Structural Geology
Stereographic Projections
Surfaces, or a pole perpendicular to the surface, are
oriented by their intersection with the surface of a bowl
The Stereonet Grid.
The stereonet grid is used as a protractor, and in structural
geology, it is used to plot attitudes of planes and lines.
Plotting Planes using the Stereonet Grid.
The stereonet grid is a
protractor which can
measure strike and dip,
and trend (bearing and
and plunge) – the attitude
of lines and planes. Since
the equatorial plane is
horizontal, its edge (called
the primitive great circle)
measures the strike or
bearing.
The great circles can
measure dip or plunge.
Plotting Planes using the Stereonet Grid.
Plotting is done on
transparent paper laid
over the stereonet that
can be rotated around a
thumb tack poking up at
the center of the net.
A tick mark coinciding with
the 0 on the primitive
circle is labeled N. E, S
and W occur at 90, 180
and 270o respectively.
1: Plotting a Plane using the Stereonet
Suppose a sandstone layer has strike and dip N80W 40S
Visualize the plane in
space and the way it
cuts the sphere.
Plotting a Plane using the Stereonet
The layer is N80W, 40S
Place the overlay on the net
and mark N, S, E, & W on the
overlay.
With the overlay positioned
with N at 0o, Count 80o
CCW to N80W and place
a tick mark showing the
position of the strike.
Plotting a Plane using the Stereonet
The layer is N80W, 40S
Next, rotate the transparency
so the strike is at 0. Now
you can find the great circle
with the same dip (40) as the
layer by counting 40 from
the primitive circle along the
90 – 270 diameter.
Make sure you count in from
the primitive along the
equator
Now trace that great circle on
the transparency and you
have the projection of the
layer plotted.
Plotting a Plane using the Stereonet
The layer is N80W, 40S
Now, rotate the transparency
back so North on the
transparency coincides with
0 on the stereonet.
The plot now shows the
projected position of the
layer in space.
2: Plotting a Line using the Stereonet
The line is a fold axis at bearing S42W, plunge angle 38o
Visualize the line in space plunging 38o to the SW
Plotting a Line using the Stereonet
The line is a fold axis at S42W, 38o
Next, place the overlay on the
net and mark N, S, E, & W on
the overlay.
Locate S42W and make a tick
mark at that position on the
primitive circle.
Plotting a Line using the Stereonet
The line is a fold axis at S42W, 38o
Next, rotate the tick mark to
either of the straight line
axes of the net.
Measure 38o along the line
from the primitive circle and
put a dot. This is the
projection of the point where
the line penetrates the
sphere.
Plotting a Line using the Stereonet
The line is a fold axis at S42W, 38o
Finally, rotate the transparency
so that North coincides with
0 on the stereonet. Check to
make sure your dot is in the
SW quadrant.
3: Plotting a Line on a plane
The plane (cleavage) is N70W 43 SW or (290, 43) The line
(a mineral lineation) bears S20E or 160. Plot the plane and
the line and find the plunge of the line.
Visualize the problem:
A plane (cleavage)
with a line (lineation)
lying in the plane.
Plotting a Line on a plane
The plane (cleavage) is N70W 43 SW or (290, 43) The line
bears S20E
Plot the plane on the
overlay: Put a mark at 290
Plotting a Line on a plane
The plane (cleavage) is N70W 43 SW or (290, 43) The line
has bearing S20E = 160o
Next rotate the overlay to 0
or 180, then count in 43o
along the equator to find
the great circle that dips
43o to the SW. Trace the
arc.
Plotting a Line on a plane
The plane (cleavage) is N70W 43 SW or (290, 43) The line
has bearing S20E = 160o
Rotate the overlay so that N is
at 0. Mark the primitive at the
bearing of the lineation, 160o.
Plotting a Line on a plane
The plane (cleavage) is N70W 43 SW or (290, 43) The line
has bearing S20E = 160o
Rotate the overlay so that the
160o mark is at the 0-180 axis
of the net. The line from the
center of the net to the 160
mark is the trend of the
lineation. Where that trend
line intersects the arc of the
plane is the projection of the
lineation that lies in the plane.
The plunge on the lineation is
measured from the primitive to
the point on the arc.
Before we go on, let’s
review Rake (aka Pitch)
The Rake of a Line on a plane
The plane (cleavage) is N70W 43 SW or (290, 43) The line
has bearing S20E = 160o
We plotted the plane and the line.
a. Rotate the transparency so the plane lies along a great
circle.
The rake is the angle measured from the primitive along the
great circle, here 58o.
b. Return the
Transparency to
North = 0o
4: Apparent Dip knowing True Dip
The attitude of a dike is 205, 65 (N25E, 65SE). What is its
apparent dip on a vertical quarry wall trending 350?
Visualize the problem (a) and plot the dike (b)
Apparent Dip knowing true dip
The attitude of a dike is 205, 65 (N25E, 65SE). What is its
apparent dip on a vertical quarry wall trending 350?
Plot the quarry wall [c]. The intersection of the dike with the
wall defines a line parallel to the apparent dip.
Apparent Dip knowing true dip
The attitude of a dike is 205, 65 (N25E, 65SE). What is its
apparent dip on a vertical quarry wall trending 350?
d. Rotate the transparency so the wall is along the axis. The
apparent dip angle is from the primitive to the intersection,
52o.
True Dip from two Apparent Dips
Question: A layer intersecting a road cut trends 256, 32. The
same layer intersects the road cut around a bend and
trends 125, 27. What is the strike & dip of the layer?
Visualize the problem (a) and then plot the intersections for
the road cuts and the layer (b & c)
Mark 256, rotate it to vertical
Count in apparent dip 32o
Mark 125, rotate it to vertical
Count in apparent dip 27o
True Dip from two Apparent Dips
Question: A layer intersecting a road cut trends 256, 32. The
same layer intersects the road cut around a bend and
trends 125, 27. What is the strike & dip of the layer?
Because the two road cut intersections lie on the dike, rotate
the transparency so the two points line on the same great
circle (d). Draw the great circle.
True Dip from two Apparent Dips
Question: A layer intersecting a road cut trends 256, 32. The same layer
intersects the road cut around a bend and trends 125, 27. What is the
strike & dip of the layer?
That great circle is the attitude of the dike; measure its dip
along the equator from the primitive to the great circle (d).
Now rotate the transparency so that its N coincides with 0 on
the net. Measure the strike and note its dip direction (e),
here about N77W 54S