1. Deformation of the Crust

2. How Rocks Deform Deformation
= bending, tilting, or breaking Earth’s crust.

3. 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.

4. 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

5. Sinking, uplift, sinking, uplift, etc.

6. 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

7. 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

11. Ductile: bend without breaking

12. 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

13. 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

14. Normal Fault (tensional stress)

15. Reverse Fault or Thrust Fault (compressional stress)

16. Strike-Slip Fault (shear stress)

17. 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.

18. Plate Tectonics and Mountains
Collisions: continental and oceanic crust
Melting may also form volcanic mountains
Cascade range, N. America
Andes, S. America

19. Cascade Mountains
Mt. Hood
Mt. Jefferson
Three Sisters

20. Plate Tectonics and Mountains
Collisions: oceanic and oceanic crust
Melting may form an arc of volcanic mountains.
Mariana islands

21. Aleutian Islands

22. Plate Tectonics and Mountains
Collisions: continental and continental crust
Forms uplift mountains
Himalayas

23. Himalayas

24. Folded Mountains Occur when two continents collide Form high mountains
Alps, Himalayas, Appalachians, Urals.

25. 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.

26. 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.

27. The Grand Tetons (Fault-Block Mountains)

28. 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