At first glance, a stereonet appears to be graph paper gone awry. In fact, it is an elegant and powerful tool for collapsing a three-dimensional universe into a two-dimensional one, and performing calculations on it. It is used in geology because analyzing geological structures involves solving three-dimensional geometry problems. On this page you will find resources to get you started using a stereonet, and to help you with applying the stereonet to solving common problems.
Videos by Jacqui Houghton
How to find the fold axis and axial plane of a fold from bedding data video by Jacqui Houghton
This video shows how to plot a plane with a strike of 030° (or N30ºE) and a dip of 50°W. Geological structures that are represented as planes include bedding surfaces, faults, and cleavage. A quarry wall or a road cut would also be plotted as a plane.
Lineations are structures with one dimension much longer than the others. A sheet of paper is a good analogy for a plane, but a pencil is a good analogy for a lineation. Lineations that you might plot on your stereonet include slickenside lineations on fault surfaces, the intersection between two planes (e.g., when cleavage cuts bedding), apparent dips, mine shafts, and drill holes. In the video, a lineation is plotted with a plunge of 31º and an azimuth (or trend) of 256º. If you are unsure of whether the feature you are plotting is a lineation or a plane, one way to tell is from the notation. Usually, the orientation of a lineation is written in the order plunge/trend, whereas the orientation of a plane is written in the order strike/dip, but this isn’t a hard and fast rule. A more reliable way to tell is that dips always come with a dip direction (e.g., 31ºNW or 31ºSE) but plunges do not.
A pole is a special kind of lineation that is perpendicular to a plane. A pole is like the mast on a ship, which is always at right angles to the ship’s deck (a plane). The pole to a plane usually does not represent a physical structure. Instead it is used for convenience, because the task of analyzing the orientation of a collection of planes is much easier when those planes are represented on the stereonet as a tidy collection of dots rather than as an unwieldy yarn-ball of curves.
The two sets of parameters that tell us the orientation of a fold are (1) the strike and dip of the axial plane, and (2) the plunge and trend of the fold axis (the hinge of the fold). If a fold is small and the hinge is visible, then you could measure these parameters with a compass and some squinting. If it isn’t possible to get direct measurements, then you can still get data about the orientation of the fold by measuring the strike and dip of the folded beds at different locations along the fold limbs.
This video begins at the point where the poles to the bedding planes have already been plotted. From there, the steps are:
- Find the plane that best fits the poles. This is sometimes called the π-circle.
- Find the pole to the π-circle. This pole is called the π-axis, and it represents the fold axis.
- Find the midpoint of the spread of bedding plane data along the π-circle.
- Find the plane that connects the midpoint to the fold axis. This is the axial plane.