Geomorphology
Rana Faizan Saleem (Roll.No 10)
Structural Geology
Tectonic plates moves and rocks bulge up and different structures,forms and
arrangements produces,the study of these structures is called Structural Geology.
Orogeny
Due to collision of Plates,plane surface of crust looks like mountains ,this process is
called Orogeny.
Crystal Shortening
Crust plates become short/bulge up due to movement of tectonics plates.this process is
called Crustal Shortening.
Stress and Strain
Stress - defined as force per unit area
Strain - a change in size, shape, or volume relative to initial conditions
Three types of stress
 Compression: stresses are directed inward - produces thrust faults, reverse
faults, or folding
 Tension: stresses directed outward - produces normal faults
 Shearing: stresses are lateral (horizontal) - produce strike-slip faults
Types of deformation
 Elastic deformation: materials return to original state after deformation
 Brittle deformation: materials are permanently deformed (ruptured) after
reaching their yield points
 Plastic deformation: materials are deformed and do not return to original
shape, but have not ruptured
Orientation of the rocks
 Strike - horizontal line along the rock.
 Dip - angle that the rock is inclined relative to the horizontal.
Using strike and dip we can uniquely define any rock outcrop orientation that we find.
Factors affecting rock deformation
 Intensity of applied stress
 Lithostatic pressure: pressure due to overlying rock (confining vs.
directed); increased pressure allows for plastic rather then brittle failure
 Temperature: higher T allows increased deformation and a more plastic
behavior
 Time: stress rate affects strain. If material is subjected to a high stress rate
then it experience brittle failure rather than plastic or elastic failure.
Deformed rocks in the field
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Most apparent in sedimentary rocks
Importance of deformation
Determines past plate motions
Determines other past geologic events
Locates specific natural resources
Rock orientation: strike and dip
Folds
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Synclines: rocks folded into troughs; youngest rocks in the center.
Anticlines: rocks folded into ridges; oldest rocks in the center.
o Parts of a fold
 Limbs - sides of the folds
 Axial plane - imaginary plane dividing fold into two symmetrical
halves.
 Anticlines and synclines are structures in rocks, not surface
landforms
Types of folds
 Symmetrical or open folds - axial plane is vertical; halves are
symmetric
 Asymmetrical folds - axial plane is inclined; one limb dips more
steeply than other limb
 Recumbent folds - axial plane is close to horizontal
 Overturned folds - axial plane has rotated more than 90 degrees
 Plunging folds - folds are themselves tilted
 Basin - synclinal in cross-section at any orientation
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Dome - anticlinal in cross-section at any orientation (also known
as pericline or quaquaversal fold)
Plate tectonics and folding
Faults
 Fractures: any crack or break in the rock
 Joints: fractures with no relative movement
 Faults: fractures with relative movement
 Fault blocks: the rocks on either side of a fault
 Fault plane: plane on which displacement occurred
 Evidence of faults
 Visible displacement of rocks
 Pulverized rock
 Slickensides
 Discontinuity of rock sequences
Types of faults
 Strike-slip faults
 Example: San Andreas Fault
 Distinctive landforms (linear valleys, chains of lakes, sag ponds,
topographic saddles)
 Dip-slip faults
 Hanging wall - block on which a miner would hang their
lamp
 Footwall - block on which a miner's foot rests
 Normal faults
 Graben - downdropped block between two normal faults
that dip in opposite directions
 Horst - upthrown block between two normal faults that
dip in opposite directions
 Reverse faults - hanging wall moves up; footwall moves down
 Thrust faults - a reverse fault which has a very shallow dip (~ < 15°)
 Bedding Plane-The definition of a bedding plane is the line separating one
layer of compressed rock from the next layer of compressed rock.
Plate tectonics and faulting
 Normal faults: mid-ocean ridges and continental rifts
 Reverse and thrust faults: convergent plate boundaries
Exogenetic and Endogenetic Process
Exogenetic:
The process that operate outside the earth surface,called Exogenetic process.
e.g.Rain,Wind,Snow etc.
Endogenetic:
The process that operate inside the earth surface,called Endogenetic process.
e.g.Volcanoes,Earthquakes etc.
Weathering
The gradual breakdown of pre-existing rocks due to natural processes.
Types of Weathering:
(1) Mechanical Weathering:
Mechanical weathering is the process of breaking big rocks into little
ones. This process usually happens near the surface of the planet.
Frost Wedding:
Scientists have observed a process called freeze-thaw. That
process occurs when the water inside of rocks freezes and expands.
That expansion cracks the rocks from the inside and eventually
breaks them apart. The freeze-thaw cycle happens over and over
again and the break finally happens. Another word for it is frost
wedging.
Salt Wedding:
Another type of mechanical weathering is called salt wedging.
When it rains and water flows everywhere, it usually has ions and
salts dissolved inside. Have you seen salt water dry up? Salt
crystals are left over. Those crystals happen in nature, too. The
water flows in a rock and evaporates. Crystals slowly begin to grow.
Those crystals act like a wedge and crack the rocks.
Frost Heaving:
the upthrust and cracking of a ground surface through the freezing and expan
sion of water underneath. Alsocalled: frost heaving.
Onion skin weathering or Exfoliation:
Onion skin weathering is a geological process that occurs when a rock's
outer layer peels off due to extreme variations in temperatures. This type
of rock weathering typically occurs in desert areas where rocks are subjected
to extreme heat during the day, which causes rocks to expand.
When temperatures drop during the night, the rocks contract.
(2) Chemical Weathering
Chemical weathering occurs when rocks are broken down by a chemical
change.
Reactions such as oxidation, hydrolysis, and acidification can
happen when all of the elements are together. Oxidation makes
rocks softer. It is similar to an iron bar rusting. Since there is a lot
of iron in many rocks, oxidation often happens. Hydrolysis usually
causes rocks to expand and then mechanical weathering can begin.
These chemical reactions are happening all of the time. When you
see rocks next to each other that are different colors (often shades
of red) then you know chemical reactions have taken place.
(3) Biological Weathring:
Biological weathering is the weakening and subsequent disintegration of
rock by plants, animals and microbes
Erosion
The process of eroding or being eroded by wind, water, or other natural
agents.
Erosion is the general name for the processes that break down rocks
(weathering) and the processes that carry away the breakdown products
(transportation).
The physical processes of erosion are called corrasion or mechanical
erosion.
The chemical processes are called corrosion or chemical erosion. But most
examples of erosion include some of both corrasion and corrosion.
Types of Rocks:
Hydrophobic- that repell water.
Hydrophilic- that attract rocks.