Deformation of the Crust
5.1 How the crust
is deformed
5.2 The results of
stress
5.3 Mountain
formation
Faulting
• This tends to occur under cooler conditions
and lower pressure near the Earth’s surface
• When a break occurs in rock with no
movement it is called a fracture
• When a break occurs and the rocks do move
it is called a fault
Faults
• The fault plane is the surface of the fault
along which any motion occurs
• The part of the fault above the fault plane is
the hanging wall and the part of the fault
below the fault plane is the footwall
• There are 4 types of faults: normal, reverse,
thrust and strike-slip
Small normal faults, Malta
Normal fault
• The hanging wall
moves downwards
with respect to the
footwall
• This happens where
the Earth is under
tension, for example
along divergent
boundaries
– Great Rift Valley
(Africa)
Normal faults in volcanic ashes and paleosoils, El Salvador.
Bishop Tuff lake sediments, Owen Valley, CA.
This mountain of nicely layered limestones shows several sets of normal
faults offsetting the prominent white band near the top of the mountain. If
you focus on each fault, the offset of each hanging wall from each foot wall
indicates normal fault motion.
Round O Quarry, Lancashire, England
Reverse fault
• Occurs when the hanging wall moves up
with respect to the footwall
• Caused by compression
• For example when two plates collide the
compression causes the fault to thicken and
rocks to shorten
Fault planes are nearly vertical but so tilt to the left
in a steep series of reverse faults.
Small reverse thrust on a road cut in Japan
Thrust fault
• A reverse fault where the fault plane sips below
45o or less from the horizontal
• Common in the Rockies and Alps
• Sometimes called a blind fault meaning there is no
deformation at the surface (no visible break)
– Difficult to detect or locate
– Sometimes the only clue is what appears to be a fold at
the surface that is actually the thrust fault pushing
overlying layers upwards
Thrust fault in the Andes, Chile.
Onondaga limestone, New York state.
Monos de Agua Canyon, Chile
Major thrust fault – White Rock, Wyoming
Strike-slip fault
• This occurs when rocks on opposite dies of
the fault plane move horizontally past each
other
• Often occurs are transform boundaries
– San Andreas Fault
– very long strike-skip faults are found in the
Himalayas (thousands of km long)
This fault can be recognized from the air as a line with dissimilar
surface features on either side because the crustal blocks have shifted
horizontally relative to each other; the movement is mainly horizontal
making this a strike-slip fault.
Joints
• These are breaks in the bedrock but unlike faults
joints are breaks that appear where no apparent
movement has occurred
– They are the results of the same stresses the lift, tilt and
fold rock layers into mountains
– One of the most common rock structures, appear on the
surface of a rock outcrop as a line or groups of lines
– They provide channels for the flow of fluids such as
water and magma (can form vein deposits)
Looking down (note shoe)
Joints in a rock unit exposed in a cliff
Columnar joints in basalt caused by systematic
fracturing during cooling of basaltic lava, Turkey.
Joints in a sandstone unit widened by water
erosion, Henry Mountains, Utah.
Mountain building
• Mountains form along continental margins
where crusts meet
– Andes and Rockies – oceanic and continental
– Himalayas – two continental crusts
– Island arcs (eg. Mariana Islands) – two oceanic
plates
Mountain building
• Stress is applied to rocks at convergent plate
boundaries
• Compression, tension and shearing all
contribute to the process
– Fracturing (faulting) near the cooler surface and
folding in warmer deeper layers
– Also the type of material the rock is formed
from will influence how it reacts
Fold and thrust
Continental margins
• This is a boundary between continental
crusts and oceanic crust
• Active margins occur along plate
boundaries (subduction zones)
– For example there are active continental crusts
along the west coasts of North and South
America
• This is one of the reason for the many earthquakes
and volcanoes that affect South America
The convergence of the Nazca and South American Plates has
deformed and pushed up limestone strata to form the towering
peaks of the Andes, as seen here in the Pachapaqui mining area
in Peru.
Continental margins
• Passive continental margins are stable areas
because they do not occur at plate margins
– For example, along the east coast of North
America eroded sediments from the
Appalachians have formed a wedge 250 km
long and 10 km deep
• In the future the material forming passive margins
my become involved in mountain building as plates
move and collide in the future
Continental margin of the northeastern United States
Mountains
• Mountain range – a group of adjacent
mountains with the same general shape and
structure
– Cascades, Sierras
• Mountain system
– Group of adjacent mountain ranges
– Rockies, Andes
Mountain Ranges
Grampian Mountains, Australia
Mountains
• Mountain belt
– Most of the world’s mountains occur in belts
– E.g. North American Cordillera runs from
Alaska to Mexico
– They are regions where mountains are being
formed or have been formed in the past
• For example, the Appalachians formed part of a
previous plate boundary millions of years ago
Folded Mountains
• When two continental crusts collide
rock can fold and crumple into folded
mountains
– Form some of the highest mountains in the
world such as the Himalayas, Rockies,
Urals, Alps, Appalachians etc.
Folded Mountains
• For this to happen the ocean basin between
the continents must close
• This brings the two continental crusts into
direct contact
• No more subduction because the crusts now
have equal densities
The Himalayan mountain range
dramatically demonstrates one of
the most visible and spectacular
consequences of plate tectonics.
When two continents meet head-on,
neither is subducted because the
continental rocks are relatively light
and, like two colliding icebergs,
resist downward motion. Instead,
the crust tends to buckle and be
pushed upward or sideways.
Folded Mountains
• Form plateaus – uplifted areas form where
thick horizontal layers are pushed up gently
enough that they don’t break and remain flat
rather than bending
– Tibetan Plateau next to the Himalayas
– Plateaus can also form from igneous rock
extrusions
The collision between the Indian and Eurasian plates has pushed up the
Himalayas and the Tibetan Plateau. The cross sections show the evolution of
the Himalayas and the displacement of slivers of continental crust during this
collision. The reference points (small squares) show the amount of uplift of an
imaginary point in the Earth's crust during this mountain-building process.
Tibetan plateau
Dome Mountains
• A nearly circular folded mountain
– Individual insolated structures, not part of
mountain belts, usually in areas of flat lying
sedimentary rock
• Formed by igneous intrusions (plutonic
dome mountains) such as laccoliths
– Some on the border of the Colorado plateau and
the Rockies
Dome Mountains
• Tectonic dome mountains are formed by
uplifting forces that arch the rock layers
upwards
– Adirondack Mountains, Black Hills
Located in New York, near Ontario/Quebec
Tectonic Dome Mountains
Black Hills, South Dakota
Black
Hills
Fault-Block Mountains
• These mountain form from the forces of
tension instead of compression
– For example in some parts of western North
America the crust is slowly being uplifted
– This causes the crust to stretch and crack
forming normal faults along the surface
– Examples include the Sierra Nevadas and
Tetons (Wyoming)
Road cut near Denver Colorado
Hart Mountain (a massive block fault ridge) at the north
end of Lake Crump, Oregon.
The Teton Range is one of Earth’s most impressive examples of fault-block
mountains, along which movement is estimated to average about one foot per
300–400 years. Although the Tetons are the youngest mountains of the Rocky
Mountain chain, they contain some of its oldest rocks, dating back about 2.8
million years.
Horsts
• Forms when a block is forced upwards due
to tension between two normal faults
Grabben
• Forms when tension
forces rocks apart
and between normal
faults on either side
of the valley large
blocks of crust have
dropped
– Great Rift Valley,
Death Valley
Looking down on Death Valley
from Zabriskie Point