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)
- 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.
- Syncline - Youngest 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.
- Continental & Continental Crust Collisions
- Neither plate subducts. Instead, the rocks deform to create mountains.
Types of Mountains
- Folded Mountains
- Formed when rock layers are squeezed together and uplifted.
- Examples: Alps, Himalayas, Appalachians, and Ural Mountains.
- 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.
- Can occur along divergent boundaries.
- Can occur at hot spots (volcanic areas found far from plate boundaries).
- Pg. 312-313 #5-24, 34-36.