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.

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).

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.

Optical Illusion Offset

The other day while measuring joint orientation in the Weverton formation at Thoroughfare Gap, I stumbled upon this felsic vein down section in the Catoctin. We had already seen some minor faulting in the outcrops leading up to this one so it wasn’t a big surprise to find another one. From this angle it looks like the vein was offset laterally to the right, but when it is viewed from another angle the offset disappears. Just a case of tricky jointing.

Here is the vein looking offset…

…and here the offset is gone.

This could be a good lesson in seeing everything in 3D space, and “apparent” vs. “true”. Enjoy the video below as another example.

My AGU Day 1

I have decided to throw my contribution onto the pile and talk about my personal experience at the AGU Fall Meeting. Yesterday was my first day at the conference, and actually my first day at any large geologic conference period. The excitement I felt before arriving quickly turned to being overwhelmed once I saw the line of people waiting to register. I should have been tipped off about the massive amount of people that would be here once I started spotting people with hiking boots and posters in the Detroit airport. I don’t think I gave enough weight to just how many people attend this meeting, something that was driven home further when I went to look at the posters.

So many posters. So many people looking at posters.

I spent the first half of my day walking around with my friend Kristy looking at posters and trying to acclimate myself to the surroundings. I also played a game of Name Tag Bingo in my head by filling out my card with organizations and schools from across the world. I won every time. I do think it is funny that everyone stares at your name tag as you pass. My eyes are up here, guys.

(I am very guilty of name tag glaring)

Luckily the overwhelming size and lack of personal organization dissipated in the afternoon. I attended the section of “Water at the Ice Bed Interface” and watched a handful of very interesting talks. I personally enjoyed a strong yet concise talk given by Neil Humphrey from the University of Wyoming concerning subglacial channel flow connectivity. It was also interesting sitting across the aisle from people like Richard Alley whom I had watched give congressional testimonies and host his own television show.

Afterwards I was able to have inspiring conversations with faculty about research and where the science of geology may be going in the future. While I missed out on the Social Media Soirée, a fault of my own doing, I am reenergized and excited to head back today and do it all over again. I have more sections planned for this morning and will see how the rest of my day plays out from there.

*Please excuse some of the writing in this post. It is pretty early and I am writing this on a phone.

Memorial Pegmatite

Being both a bicycle and monument enthusiast, recently I have been combining the two and taking a self-guided bike tour of my hometown, Washington, DC. Just for kicks I am putting together a personal collection of pictures of my bike with the monuments. Though I see the monuments often I still enjoy them and appreciate getting to share them with people for the first time. Yesterday I made my way over to a few memorials including the FDR Memorial along the Tidal Basin. I remember when this particular one opened up and enjoying it because it is a collection of waterfalls and statues, and that it brought a different approach than the other monuments in DC. While there, a few of  the rocks found in the walls jumped out at me; in particular this pegmatite dike.

*Bike as a sense of scale borrowed from Alan at Not Necessarily Geology

While the surrounding parent rock is pretty evenly distributed between minerals this dike is visibly dominated by feldspars, most notably the bright pink orthoclase. The other mineral looks to me like plagioclase with a green tint to it. I felt it might be frowned upon to scratch the memorial, but the two cleavage planes are apparent. Below a crystal of plagioclase has fractured and infilled with quartz.

Unfortunately I don’t know the locality where this rock was taken from, but I can appreciate the unique aspect and added beauty it brings to the memorial. Keep an eye out the next time you are visiting the monuments maybe there are some other hidden geologic gems to be found, and if you want a tour send me a message I am always willing to take another ride around the city.

California Folds

Here is a picture my friend Heather sent me on her drive out to California a few weeks ago. These chevron folds are so impressive she had to send me a picture. The picture was taken near the Calico Ghost Town in Southern California east of Los Angeles. In this area the rocks are Miocene aged nonmarine sedimentary rocks including sandstones, shales, and conglomerates which would explain the pronounced differential weathering. Each cliff is about 10 – 15 meters tall. Notice how the folds continue on the outcrop in the distance. I may need to make a trip out west to check out some of these folds…and ghosts.

Thanks Heather!

Glacial Feature: Cirques

This past summer I found cirques to be one of the more visually outstanding and impressive features. The appealing visual aspect may have to do with the propensity of cirques to be found in mountaintop scenery, or maybe even their juxtaposition of inward depressions next to jagged peaks. At the same time, it is hard to see one and not consider their geologic significance.

As with many of my recent adventures and subsequent blog entries, my first known encounter with this particular glacial feature was in Ireland along the western coast, though not the coastline itself. Our glacial project found us in the middle of Big Daddy Cirque near the waters of Doo Lough in county Clare; a beautiful piece of countryside with an unfortunate and sad history. The magnitude of the location resonated with me enough so that I found myself actively searching them out along the hike through the Alps, where there were plenty more to take in.

