Download 10) Folds and Faults Notes

Folds and Faults
Rock Stress
Folding
Faults
Isostacy
Forces and their effect
There are two things that are important when
dealing with to forces molding the earth’s
surface: rock stress and rock strain.
Rock stress are the forces that can act at the
earth’s surface
Rock strain is the effect on the rocks at the
surface as a result of those stressors
If you think of a spring: stress would be the
weight pulling down on the end, strain would
be the stretching of the spring.
Stress
Stress is the force applied to rocks due
to moving of the Earth’s plates.
There are 3 types of stress:



Compression – occurs at converging plates;
rocks generally thicken and shorten
Tension – occurs at diverging plates; rocks
generally thin and lengthen
Shearing – occurs at transform plate
boundaries; rocks are sliced into parallel
blocks
Strain
When anything is subjected to a force,
that object is deformed due to the
stress.
When rocks are subjected to stress,
they can either bend (fold) or break
(fault).
Whether the rocks fold or fault depends
upon the elasticity of the rock.
Elasticity
Something that has a high elasticity will take a lot of
force and can deform in any direction then spring
back to its original shape
Something that has a low elasticity will take some
force but cannot deform very much before it reaches
its breaking point.
All objects whether they are rock or elastic bands
have two different points: a deformation zone in
which they have been strained to the point where
they will not return to their original shape, and a
fracture point where the strain has reached a level
that breaks the object in two.
Folds
Rocks fold due to compression at
convergent plates.
The most common types of folds are
tightly folded arches and troughs.
The arches are called anticlines.
The troughs are called synclines.
Anticlines and Synclines
anticline
syncline
Faults
Easily mistaken to be plate boundaries, faults
are often found near and caused by
boundaries
A fault is a crack in a rock where the rocks on
either side of the crack slide and rub against
each other
There are 2 main types of faults:


Strike-slip faults
Dip-slip faults
Strike-slip Faults
occur at transform plate boundaries
where the fault blocks (rocks on either
side of the crack) move horizontally
past each other.
Ex. the 1000 km long San Andreas fault
in California.
San Andreas
Fault
Dip-slip Faults
In dip-slip faults the fault blocks move
vertically past one another.
The block above the fault is called the
hanging wall.
The block below the fault is called the
footwall.
footwall
hanging
wall
Dip-slip Faults, cont’d
There are 2 types of dip-slip faults:

Normal dip-slips – the hanging wall moves
downward. This is associated with
tensional stress.
Normal Dip-slip
Normal Dip Slip
Dip-slip Faults, cont’d
There are 2 types of dip-slip faults:


Normal dip-slip – the hanging wall moves
downward. This is associated with
tensional stress.
Reverse dip-slip – the hanging wall moves
upward. This is associated with powerful
compression.
Reverse Dip-slip
Reverse Dip Slip
Fault Summary Table
Fault Type
Stress
Plate Boundary
Fault Summary Table
Fault Type
Stress
Plate Boundary
strike-slip
shearing
transform
normal dip-slip tension
diverging
reverse dip-slip compression
converging
Isostasy
Isostasy is an equilibrium or balance of
all rock and ice formations on the
surface of the earth.
Think back to the earth’s structure
What makes earth a layered or
differentiated planet?
Floating Mountains
The crust material is less dense than
the upper mantel or asthenosphere
material, so it floats on top like an
iceberg.
Isostasy
Isostasy is responsible for much of the
earth structures we see around us.
Isostatic adjustment and crustal
rebound are just two ways in which
isostasy works to create the mountains
and valleys we see around us.
Handout
Go to pg 356 in your text to answer the
questions on rock stress
And go to pg 424 in your text to answer
the questions on isostasy.