Upper Flow Regimes

One of the aspects of sedimentary geology that I enjoy is the connection between sediment transport and deposition. Particularly the dynamics of the moving fluid (usually air or water) and its effects on the material being "pushed" along the base of the moving fluid.

Things like ripples in sand are a result of this dynamic relationship. I've got some more detailed posts on different kinds of bedforms, but today I want to share a video of one particularly intriguing type: antidunes.

Any bedform is going to be a byproduct of the depth and velocity of the moving fluid plus the type of material it is flowing over. For most sand, slowly flowing water will produce small ripples - you have probably seen these in the sand along the shore of a lake or river.

However, once you start trying to move fluids faster and faster over this surface, the fluid exhibits some interesting behaviors. The ratio between the velocity of the fluid to gravity and the flow's depth is referred to as the "Froude" number F_r. I've heard it pronounced "FROOD" and "FROWD." I like the sound of the former myself.

F_r = (mean velocity) / (g * depth)^1/2

For low velocities, or deep streams, this number is small (typically less than one). But, when a shallow stream is flowing very fast, it can be higher than one. When F_r < 1, it is referred to as "subcritical flow." When it is equal to 1, it is called "critical flow," and if F_r > 1, it is called "supercritical flow." Most ripples and dunes form as a result of subcritical flow. With supercritical flow, however, we start to get other bedforms, including antidunes.

Antidunes are relatively easy to create in an artificial flume. They are more difficult to see in nature:



The above video was taken by a friend of mine while in Hawaii this winter. Water was draining down the beach towards the ocean. The shallow stream started picking up speed and began to form antidunes - which appear in the flowing water as rapids that migrate upstream. Recognizing these processes in the sedimentary record requires some understanding of how they work. I'll have to spend some more time talking about sedimentary structures.

I want to thank Kelly McCullough for capturing the above video - my camera had started to act a little funny and I didn't get many good pictures. In this case, video was the right tool for the job. Kelly's a sci-fi author and all-around neat guy. You should check it out sometime.

Snails emitting light waves

I've been rather busy with teaching and a few other projects, so updates have been rather sparse and fluffy. I've been thinking about snails again (a reprieve from boring my wife with tales of wave functions and magnetic flux equations). Last year, quite by accident, I discovered that the shells of many of my snails fluoresce a dull orange under long-wave ultraviolet light.



The image above is of a set of Oreohelix sp. shells that I had collected a couple years ago. This is part of a rather large collection of a few thousand shells that I've amassed. I suspect they will lead to some additional posts and manuscripts in the next few years. One long-term study will be to investigate why these shells fluoresce a bright orange along parts of their shells, dull blue elsewhere, and on the inside emit this pale blue-green color. I've seen shells fluoresce in parts, and not others, but I have not observed the stark contrast in fluorescent regions before. I suspect it has something to do with the organic molecules bound with the calcium carbonate, but what the "activator" is, I have little idea. Some sources suggest that divalent Manganese causes an orange or red color, although how a snail could decide to use Manganese in one part of the shell and not the other. Nor have I found much in the literature that would suggest some cause of the light blue-green. My thoughts regarding the proteins as the culprit stems from the lack of strong banding on some of the more bleached shells.

For those of you that are not familiar with the term, fluorescence is the property some compounds possess whereby they absorb UV light. The UV light energy (wavelengths too small to be detected with our eyes) excites electrons in the outer orbitals of some "activator" atoms, causing them to jump to a higher energy level. As the electrons fall back to a lower energy state, they emit light energy at longer, visible wavelengths. A small bit energy is dissipated as heat, but the rest is emitted as visible light. Day-glo highlighters, blaze orange safety materials, and even laundry detergents contain fluorescing compounds (so the clothing emits more visible light and appears "brighter" when they come out of the wash). Even scorpions will fluoresce.

Easter Trilobites

I went fossil-hunting today, and picked up some trilobites. I promptly handed some of them out to friends and family. I think they make a much better gift tradition than chocolate bunnies and jelly-beans.

Brief update on the Geoblogosphere

I'm still making figures and editing photos for the Hawaii post. For now, here are a few items floating about the 'tubes:

• Ron Schott's sandy meme. Ron's got some nice photographs of sand from different places. His challenge was to tell him where it came from. I left a few guesses, but perhaps you have other thoughts.


• Callan's NOVA blog. Some Q&A with Michael Welland, the author of "Sand." Callan's students have asked some interesting questions about "sand."


• Clastic Detritus: anthropogenic forcing of fluvial systems. Brian has some cool pictures and discussion about human-induced changes in a big river system. Neat stuff.


• Michael Welland's Sandglass. Speaking of "sand," author Michael Welland wrote an entire book devoted to the subject. Being a clastically trained sedimentologist, sand is near and dear to my heart. I haven't read the book yet, but Clastic Detritus has a review and interview here and here.
  

Oh, and don't forget to cruise on over to the Hairy Museum of Natural History for some kewl links about fossils...

2009 in Review

I'm back from a nice long vacation in Hawaii. My wife and I figured that since we've been married a few years and now I finished my PhD, we should take our honeymoon (she finished her PhD just after we got married). I'd be hard-pressed to think of a better geo-holiday-cation than poking around Hawai'i, Maui, and Kaua'i for two weeks.

So I have many geology-themed Hawai'i posts in the works, plus lots of warm, tropical photos to regale you with. I took about 2,500 photos over 14 days. That's nearly 179 per day. It's no small task to sort through 14 gigabytes of holiday snaps, but I'll be sure to share some of my favorites. Here's one:


It's a 5-photo stitched panoramic view of Haleakala "crater." The crater is actually two wide erosional canyons that have joined together near the summit. There are numerous cinder and spatter cones along the crater floor, which makes for an other-worldly hike through this landscape.


I figure before I get too far involved in a self-indulgent Hawai'i geology fest, I'd wrap-up the biggest geology places/events that I experienced this year. I expect that I'll post the list now, and then spend a few posts going into detail.

So here's the list, ranked somewhat by personal importance:

10. San Raphael Swell: Coyote Gulch
9. Panther Tongue
8. Western Wisconsin Trilobites
7. Porcupine Mountains, MI
6. San Raphael Swell: The Wedge
5. San Raphel Swell: Wild Horse Canyon
4. Airplane Flights over the US
3. Grand Teton NP
2. Mt. Saint Helens/GSA
1. Hawai'i

And of course, I give pride of place to the fact that I finished my PhD this year. It's such a relief to be done. I do feel some measure of pride. However, when I see the work being done by people like Dr. Steve Austin (backstory), I realize that - in the end - it's just a piece of paper. Any schmuck can get one if they stick with things long enough. It doesn't convey me some magical power, or make my ideas any better than the next person's. But it does show that I am stubborn enough to see something through to the end, and outlast my peers. So I guess that's something.

On Vacation...

I've been rather busy having fun. I have lots of photos to go through, and some good geo-rific things to share. But that will probably have to wait.