Like I mentioned above, cirques are glacial features found in alpine settings and often near the mountaintop itself. The feature is an eroded out, bowl-shaped area that was the birthplace of an alpine glacier. As moisture travels over the mountain, if cold enough, snow will collect on the more shaded side and sit. As long as more snow accumulates than melts it begins to compact from the pressure forming into ice, and eventually a glacier. Cirque glaciers are constrained in their boundaries which are defined or restricted by the topography. Therefore as the snow collects and compresses the ice pushes back into the mountain creating an amphitheater type feature. Below is a picture of Big Daddy Cirque taken from the lip of the cirque, and another taken from afar.

In the close up photo the curved walls and flat bottom are apparent from where the rounded glacier sat. In the second photo the scope of the feature is revealed and the overall “bowl” shape is more noticeable. Here is an annotated version of the second photo highlighting the lip and top of the cirque. The blue X marks where the first photo was taken, and the red arrow points “inward” towards the center of the cirque.

Cirques were formed as erosional features as the alpine glacier propagated from the center and dug into the mountainside carving out a rounded surface. The conventional thought was that movement of the glacier was from sliding and rotating in the recessional surfaces of the mountain. But, in a paper published in the October 2010 issue of the American Journal of Science, a team of glaciologists suggest that internal shear of the ice is the driving factor behind propagation, and the rotation is also a result of internal deformation. Basal sliding from base meltwater does occur, though, only at a minimum.

To help visualize the process I have annotated some pictures of a cirque I found in the Alps. Here is the cirque as it exists today:

The annotations are a little hard to see so feel free to follow the link to a larger image. Thanks.

Moisture travels over the mountain and collects as snow and ice on the shaded side of the mountain.

As long as the rate of accumulation of snow is greater than the rate at which it melts (ablation zone) then the snow will compact under pressure to form a glacier. The glacier erodes away at the mountain creating the amphitheater shape depression.

Deviatoric stress from pressure is the driving force that propagates the glacier further down the slope out from the center, and the glacier continues to erode the landscape.

While cirques are significant for being the starting point of glacial expansion, they are just as significant for being the last gasp of glacial retreat. Just as the glacier propagates outward from the cirque, as the glacier melts it regresses back to the same spot before melting away. Evidence for this retreat is seen in features like recessional moraines. So, looking at the cirques above we can appreciate the handy work of a glacier that used to be.

References:

Sanders, J.W., Cuffey, K.M., MacGregor, K.R., Kavanaugh, J.L., and Dow, C.F., 2010. Dynamics of an Alpine Cirque Glacier. American Journal of Science, v. 310, pp. 753 – 773.

Do You See What I See? #2

After another one of my trademark hiatuses, I am now back and hopefully more composed, motivated and ready to post with more frequency. Forgoing anymore excuse ridden sentences let’s jump right into the geology.

It’s time for the next installment of “Do You See What I See?”. This time I’ll give you two pictures two consider; both of the same feature but taken at slightly different angles. Careful though, this is not going to be hard rock geology (big hint).

This feature was found at high elevations in the Alps Mountains while I was traversing this past July. My friends and I had just hit the summit and were now on the arduous decent into town miles and thousands of meters in elevation away. Where this picture was taken though was along a relatively nice and flat portion of the trail (another big hint). What we stumbled upon here were a couple of recessional moraines created by sediment accumulation at the edge of a preexisting glacier. Here are some annotated pictures to help visualize. The numbers on the moraines dictate the order in which they were deposited.

The name moraine actually originated in the Alps to describe ridges of debris found at the edge of glaciers. As the glacier that was previously here retreated during melting, the sediments within the glacier were transported and dumped along the terminal edge from meltwater within the ice. These sediments, described generically as till, are various in grain size and if the moraine was cut open it would be seen that the sediments are unstratified. There are a few types of moraines that also include terminal and lateral. All moraines are found along the edges, but the different varieties describe where or when they were deposited there. Lateral moraines are found along the lateral edges of the glacier since debris flows there as well. Terminal moraines are found along the downstream edge like recessional moraines, but are deposited on the front of advancing glaciers. Because of the advancing glacier the terminal moraines are often eroded away by the very feature that put them there in the first place.

If you had stood where this picture was taken and looked back towards the summit you would see that the trail takes you down through a large cirque where the depositing glacier originated.

More Pictures from Ireland

Here are some more pictures from my geology field camp for everyone to enjoy. The weather has been even more rainy than usual according to the locals, and I am starting to feel a bit soggy. Nonetheless it is a beautiful country and I am very happy to be here.

Enjoy the pictures.

Great s-folds in schist

Very well defined xenolith in this rock

Big Daddy Cirque near Doo Lough

Look at these great en echelon gashes!

Hopefully I will be able to take more pictures when it stops raining. When I do I will put them up here.

Take care.