 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.
Rock Type

Mineral composition and texture affect how it deforms.

Strong internal molecular bonds = brittle fractures
 Sedimentary and Metamorphic rocks contain areas of weakness = ductile deformation
Time

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
310)

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.
Folds

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
Monoclines

Monocline – large step-like folds in otherwise horizontal sedimentary strata

Layers folded over a large faulted block of underlying rock.
Faults
 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

Faults are classified according to the type of movement that occurs along the fault
Normal 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
Strike-Slip Fault

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.
Volcanic Mountains
 Form along plate boundaries and at hot spots
Folded Mountains

Mountains formed primarily by folding caused by compressional stress, EX: Alps

Thrust faulting is also important in formation 9 fold- and – thrust belts)
Fault-Block Mountains

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
Plateaus

Relatively high elevation and level surface

Broad area of the crust is uplifted vertically
Domes

Broad upwarping producing a roughly circular structure

Shape of an elongated oval
Sedimentary Basins

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
Continental Accretion

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.
Terranes

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.