Brittle vs. Elastic

Okay, so we're wading into some rock mechanics with the intro class. One of the most important characteristics is how rocks respond to stress. If you've ever hit a rock with a hammer, you're probably aware of brittle behavior - where the rocks break into smaller pieces. If you've got the right kind of rocks (typically rocks under higher pressures and temperatures - like you see deep in the crust/mantle area) they might bend and flow. There are some other things that affect this brittle/ductile behavior. How quickly the stress is applied, for example. Hit the rock with a hammer and you might get broken pieces. Apply the same stress over a very long time and you might get the rock to bend.

This is rather hard to do with rocks, however. Silly Putty is a much more convenient material for those of us stuck on the earth's surface. Pull the silly putty slowly and it will stretch and flow. Pull on it quickly and you might get it to snap and break. Or, you could hit it with a hammer, which is not as easy as it sounds...

AW#43: Geological Illustrations

This month's accretionary wedge is hosted by "In the company of plants and rocks," which is a great title for a blog, BTW. The theme is "favorite geological illustration" and I've been having some trouble settling on one illustration. So I'm going to provide three.


This first illustration is not so much about the actual illustration, but the ideas it conveys. One of the great aspects of plate tectonic theory is that it explains so many different geological phenomena. It explains the patterns that we can observe and it allows for very powerful predictions regarding evidence (data, outcrops, etc) that we have yet to discover, and events that have yet to occur. This simple idea that certain parts of the earth's crust can be in motion relative to other parts of the crust was wildly revolutionary - it took many separate lines of evidence to pull together our current ideas regarding plate tectonics. Some 100 years ago when Wegner suggested continents plow through the ocean crust like giant icebreakers, he was criticized for lacking a mechanism to provide the necessary forces and allow for the crust to move. But today, we take this cross-sectional view of ocean crust subducting beneath continental crust as second nature. Yet just 50 years ago it would have been considered somewhat fancy and "cutting edge."




This second illustration is from my dissertation chapter about sedimentary patterns in the upper mississippi valley. If there's something other than a cross-section to get a geologist's blood going, it's a block diagram. It's like someone used a giant pie knife to carve out a block of the earth and hold it up for all to see. This particular diagram represents a great deal of sweat and tears on my part to finish my PhD.



This last example (one of my absolute favorites) is a cartoon my dad did for me right after I started working on my PhD. I think he found the idea of "Ice Age Land Snails" amusing in general. Because who wouldn't want to see giant land snails roaming the frozen tundra, attacking cavemen? I think the comment I like most is that he had a hard time making the radula (snail's rasping mouthparts) look threatening.

Some more fluid-y stuff

Drop a steel ball with a hydrophobic coating into water and watch what happens:

Water is wet, part II

Trying out the Nanofabric in a slightly different way - getting a drop of water to roll down an incline.

Dropping a coin in water

Here's the deal: you want to drop a coin into a container of water. Maybe you're bored. Maybe you're trying to win a bet about hitting a submerged target. What's the best way to drop the coin so that you can hit that target? The easiest way is to drop the coin perfectly flat, with its faces parallel to the water's surface. Provided there aren't any currents or turbulence in the water column below, the coin should drop straight down.

An unexpected result

Popping water balloons is fun. But when I set up a water balloon to pop when it hit the top of an upward trajectory, I didn't expect the result I ended up with.



I really have no idea if I could replicate that or not. But you know I'm gonna try. I also tried a setup with the water balloon swinging from the side.

Feb 21st

For those of you across the Atlantic, today's date is a lovely palindrome 21-02-2012.

For us here in the states, it's not quite as symmetric.

Jargon Battles!

So Ron and Callan are having a good-natured discussion about the term "xenobomb." This is a good example of how we as scientists use language. For use in the study of the natural world, the words we use to describe objects and processes must refer to something specific (not vague) and also be accurate (reasonably).

Although I have nothing more specific to add to their web discussion. The general thought on twitter is that some kind of "xenolith-cored bomb" term is most specific and accurate. Either way, the sample in Callan's lab is a gorgeous olivine peridotite. You don't often get such lovely chunks of the mantle brought near the surface for us to see.

