Transcript Forces and Structures ppt

Forces and Structures
Essential Questions
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What is folding?
What is faulting?
How are mountains made?
How are volcanoes made?
What are the different types of
volcanoes?
What is folding?
Fold
• when one or a stack of originally flat and planar
surfaces, such as sedimentary strata, are bent
or curved as a result of plastic (permanent)
deformation
• form under varied conditions of stress,
hydrostatic pressure, pore pressure, and
temperature
• commonly formed by shortening of existing
layers, but may also be formed as a result of
displacement on a non-planar fault, at the tip of
a propagating fault, by differential compaction or
due to the effects of a high-level igneous
intrusion
Fold classification
• Folds are classified by:
– Size
– Fold shape
– Tightness
– Dip of the axial plane
Parts of a fold
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Hinge
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– where the flanks join together
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the point on a limb at which the
concavity reverses
– on regular folds this is the midpoint of the limb
Limb
– the flanks of the fold
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Crest
– the highest point of the fold
surface
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Trough
– the lowest point of the fold surface
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plunge
– angle that the fold axis makes with
a horizontal line when the fold axis
is not horizontal
Hinge line
– hinge points along an entire folded
surface
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Axial plane
– the surface defined by connecting
all the hinge lines of stacked
folding surfaces
Hinge point
– the point of minimum radius of
curvature
Inflection point
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Fold axis
– is the closest approximation to a
straight line that when moved
parallel to itself, generates the
form of the fold
Types of folds
• Monoclines
• Anticlines
• Synclines
Monoclines
• the simplest types of folds
• occur when horizontal strata are bent
upward so that the two limbs of the fold
are still horizontal
Anticlines
• are folds where the originally horizontal
strata has been folded upward
• the two limbs of the fold dip away from the
hinge of the fold.
Synclines
• are folds where the originally horizontal
strata have been folded downward
• the two limbs of the fold dip inward toward
the hinge of the fold
Classification of folds
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Symmetrical fold
Asymmetrical fold
Isoclinal fold
Overturned fold
Recumbant fold
Chevron fold
• Symmetrical fold
– the two limbs of the
fold dip away from the
axis with the same
angle
• Asymmetrical fold
– the limbs dip at
different angles
• Isoclinal fold
– the limbs are parallel
to each other
• Overturned fold
– the strata on one limb
of the fold becomes
nearly upside
• Recumbant fold
– an overturned fold with
an axial plane that is
nearly horizontal
• Chevron fold
– has no curvature in its
hinge
– straight-sided limbs
– form a zigzag pattern
What is faulting?
Fault
• is a planar fracture or discontinuity in a
volume of rock, across which there has
been significant displacement
• large faults within the Earth's crust result
from the action of tectonic forces
What causes faults?
• Stress
– a force applied over an area
– Uniform stress
• a stress where the forces act equally from all directions
– Tensional stress (or extensional stress)
• which stretches rock
– Compressional stress
• which squeezes rock
– Shear stress
• which result in slippage and translation
Types of Stress
Parts of a fault
• Hanging wall block
– the block above the fault
• Footwall block
– the block below the fault
• Fault line
– the surface trace of a fault
– the line of intersection
between the fault plane
and the Earth's surface
• Grabens
– down-dropped blocks
• Horsts
– uplifted blocks
Slip, heave, throw
• Slip
– the relative movement of geological features present
on either side of a fault plane
– is a displacement vector
• Throw
– the vertical component of the dip separation
• Heave
– the horizontal component of the dip separation
• Dip
– gives the steepest angle of descent of a tilted bed or
feature relative to a horizontal plane
Types of faults
• Dip Slip Fault
– where the slip on the fault plane is
approximately vertical
• Strike-Slip Fault
– where the slip on the fault plane is
approximately horizontal
• Oblique-Slip Fault
– has non-zero components of both strike and
dip slip
Types of Dip Slip Faults
• Normal Fault
• Reverse Fault
• Thrust Fault
Types of Dip Slip Faults
• Normal Fault
– where the hanging-wall block has moved
down relative to the footwall block
– result from horizontal tensional stresses
Horst and Graben Normal fault
Due to the tensional stress
responsible for normal faults, they
often occur in a series, with
adjacent faults dipping in opposite
directions
A normal fault that has a curved
fault plane with the dip decreasing
with depth can cause the downdropped block to rotate
Types of Dip Slip Fault continued…
• Reverse Fault
– the hanging-wall block has moved up relative
the footwall block
– result from horizontal compressional stresses
in brittle rocks
– the dip of a reverse fault is relatively steep,
greater than 45°
Types of Dip Slip Fault continued…
• Thrust Fault
– a reverse fault
– the dip of the fault is less than 15o
– can result in older strata overlying younger
strata
– typically form ramps, flats and fault-bend folds
Strike-Slip Fault
• where the relative motion on the fault has
taken place along a horizontal direction
• result from shear stresses acting in the
crust
Types of Strike-Slip Fault
• Sinistral
– Strike-slip faults with left-lateral motion
• Dextral
– Strike-slip faults with right-lateral motion
• Transform fault
– such faults form a plate boundary
Types of Strike-Slip Fault
continued…
Oblique-Slip Fault
• a fault which has a component of dip-slip
and a component of strike-slip
• some occur within transtensional and
transpressional regimes
• others occur where the direction of
extension or shortening changes during
the deformation but the earlier formed
faults remain active
How are mountains made?
