
  • Continental & Continental Crust Collisions
    • Neither plate subducts.  Instead, the rocks deform to create mountains.
      • Example: Himalayas
Types of Mountains
  • Folded Mountains
    • Formed when rock layers are squeezed together and uplifted.
      • Examples: Alps, Himalayas, Appalachians, and Ural Mountains.
  • Plateaus
    • Large, flat areas of rock, high above sea level.
    • Formed by same forces as folded mountains - thick layers of rock are slowly uplifted so that they don't fold or fault.
    • May also form when layers of lava build up and harden, or surrounding areas are eroded.
  • Fault-Block Mountains
    • Formed when the crust is broken into large blocks, and some of the blocks drop relative to others.
      • Examples - Sierra Nevada Mountains
      • Graben - Long, narrow valley formed by the same forces as fault-block mountains.

  • Dome Mountains
    • A circular structure, made of rock that slopes gently away from a central point.
    • Formed when magma rises through the crust, and pushes up on the rock layers above it.
      • Examples - Adirondack Mountains, NY and Black Hills, SD
  • Volcanic Mountains
    • Formed when magma erupts on to the surface.
    • Can occur along convergent plate boundaries.
      • Example - Cascade Range
    • Can occur along divergent boundaries.
      • Example - the Azores
    • Can occur at hot spots (volcanic areas found far from plate boundaries).
      • Example - Hawaii

Chapter Assignment
  • Pg. 312-313 #5-24, 34-36.