Structural Geology Stereographic Projections

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

Visualize the plane in space and the way it cuts the sphere.

Suppose a sandstone layer has strike and dip N80W 40S

Plotting a Plane using the Stereonet

With the overlay positioned with N at 0o, Count 80o CCW to N80W and place a tick mark showing the position of the strike.

The layer is N80W, 40S Place the overlay on the net and mark N, S, E, & W on the overlay.


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.

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.

The layer is N80W, 40S

2: Plotting a Line using the Stereonet

Visualize the line in space plunging 38o to the SW

The line is a fold axis at bearing S42W, plunge angle 38o

Plotting a Line using the Stereonet

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.

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.



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

Visualize the problem: A plane (cleavage) with a line (lineation) lying in the 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.

Plotting a Line on a plane

Plot the plane on the overlay: Put a mark at 290

The plane (cleavage) is N70W 43 SW or (290, 43) The line bears S20E


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.

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.



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)

http://en.wikipedia.org/wiki/File:StrikeDipPlungeRake.png

The Rake of a Line on a plane

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 DipThe 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 dipThe attitude of a dike is 205, 65 (N25E, 65SE). What is its


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 dipThe attitude of a dike is 205, 65 (N25E, 65SE). What is its


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 DipsQuestion: 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 DipsQuestion: 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 DipsQuestion: 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

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Structural Geology Stereographic Projections