File - RHS Earth Systems

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Chapter 11 Notes

Mountain Building

11.1 Forces in Earth’s Crust

 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.

11.2 Folds, Faults, and 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.

11.3 Mountains and Plates

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.

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