Fun with Stereonets: Finding the Axial Hinge

Since I graduated at the end of the Fall semester I have had the opportunity to be the TA/Lab Instructor for George Mason’s Structural Geology class. While I could spend a whole post discussing the rewards of being on the other side of the desk I will instead go over one of the prominent topics discussed in the class: stereonets.

For the sake of time I am going to make a large assumption here and proceed like anyone reading this has a basic understanding on what stereonets are and how to use them.

What I am hoping to accomplish with this blog entry is giving step by step instructions on how to find the axial hinge of a fold by using stereonets. Since we create them manually in the lab using Equal Area stereonets and tracing paper, I will explain the steps the same way.

Let’s say we have a fold with the all the necessary parts: two limbs, an axial plane, and the axial hinge. Let’s also say we already have the strike and dip measurements for each limb with Limb A oriented at 225, 66 and Limb B oriented at 016, 50. Here is a picture of just such a fold.

With these two measurements we can determine the orientation of the axial hinge without having to climb inside and attempt to measure that thing upside down.

First we need to plot the two limbs, and knowing that fold limbs are planar features we’ll mark them down as great circles on our stereonet. We start by marking 220 degrees for Limb A.

Next we need to rotate our “tracing paper” so that our 220 mark is oriented to the North. From here we can find our dip measurement of 56 degrees on the horizontal axis, and then draw in the associated great circle.

Super. Now we rotate the tracing paper back to its original position, and our Limb A is plotted.

Now we need to plot Limb B of the fold by following the same process: mark 016 degrees, rotate to North, mark the dip and draw the circle.

Understanding that the axial hinge is the point of maximum curvature or bending in a fold, it is going to be located at the intersection of the two limbs or great circles in this case. Since the intersection of two planes forms a line, we will plot the hinge as a dot on the stereonet.

Here are the two limbs plotted together.

Let’s go ahead and mark the point of intersection between our two circles.

From here we can check the the direction of the hinge by looking at its location on the stereonet. Looks to me like it is trending at 032 degrees. To find out how much the hinge is plunging, if at all, we can rotate our tracing paper one last time to the horizontal axis and measure it there.

According to our stereonet the hinge is about 18 degrees from horizontal. Therefore our antiform is trending 32 degrees from North and plunging 18 degrees. (T&P:  18 –> 032)

I hope this was helpful. If so, maybe I’ll explore some other uses of stereonets later.

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Aerial Geomorphology (with snow)

Last week Callan Bentley decided to go digging through some old images and his action inspired me to do the same. Fortunately for me, I knew exactly the purpose of the images I had set aside. I took this picture with my iPhone while flying back from the AGU Meeting in San Francisco last December, and was struck by the saw-tooth appearance seen in the landscape.

What I am describing as “saw-tooth” appearance are the eroded V-shaped notches in the bedrock and the snow. Using what a professor of mine described as the “Power of Geomorphology” we can infer a few things about this landscape. The orange annotated notches below all point to the left side of the picture indicating that the homoclinal ridges are dipping the same direction. The fact that the same phenomena is shown in the snow covering is just a nice added bonus.

Highlighted in blue is the dendritic drainage pattern indicating that the melt channels are working their way into a homogeneous material, in this case snow, and heading down slope to the east (since I was facing south when I took this picture…I think).

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Glacial Feature: Cryoconite Holes

The other day while reading a paper on the Dry Valleys of Antarctica I came across a new (to me) geologic feature: cryoconite holes. These vertical holes are often found in the ablation zone of glaciers and form in an interesting way. Aeolian sediments collect in little melt pools which then increase the absorbency of solar radiation relative to the surrounding glacier. The increased rate of melting created from the solar radiation forms these long, cylindrical features that are filled with water. In areas like the Dry Valleys the cold air temperatures will freeze the surface giving the hole an ice cap. It’s a self formed bottle of water.

Below is a sketch I made of the formation process while reading about them.

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