Download Deformation of the Crust

Deformation of the Crust
5.1 How the crust
is deformed
5.2 The results of
stress
5.3 Mountain
formation
Deformation
• Many features of the earth result from
deforming of the the crust
–
–
–
–
Bending
Folding
Faulting
Plate tectonics
These rocks on the coast of New
Zealand have been deformed by stress
Isostasy
• The balance of two forces acting on the
Earth’s crust
– The mass of the crust pushing done and the
force of the mantle pushing up
– Isostasy occurs when the two forces are
balanced
– This is when deformation occurs
Isostasy is the vertical
movement of the crust to to
bouyancy in the mantle. Just
like blocks of wood in water
float higher the thicker they
are, the crust rises and sinks
because it is lighter than the
underlying mantle.
Variations in elevation are due
both to thickness and density.
The continents stand high
because continental crust is
thick and light. The ocean
basins are low because oceanic
crust is thin and dense.
Isostasy
• This type of deformation is common in areas
where mountain or glaciers exist
– For example the building up of the Himalayas pushes
downwards on the crust
– Erosion washing away the Appalachian Mountains
changes their mass and as the result the area is rising
– Hudson’s Bay continues to rebound since the mass of
the last glacier (ice age) receded
– Sediments washing down a river into the ocean add to
its mass causing it to sink, allowing more deposits to
settle etc.
It's probably no accident that
two of the largest
epicontinental seas (seas
extending deep into the
continent) on Earth are Hudson
Bay and the Baltic, both dead
center in areas of active isostatic
uplift. In all likelihood, the crust
in these regions is still
depressed and has not finished
rising, and when uplift is
complete both seas will mostly
or entirely disappear. Gravity
measurements suggest that the
crust in the Hudson Bay region
has another 100 meters still to
rise.
Isostasy
• Up and down movements of the crust to
reach a balance is called isostatic
adjustment
– This causes stress in the rocks of the crust and
includes: compression, tension and shearing
• All three forces contribute to mountain
building
– Different kinds of rock respond differently to
various types of stress.
Compression
• When forces are pushing on either side of
the rock
• Squeezing makes layers thicker and shorter
Tension
• When forces pull rocks apart
• Rock layers tend to become thinner and
longer, layers may fracture
Shearing
• Forces push rocks in opposite horizontal
directions
• These rocks could bend, twist or break
• Common in earthquakes
Folding
• This occurs when rock becomes
permanently deformed but does not break
– Layers may have cracks they they don’t break
– Fold layers look wavelike and have various
thicknesses
– They can be small and localized or cover large
areas
• There are three types of folds:
– Anticline, syncline and monocline
Isoclinal folds
Tight isoclinal
folding, the
upfold on the
left is the
anitcline and on
the right is the
syncline
Recumbent folds
Recumbent fold
Black Hills, South Dakota
Overturned fold
At a small scale (outcrop-sized), contorted folds are found in metamorphic
rocks such as gneisses. The rocks actually soften (see below) as they are heated
during deep burial. This type of folding, called ptygmatic, is illustrated here.
Open folding showing
buckling and bending
Anticline
• Upwards fold where the
lowest layer is in the
centre
• The sides of the fold are
called limbs
– The steepness of the limbs
reflects the amount of
force applied
– Generally forms a ridge
Anticline on a road cut near the Canadian
Rockies
Syncline
• A downward fold
where the
youngest area is in
the centre
• Generally forms a
valley
Syncline on a road cut near the Canadian
Rockies (same area as anticline shown
earlier
Lulworth Cove Dorset
Synclines and anticlines commonly found next
to each other (Canadian Rockies)
A small anticline next to a syncline
Folds can be extreme in their extent of crumpling. This is especially
common in weak, thin-bedded shales, as shown here.
Monocline
• A fold where both limbs remain horizontal
The House Rock Valley which runs between the reddish escarpment of
the Vermilion Cliffs (right) and the green East Kaibab Monocline (left),
is visible far to the north. (Grand Canyon)
Monocline in the Waurn Ponds Limestone Member of the Jan Juc Formation
(Oligocene to Miocene) at Waurn Ponds. Victoria State. Australia
Island monocline, Colorado National Monument
Another view of the same monocline
Folding
• The Appalachians are good examples of
folded mountains with many anticlines and
synclines
• Folding stresses often begin deep in the
Earth and the pressure increase heat making
rocks more malleable