There is an excellent surfing location on Kauai where the large beach boulders are almost all cored by these olivine xenoliths. Though if you go, don't walk around with your big telephoto lens like I did - the surfers value the secluded, private nature of this beach. Fortunately, I was forewarned by a local and kept myself as inconspicuous as possible. (Can that be done with a 400mm lens?)

Here's an example - Hawaiian quarter for scale. Note also, the rusty-colored surface of the olivine crystals. Olivine is an iron-magnesium silicate and weathers into iron, magnesium oxides and silica (SiO₂ in solution).

Thurs-Demo:The One that Glitters

I've been thinking of a pop culture reference for today's post. I was mulling the thought of using a Tolkien-Aragorn reference, or perhaps a Twilight-Vampire comment. Instead, I'll just let you know that pearlescent paint/ink has some great potential for demonstrating fluid dynamics.



Mix a water-based pearlescent paint (this one is probably made with powdered aluminum) with water and stir. Watch lovely Von-Karmen vortices whirl about. See the transition from laminar to turbulent flow. All this can be yours for a few bucks worth of paint.



Or, if you're so inclined, mix a very thin suspension in water and then use the glittery parts to help track where the water goes.

This is kinda like painting

Is this what a scientist looks like?

I certainly think so. To find out what happens next, check out the video below:

This was pretty cool: candle burning "Underwater"

I was experimenting with dropping steel balls into water. Some of them coated with hydrophobic stuff, including soot and nose oil. No, that's not a fancy brand name, it just what it sounds like. But after the fun of balls hitting water, I thought it would be interesting to drop a candle into water. I was hoping that the cavity formed by the hydrophobic material hitting the water would allow for the flame to continue burning when the candle went below the surface of the water. I managed to get it just about perfect on the first try.



Not a bad end to the day.

Late to the Party

We went to my parent's house today to have some lunch and catch up. I had intended to get some pictures of their kitchen countertops earlier, in time for AW #42, but that's how things go...


Felsic pegmatite (slight metamorphism/alteration?) with strawberry for scale.


Felsic pegmatite with metallic inclusion and oxidation halo, mustard and relish for scale.

Not all LEDs are created equal

As many of you are aware, I've been shooting quite a few high speed videos lately. The one real limitation I have is with light. These high speed cameras are just gobbling up light like it was going out of style. Here's an example. Take your camera - if it has manual controls, go ahead and dial in an exposure of 1/1000 sec, an f-stop of around 2.8 (although for decent depth of field, I like to get around f5.6 or more) and then try and take a picture of something indoors. For our videos, we're using a pair of 500W tungsten lights. They toss out plenty of light for speeds under 5,000 fps - but they get really, really hot. 200+°C hot. Plus, a 500W light on a 120-volt circuit draws about 4 amps of current. You don't have to add too many lights before you're really sucking up a lot of electricity (and converting way too much of that into heat instead of light).

So LEDs are a really attractive option. They don't necessarily draw as much electricity and they are much more efficient. Running much cooler. They are a bit more expensive - probably about two or three times as expensive as tungsten. There are cheaper LED light panels, but the "color" of their light isn't matched to that of tungsten or sunlight - so using them together sometimes yields weird colors in the subject. LED stands for "light emitting diode." A diode is a component of an electric circuit that only allows the electric current to travel in one direction. So for a DC (direct current) circuit, the flow of electricity is in the same direction and there's not much to see. But, the electricity that "comes out of the walls" to power our appliances, lights, and televisions is AC (alternating current). The current switches direction (alternates) 60 times a second. In once cycle, 1/60th of a second, the electricity flows in one direction, and then reverses direction again. So an LED would light up for half of that cycle, then go off, then go back on again for half of the following cycle, and so on...

Good LEDs are set up to deal with this brief interruption and remain lit. I had thought that even cheap LEDs would remain lit sufficiently to provide a source of light that doesn't flicker (at 60 times per second). Shooting at anything above 30 frames per second renders any normal AC light source useless. Imagine my dismay when I looked at some AC-powered LED lights. One was an LED "worklight" and the other was a single panel for mounting under a cabinet. (cost around $20-30).



The under-cabinet light is flickering, while the worklight remains steady. This means that finding an economic set of lights is going to take some additional research to make sure they don't flicker...

Thurs-Demo: The One with Room for Cream

Getting one drop to hit another takes some timing. The higher viscosity of the cream makes it easier to get short spacing between the drops.