14 mountains higher than 8,000 meters, 4 are in Pakistan’s Karakoram Range
Plate motions cause rock to bend and break over time.
Deformation of Rock
Deformation – any change in the original shape and/or size of a rock body.
Most occurs along plate margins
Plate motions and interactions at boundaries causes forces that deform rock.
Occurs because of stress in a body of rock.
Stress – the force per unit area acting on a solid.
Rocks under stress will begin to deform.
Strain – change in shape or volume of a body of rock as a result of stress.
Gradual stress causes rocks to deform elastically
Factors that affect the deformation of rock include temperature, pressure, rock type and time.
Temperature and Pressure
Deform permanently in 2 ways – brittle and ductile deformation
Brittle – Brittle Failure or Brittle Deformation. Ex: glass, china plates, bones.
Ductile Deformation – solid state flow, produces a change in size and shape without fracturing the object. Ex: clay, bee’s wax, caramel candy.
Ductile Deformation of a rock is similar to a train flattening a penny.
Mineral composition and texture affect how it deforms.
Strong internal molecular bonds = brittle fractures
Sedimentary and Metamorphic rocks contain areas of weakness = ductile deformation
Small stress applied over a long period of time eventually causes deformation.
EX: Marble benches sag over time due to weight
Types of Stresses
Stress in the rocks in the lithosphere
3 types cause deformation – tensional stress – rocks being pulled in opposite directions compressional stress – Squeezed or shortened shear stress – causes body of rock to be distorted
Principle of Isostasy
Lithospheric plates can also move up and down
Usually at plate boundaries, EX: mountains
Some are in the interior of continents
What is Isostasy?
Isostasy – concept of a floating crust in gravitational balance ( blocks Picture page
Iso = equal and stasis = standing
Isostatic adjustment – when a new level of gravitational balance is reached.
Isostatic Adjustment for Mountains
Weight added to crust – it subsides. Weight removed it will rebound
Accounts for vertical movement
Crust rises, erosion increases, and deformed rock is carved into mountains.
Compressional Stress can build flat-lying sedimentary and volcanic rocks
3 main types of folds – anticlines, synclines, and monoclines
Anticlines and Synclines
Anticline – formed by upfolding or arching of rock layers.
Synclines – downfold or troughs
Dip – the angle that a fold or fault makes with the horizontal
Symmetrical fold – both limbs of anticline have the same dip
Asymmetrical fold – one limb is steeper
Overturned fold – one limb is tilted beyond vertical
Monocline – large step-like folds in otherwise horizontal sedimentary strata
Layers folded over a large faulted block of underlying rock.
Hanging wall – rock surface immediately above the fault
Foot wall – rock surface below the fault
4 major types of faults – normal faults, reverse faults, thrust faults, and strike slip
Faults are classified according to the type of movement that occurs along the fault
Occur due to tensional stress
Occurs when the hanging wall block moves down relative to the footwall block.
Steep dips – about 60 degrees
Movement in a vertical direction
Results in lengthening or extension of the crust
Reverse Faults and Thrust Faults
Occur due to compressional stress
Reverse fault – hanging wall block moves up relative to the footwall block – high angle with dips greater than 45 degrees
Thrust fault - reverse faults with dips lass than 45 degrees.
Shortening of the crust
Produced by shearing stresses
Movement is horizontal and parallel to the trend.
Large size and linear
Produce a trace that is visible over a great distance
Zone of roughly parallel fractures
EX – San Andreas Fault
Types of Mountains
Mountains are classified by the processes that formed them
4 major types – volcanic, folded, fault-block, and dome
Orogenesis - Collection of processes involved in mountain building ( oros – mountain and geny – born)
Mountain range – group of mountains that are similar in shape age and structure.
Form along plate boundaries and at hot spots
Mountains formed primarily by folding caused by compressional stress, EX: Alps
Thrust faulting is also important in formation 9 fold- and – thrust belts)
Results from normal movement along faults
Form as large blocks of crust are uplifted and tilted along normal faults
Graben – A valley formed by the downward displacement of a fault-bounded block(German word for ditch or trench)
Grabens produce an elongated valley bordered by uplifted structures called horsts.
Plateus, Domes, and Basins
Up and down movements of the crust can produce variety of landforms.
Within the crust factors like plate motion, folding, faulting, igneous activity and
isostatic adjustment create landforms.
Crust bends downward it is downwarped
Crust bows upward it is upwarped
Relatively high elevation and level surface
Broad area of the crust is uplifted vertically
Broad upwarping producing a roughly circular structure
Shape of an elongated oval
Downwarped producing a roughly circular structure
May form along edges of continents
Convergent boundaries with basins may disappear if plates come together.
Convergent Boundary Mountains
Most mountains occur at convergent plate boundaries
Ocean –Ocean Convergence
Convergence of 2 oceanic plates produce volcanic mountains.
Converge in a seduction zone.
Ocean – Continental Convergence
Convergence of oceanic plate and a continental plate produces volcanic and folded mountains.
Example: Andes Mts.
Accretionary wedge – a large wedge shaped mass of sediment that accumulates in a subduction zone. Sediment is accreted to the overriding crustal plate.
Continent – Continent Convergence
Folded mountains form at collisions of 2 continental plates.
Divergent Boundary Mountain
Usually on the ocean floor
Fault block mountains made of volcanic rock
Mountains are elevated due to isostasy
Non Boundary Mountains
Far from plate boundaries
Can form over hot spots
Example – Hawaiian Islands
Size and shape of a continent changes over time
Can change by accretion
Accretion – fragments of crust collide with a continental plate and become stuck or embedded into continent.
Accreted crustal blocks
Any crustal fragment that has a geologic history distinct from that of adjoining terranes
Mountains from accretion
Accretion of larger crustal fragments may result in mountains
Mountains are smaller than Continent- continent collision.