Inspired by a recent search string

I was looking through my visitor log, and while I don't have as many weird and wonderful search engine hits as other blogs, I did see a phrase that fits with some of the sedimentology topics I've been thinking about for a blog post. This particular phrase:

Why does wet beach sand look dry in front of your footprint?

Is one of those fun, yet hard-to-conceptualize questions that requires a little understanding of how granular materials behave. In particular, the behavior of saturated sand. The process that makes the wet beach sand look "dry" when you step on it is called dilatency. You step on the saturated sand and some of the sand gets "squished" beneath your feet. Some of the sand is compressed downwards - but, due to the granular behavior of sand, some of it is forced sideways (in a process called "lateral yielding") and this sand forces some additional grains to get shoved upwards. The spot away from your foot is not being forced down, so it's easier for the sand to be pushed upwards to make room for all this sand forced out sideways.

The small spaces (interstices) between sand grains that are forced out and up become larger (dilatency) - more space between grains means that the water can't fill those other spaces and the sand appears dry. See the illustration below:


Since water fills all the open space between the grains, the sand appears dark and wet. You push down on the sand with your foot, and force the sand outwards:


The grains forced out and up now appear dry - there isn't water between those grains (thanks in part to gravity and the lack of sufficient surface tension in water to pull it up into those spaces). If you stop moving, or take your foot away, you may notice that some of the sand "sinks" back downwards as the grains re-arrange themselves again, allowing the grains to settle together and the water re-fills those empty spaces.

If you try this on mud - it doesn't have the same effect. Clay particles behave quite differently compared to sand-sized particles. But that's a post for another time. For now, it's important to note that the behavior of granular materials like sand depend largely on the contacts between individual grains. This "internal friction" helps sand stay in a pile (it's angle of repose), but when they are saturated (when all the voids are filled with water), this friction essentially disappears. The sand is now very susceptible to external stresses (like your feet) and move easily. Thus, when anyone claims that the Coconino (and other large sandstone formations) was deposited in sea water, then eroded prior to lithification has either not walked on the beach, or does not fully understand the behavior of saturated sands.

Which reminds me - I need to spend some time talking about things like "lithification" and "consolidation" since these terms are often mis-used or conflated. They are two different processes - sometimes related, but they imply rather different behaviors.

Origin's Sesquicentennial

150 years ago today, Darwin published "The Origin of Species." It was one of those seminal moments in science where an individual managed to put into words how we might understand the way the world works. I don't think Darwin got everything "right," nor was he working in a vacuum. His insights were built on observations both of his own and others. But he did eloquently illustrate the concepts of natural selection and how it might work and explain the diversity of life we see today.

It's too bad we have so many people (some of them well-intentioned, some of them much less so) opposed to the concepts that have been borne of this work.

Ornaments

Another bonus of using strong, tiny magnets? Putzing around with 'em to make geometric shapes.



Happy holidaze. Bonus: they spin quite freely.

Chaos and Sedimentology

I'm rereading the book "Chaos." Last time I read it I was in 10th grade, but now I've got a little more math background, so the underlying mechanics are easier for me to describe.

A few things have leaped out at me:

1. The concepts related to chaotic systems, such as the "Butterfly Effect" didn't take long to enter and become part of a "public consciousness" regarding the world around us


2. I'm not sure if this is because I've not finished rereading the book, or its place as an early summary of the work, but I think they're overselling the concept of "dependence on initial conditions."


3. One of the more powerful ideas here is in scale independent behavior - especially from a geologic standpoint (see the post in Clastic Detritus regarding sediment plumes in the GoM for an example).


4. For my own scientific work, I think I may need to change my thinking about chaotic systems and fractals.

I think more attention needs to be paid to the "boundary conditions" controlling chaotic systems. Especially with regards to sediment deposition and transport. While a turbidite displays chaotic behavior, there are often predictable results from these systems - Bouma spent a great deal of time describing the general pattern often seen in turbidite deposits. This holds for other sedimentary systems: you may see all sorts of variation within an eolian deposit, but you're also not likely to see certain things. So while any particular eolianite may display a wide variety of textures and internal structures, there are specific patterns that will usually show up (not every single time, perhaps). The system may be chaotic and display variety of internal features, but you're not likely to produce a diamict, or massive clays as a result of wind-deposition. The mechanics and source of materials form some of these boundary conditions.

Chaos and fractals are fun stuff. But, the concepts are abstract enough that it's easy to misunderstand and apply them. I'll be exploring some of these features in future posts.

Magnetic Sediments

I've been poking around with magnetic sediments recently. There's a bit "to-do" going around Quaternary geology circles regarding a potential bolide impact during the Younger Dryas -YD - (ca. 13,000 years before present - YBP). Whether or not this actually happened, and whether this has any lasting climate and ecological impacts have yet to be sorted out. However, the presence of magnetic materials in high concentrations in sediments from the YD is intriguing purely from an academic standpoint. So I've been poking around the late Pleistocene (ca. 20,000 to 10,000 YBP) sediments in the UMV, looking at what's in the magnetic fraction.

So far things look interesting - no confirmation of micrometeorites or other impact debris, but definitely lots of magnetic materials in certain layers. The difficult part is going to be analyzing these grains under an electron microscope or some big machine to see what they're made of.

Here are a few pictures of the extraction process (these are slightly modified from A. West's methods link to PDF)

The bulk sample - the small bucket contains another sample that I dried at about 100°C to determine moisture content of the portion soaking in water.

The basic method involves sticking some strong magnets in a bag...


...then placing the covered magnets into the wet sediment slurry to pick up magnetic grains.

Withdrawing the magnet reveals a big batch of magnetic grains stuck to the outside of the bag. These are placed in another container of clean water. Pulling the magnet away allows the grains to fall into the clean water.

Some of the extracted grains. Once the water evaporates, I can pick out individual grains (with a small, moistened brush or needle).

I haven't started extracting individual grains yet - I'm in the process of refining my extraction and separation methods - doing this all by hand is both slow and not as systematic as would be preferred.

Rainbow

Nothing much to say right now - I have a lecture about soil shear strength I need to prepare. I plan to put some of the points about the strength of unlithified materials into another discussion about sediments, but for now here's a lovely rainbow I saw from my front steps this summer: