Olivine Sand

All this talk of Hawai'ian volcanoes has reminded me that I have some photographs of olivine sand (from somewhere near South Point, I suspect). I suppose I could talk about the mineralogy, or the grain shape and rounding. Or I could describe the method I use to stack multiple images together to increase apparent depth of field in the image. For now, I'll just post the image:

(click to embiggen)

Sand...

I've been measuring the specific gravity of sand. More on this later, but here are the preliminary results:

Well that was different...

So, it's been a kind of a weird few days. My friends Kelly McCullough (cool author), his wife Laura (cool professor colleague), my wife and I headed over to a well-known author's house. Because this author had a lamppost. In the woods. And Laura and Kelly wanted their picture taken next to it.


Did I mention Laura and Kelly were in costume? Yeah, they were.

The entire trip was quite surreal - the author's house is situated next to a creek. I found myself seeing two worlds. The first was my world. The landscape of my day job. House: located on a high terrace. A path descends into the stream valley and the lamppost was situated on a lower terrace overlooking the stream. The second world was full of goblins and fairies and the innumerable fey creatures that rattle around in our heads when the sun is low in the sky. As we walked down the path, I felt that we were leaving the gatehouse and crossing some great threshold to descend into a place much further removed. Having two great big white dogs, a witch, and a faun.



After the photo shoot, we scrambled back up to the world of terraces, stream incision and a warm kitchen where we got to chat with the owner of the lamppost. I came home, posted the images on flickr, and then pow. Thousands of people started looking at these pictures. Orders of magnitude more people than usually view my stuff. The author tweeted and then blogged about the adventures with his lamppost - I felt like I was trying to drink from some internet firehose.

I found out that his website has its 10th birthday tomorrow. I wanted to say "thank you" for the mention, the tea, and especially the lamppost. I figured that a little note would be the appropriately midwestern thing to do. He gets a lot of sand in the mail. I find this rather amusing (although his assistant certainly does not), since I often play the role of sedimentologist in real life. But I don't think he needs any real sand. So I set up my microscope and camera attachment to take a picture of "sand." Digging around, I found some garnet sand. Garnets are considered a traditional "birthstone" for websites born at this time of year.

Happy Weblogday, Mr. Neil!

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.