Transcript STRESS, FAULTS, AND FOLDS Deformation

STRESS, FAULTS, AND FOLDS
Deformation is the bending, tilting, and breaking
of the Earth’s crust. Plate tectonics is the major
cause of crustal deformation.
Thicker and
heavier crust
sink deeper
into the mantle
where thinner
and lighter
crust will rise
higher on the
mantle.
The up and down movement of crust is balanced
by two pressures. One is the crust pressing
down and the other is the mantle pushing up.
The balancing of these two forces is called
isostasy.
The up and down movement of the crust to reach
isostasy is called isostatic adjustment. During
this adjustment the rocks in the crust are bent
causing deformation.
There are 3 basic kinds of stress that the isostatic
adjustment causes, compression, tension, and
shearing.
Compression occurs when crustal rocks are
squeezed together.
Tension is the force that pulls rocks apart.
Here rocks tend to become thinner.
Shearing pushes rocks in opposite directions.
Sheared rocks bend, twist and break.
The results of stress are folding and
faulting.
When a rock has stress put on it and does not
break it is called folding. Folds appear as wavelike structures in rock layers. Some folds are
small and can be seen in individual rocks and
some folds are huge and can only be seen from
the air.
The 3 general types of folds are anticlines,
which are upcurved folds where the oldest
rock layers are in the center,
synclines which are downcurved fold in
which the youngest layers are in the center
and monoclines in which both ends stay
horizontal but one side is lower than the other.
Rocks don’t always bend, sometimes they
break. When the rock moves and breaks it
is called a fault.
There are several different kinds of faults.
There are two sides to a fault. The side that
is above the fault plane is called the
hanging wall.
Hanging Wall
When the hanging wall moves down it is
called a normal fault. Normal faults occur
in places where there is tension or the rocks
are being pulled apart.
When the hanging wall moves up it is called
a reverse fault. Reverse faults are caused
by compressional forces.
A low angle reverse fault is called a thrust
fault because one side is being thrust onto
the other.
The last type of faults are called strike-slip
faults. Strike-slip faults slide horizontally
past one another.
If you are looking across to the other side of a strike-slip
fault and that side moves to the left of you it is called a
left lateral strike-slip fault. Strike-slip faults occur in and
around transform plate boundaries like where we live
near the San Andreas fault. This is also where shearing
takes place.