Thurs-Demo: The one from several years ago

I built this mini-basin a few years ago when I taught a sed strat course:

Wheeler Diagrams from The Mini Basin from Matt Kuchta on Vimeo.

It's based on the Desktop Delta designed and published by Thomas Hickson (University of St. Thomas) - additional info available at the SERC website.

Now that the semester is in full swing, I'll start working up some new demos. The philosophy behind these demos is that 1) they show an interesting/abstract geoscience concept and 2) don't cost too much. Typically, these demos can be made for under $20 (although sometimes I'll trick them out with some additional flapdoodles (like I did with the Earthquake Machine).

We don't need no stinkin' sieves

I've been setting up for Thursday's demo. I thought I'd show you how I often separate sand and gravel so that I have enough fine sand for some of my other projects.

Gravel Separation from Matt Kuchta on Vimeo.

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.

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.