Mountain Building & the Growth
of Continents
Earth Revealed Series
• http://www.learner.org/vod/vod_window.ht
ml?pid=318
• Complete the question sheet provided
• 30 mins
Types of Mountains
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Fold Mountains (Folded Mountains)
Fault-block Mountains (Block Mountains)
Dome Mountains
Volcanic Mountains
Plateau Mountains
Fold Mountains (Folded Mountains)
• formed when two continental tectonic
plates collide and their edges crumble to
form mountains
• the crust is uplifted forming folds on top of
the other
Fault-block Mountains (Block
Mountains
• created when faults or cracks in the
Earth's crust force materials or blocks of
rocks upward or down
• block mountains break up into chunks or
blocks and move either up or down
• usually have a steep front side and then a
sloping back side
Dome Mountains
• formed when large amounts of molten rock
or magma push the earth’s crust from
underneath
• before it can erupt the source of magma
goes away leaving the pushed up rock as
such
– rock then cools and forms a mountain
Volcanic Mountains
• created when magma pushes its way from
beneath the earth to the crust, and when it
reaches the surface, it erupts as lava, ash,
rocks and volcanic gases
• erupting materials build around the vent
through which they erupted
Plateau Mountains
• formed by erosion
• large areas of high levels of flat land, over
600 meters above sea level formed due to
earth’s internal activity
• found near Fold Mountains
Ranges versus Belts
• Ranges are groups of mountain peaks or
ridges that form areas bordered by valleys
or rivers.
• Mountain belts are long chains of
mountain ranges that can extend across
continents or along their edges.
Ranges
• Most mountain ranges are uplifted, erode
to low elevations, and are uplifted again
before they become stable
Major Mountain Ranges
• Appalachian Mountains
• Rocky Mountains
• Ozark Mountains
• In Nova Scotia
– Appalachian Mountains
– Cape Breton Highlands
– Cobequid Hills
Mountain Ranges of Canada
Cape Breton Highlands
• an extension of the Appalachian mountain chain
• 'highland' or 'plateau' of ancient rock across
northern Cape Breton Island, Canada
• comprise the northern portions of Inverness and
Victoria counties
• Elevations average 350 metres at the edges of
the plateau and rise to more than 500 metres at
the centre
• Highest peak
– White Hill
Cobequid Hills
• are considered part of the Appalachians
• range stretches from Cape Chignecto in
Cumberland County in the west through to
Pictou County in the east
• are composed of a combination of sediments,
granites, and volcanic rock all of which has been
crushed and folded by continental drift
• the highest peaks
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Nuttby Mountain (360 m (1,181 ft))
Higgins Mountain (355 m (1,165 ft)
Dalhousie Mountain (335 m (1,099 ft)
Mt. Thom
Belts
• Typically 1000 of kilometers long and 100
kilometers across (wide)
• Parallel to continental coastlines
• The taller the mountains the younger
– WHY?
Major Mountain Belts
• North American Cordillera – western edge of North
and South America
• Andes – South America
• Appalachians/Caledonides – spanning
continents
• Pyrenees – Europe between France and Spain
• Alps - Europe
• Urals - Russia
• Himalaya – Asia and includes Everest and K2
How are volcanoes made?
Volcanism
Earth Revealed Series
• http://www.learner.org/vod/vod_window.ht
ml?pid=324
• Complete the question sheet provided
• 30 mins
Parts of a volcano
• Vent
– an opening at the Earth's surface.
• Pipe
– a passageway in the volcano in which the magma rises through
to the surface during an eruption
• Crater
– a bowl-shaped depression at the top of the volcano where
volcanic materials like, ash, lava, and other pyroclastic materials
are released
• Cone
– made of solidified lava, ashes, and cinder
– layers of lava, alternate with layers of ash to build the steep
sided cone higher and higher.
Parts of a Volcano continued…
• Magma Chamber
– an underground lake of hot liquid
What are the different types of
volcanoes?
Types of volcanoes
• Composite Volcanoes
– also called strato volcanoes
• Shield Volcanoes
– also called shields
• Cinder Cones & Spatter Cones
• Complex Volcanoes
– also called compound volcanoes
Composite Volcanoes
• Dimensions:
– Height as much as 8,000 feet
– Diameter 1000sq.km
– Volume 400km3
• formed by alternating layers of lava and rock
fragments
• constructed along subduction zones
• are often so quiet they seem extinct
• form impressive, snow-capped peaks
• usually large and conical
Types of Composite volcanoes
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Concave
Pyramidal
Convex-concave
Helmet-shaped
Collapse caldera
Nested
Multiple summits
Elongated along a fissure
Composite Volcano eruption
• usually erupt in an explosive way
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usually caused by viscous magma
magma rises to the surface
it clogs the craterpipe
gas in the craterpipe gets locked up
the pressure will increase resulting in an explosive
eruption
• emits gases, ash, pumice, and a small amount of stiff, silica
lava
• can have eruptions accompanied by lahars -deadly mudflows
Shield Volcanoes
• Dimensions:
– Height often exceeding 1,500 to 2,000 feet
• Mauna Loa, the largest of the shield volcanoes its
top is over 28,000 feet above the deep ocean floor
– Diameter ~5 - 6.5 km
• huge in size
• built by many layers of runny lava flows
• produced by hot spots which lay far away
from the edges of tectonic plates
Shield Volcano eruption
• low-explosivity
• lava-fountaining
• forms cinder cones and spatter cones at
the vent
Cinder & Spatter Cones
• Dimensions:
– Height rarely exceed 250m
– Diameter 500m
• formed above a vent
– formed by Strombolian eruptions
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usually a bowl-shaped crater at the top
forming steep-sided accumulations
grow up in groups
they often occur on the flanks of strato
volcanoes and shield volcanoes
Cinder Cone eruption
• lava flows through a single vent that is
usually only up to about 1,000 feet tall
• lava fragments are ejected from a single
vent and accumulate around the vent
when they fall back to earth
• eruptions usually don't cause any loss of
life
Spatter Cone eruption
• hot erupting lava contains just enough
explosive gas to prevent the formation of a
lava flow
• the lava is torn by expanding gases into
fluid hot clots, ranging in size from 1cm to
50cm across
Complex Volcanoes
• it is because we mean the "system" of
those volcanoes is not "simple"
• consists of a complex of two or more vents
• form because changes of their eruptive
characteristics or the location of multiple
vents in an area
Complex Volcano types
• Caldera complexes
– often have a large caldera with many
subsidiary vents and deposits
– some of which could be considered
"volcanoes" in their own right.
Complex Volcano eruption
• the most explosive volcanoes
• after an eruption, the result is a caldera
(crater) caused when the area around the
vent collapses
Three Stages of Activity
• Active Volcano:
– A volcano that is erupting.
– Also, a volcano that is not presently erupting, but that has
erupted within historical time and is considered likely to do so in
the future.
• Dormant Volcano:
– Literally, "sleeping." The term is used to describe a volcano
which is presently inactive but which may erupt again.
– Most of the major Cascade volcanoes are believed to be
dormant rather than extinct.
• Extinct Volcano:
– A volcano that is not presently erupting and is not likely to do so
for a very long time in the future.