• LEARNING OBJECTIVE:
• Identify the factors that determine the strength
of rock and how it will deform, what are the three
main types of folds?
UNIT 7 (5) PLATE TECTONICS,
MOUNTAIN BUILDING
Oceanic crust is thinner but more dense than the
thicker continental crust.
Oceanic crust is mainly basalt
Continental crust is mainly granite
Continental crust will “float “ higher on the mantle.
• Elevations on the crust depend on thickness of
crust.
• Higher elevations require thicker crust to support
them.
• Large mountain held up by larger “root” of crust
that extends into mantle to support it.
• A. Principle of isostasy
• Isostasy-displacement of mantle by the crust until
equilibrium between crust & mantle is reached.
• The downward force of gravity on crust is
balanced by upward force of buoyancy in mantle.
• As more mass is added to crust, it sinks lower into
the mantle.
• As mass is decreased on crust, crust can rise.
• Erosion can decrease mass of crust.
• Isostatic rebound-process of crust rising due to
removal of overlying material.
• Orogeny- process that forms mountain ranges.
• Orogenic belts- broad, linear regions of
deformation. Most of these are found along plate
boundaries (mainly convergent boundaries).
• Deformation- General term refers to all changes
in original shape/size of rock body.
• Brittle deformation-occurs in rocks near the
surface /temperatures pressures low rocks are
brittle & fracture once their strength is exceeded.
• Ductile deformation-occur at depth temperatures
pressure high produces changes in rock without
fracturing.
• 1. Folds
• Flat-lying sediments and volcanic rocks bent into a
series of wavelike formations.
• Caused by compressional forces.
•
Anticlines -layers of rock bent upwards. Commonly,
the tops are eroded, exposing older rock within.
Synclines-layers of rock bent downwards.
• 2. Domes
• Broad upwarps in basement rock may deform
overlying cover of sedimentary strata & generat
large folds.
• A dome is a circular or elongated structure.
•
•
•
•
3. Basins
Downwards warping of basement rock.
Very gently sloping beds
Caused by accumulations of sediments, whose
weight caused the crust to subside
• Younger rocks usually found in
center, older rock on outside.
• 4. Faults
• Fractures in crust along which appreciable
displacement has occurred.
• Learning objective
• How are mountains classified and what is the
difference between folded mountains and faultblock mountains?
• Dip-Slip Faults
• The rock surface immediately above the faulthanging wall/rock below the surface the foot
wall.
• 2 types of Dip-Slip
Faults.
Normal faults occur
when hanging wall
block moves down
relative to footwall
block.
• Normal faults occur as a result of crust forces
pulling apart.
• Reverse faults hanging wall block moves up
relative to footwall block.
• Reverse faults occur when crustal blocks are
moving toward each other.
• Thrust fault is a special type of reverse fault that
occurs when fault has a dip less then 45 degrees.
• Strike-slip faults occur when the dominant
displacement is horizontal & parallel to strike or
fault surface. Large strike-slip faults are called
• Transform faults.
• 5. Joints fractures along which no appreciable
displacement has occurred. Some joints have
random orientation, most occur in roughly
parallel groups.
• Columnar joints form when igneous rock cools &
develop shrinkage fractures that produce
elongated pillarlike colums.
• 6. Orogenesis - process of forming mountain
belts.
• Mountain building
along subduction
zone.
Accretion occurs
as inactive
Volcanic arc
collides with
Continental crust.
• Fault-block mountains
• Mountains that form as normal faults.
• Horst and grabens
ABOVE REVERSE FAULTING
Left flat iron formation