Category Archives: Rock Mechanics

Squeezing Rocks with your Bare Hands

Our lab group. Photo: Chris Marone

Our lab demo group. Photo: Chris Marone

As frequent readers of the blog or listeners of the podcast will know, I really like doing outreach activities. It's one thing to do meaningful science, but another entirely to be able to share that science with the people that paid for it (taxpayers generally) and show them why what we do matters. Outreach is also a great way to get young people interested in STEAM (Science, Technology, Engineering, Art, Math). When anyone you are talking to, adult or child, gets a concept that they never understood before, the lightbulb going on is obvious and very rewarding.

Our lab group recently participated in two outreach events. I've shared about the demonstrations we commonly use before when talking about a local science fair. There are a few that probably deserve their own videos or posts, but I wanted to share one in particular that I improved upon greatly this year: Squeezing Rocks.

Awhile back I shared a video that explained how rocks are like springs. The normal demonstration we used was a granite block with strain gauges on it and a strip chart recorder... yes... with paper and pen. I thought showing lab visitors such an old piece of technology was a bit ironic after they had just heard about our lab being one of the most advanced in the world. Indeed when I started the paper feed, a few parents would chuckle at recognizing the equipment from decades ago. For the video I made an on-screen chart recorder with an Arduino. That was better, but I felt there had to be a better way yet. Young children didn't really understand graphs or time series yet. Other than making the line wiggle, they didn't really get the idea that it represented the rock deforming as they stepped on it or squeezed it.

I decided to go semi old-school with a giant analog meter to show how much the rock was deformed. I wanted to avoid a lot of analog electronics as they always get finicky to setup, so I elected to go with the solution on a chip route with a micro-controller and the HX711 load cell amplifier/digitizer. For the giant meter, I didn't think building an actual meter movement was very practical, but a servo and plexiglass setup should work.

A very early test of the meters shows it's 3D printed servo holder inside and the electronics trailing behind.

A very early test of the meters shows it's 3D printed servo holder inside and the electronics trailing behind.

Another thing I wanted to change was the rock we use for the demo. The large granite bar you stepped on was bulky and hard to transport. I also though squeezing with your hands would add to the effect. We had a small cube of granite about 2" on a side cut with a  water jet, then ground smooth. The machine shop milled out a 1/4" deep recess where I could epoxy the strain gauges.

Placing strain gauges under a magnifier with tweezers and epoxy.

Placing strain gauges under a magnifier with tweezers and epoxy.

Going into step-by-step build instructions is something I'm working on over at the project's Hack-a-Day page. I'm also getting the code and drawings together in a GitHub repository (slowly since it is job application time). Currently the instructions are lacking somewhat, but stay tuned. Checkout the video of the final product working below:

The demo was a great success. We debuted it at the AGU Exploration Station event. Penn State even wrote up a nice little article about our group. Parents and kids were amazed that they could deform the rock, and even more amazed when I told them that full scale on the meter was about 0.5µm of deformation. In other words they had compressed the rock about 1/40 the width of a single human hair.

A few lessons came out of this. Shipping an acrylic box is a bad idea. The meter was cracked on the side in return shipping. The damage is reparable, but I'm going to build a smaller (~12-18") unit with a wood frame and back and acrylic for the front panel. I also had a problem with parts breaking off the PCB in shipment. I wanted the electronics exposed for people to see, but maybe a clear case is best instead of open. I may try open one more time with a better case on it for transport. The final lesson was just how hard on equipment young kids can be. We had some enthusiastic rock squeezers, and by the end of the day the insulation on the wires to the rock was starting to crack. I'm still not sure what the best way to deal with this is, but I'm going to try a jacketed cable for starters.

Keep an eye on the project page for updates and if any big changes are made, you'll see them here on the blog as well. I'm still thinking of ways to improve this demo and a few others, but this was a giant step forward. Kids seeing a big "Rock Squeeze O Meter" was a real attention getter.

Hmm... As I'm writing this I'm thinking about a giant LED bar graph. It's easy to transport and kind of like those test your strength games at the fair... I think I better go parts shopping.

Going to the Science Fair

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Explaining doppler RADAR with an actual demo!


This past week I got to relive some of my favorite days of primary education: the science fair!  A local elementary school was hosting their annual science fair and had asked the department to provide some demonstrations for the parents and students to see. I immediately volunteered our lab group and began to gather up the required materials. Some of the setups were made years ago by my advisor. I also developed a few and improved upon others here and there. I thought it would be fun to share the experience with you.

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The line-up of demonstrations setup as the science fair was getting started.


At some point, we should probably have a post or two about each of these demonstrations, but today we'll look at pictures and talk about the general feedback I received. First, off we had four demonstrations including the earthquake cycle, how rocks are like springs, seismometers, and Doppler RADAR. I made an 11x17" poster for each demo in Adobe Illustrator using a cartoon technique that one of our professors here shared with me.

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Here is an example poster from one of the demonstrations.


For scientists, communicating with the public can be difficult. It's easy for us to get holed up in our little niche of work and forget that talking about a topic like power spectra isn't everyday to pretty much everyone. Outreach events like this present a great opportunity to work on those skills! This particular event was especially challenging for me because the children were K-5, much younger than I usually talk to. With high school students you can maybe talk about the frequency of a wave and not get too many lost looks, but not with grade-schoolers!

The other difficulty was adapting what are deep topics (each demo is an entire field of research, or several) to the short attention span we had to work with. Elementary school teachers are masters of this and I would love to get some ideas from them on how to work with the younger minds. I spent most of my time talking about the Doppler effect with the RADAR (it's the topic my lab mates were least comfortable with since we don't deal with RADAR at work generally). By the end of the science fair, I had an explanation down that involved asking the kids to wave their hand slowly and quickly in front of the RADAR and listen to how the pitch of the output changed. Comparing that to the classic example of the pitch bending of a passing fire truck siren seemed to work pretty well. I had a "waterfall" spectra display that showed the measured velocity with time, but other than trying to get the line to go higher than their friends, it didn't get much science across (though lots of healthy competition and physical exercise was encouraged).

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An excited student jumps up and down to see herself on a geophone display.


In the past, I've pointed out the value of being an "expert generalist". All of us were tested in any possible facet of science by questions from the kids and their parents. I ended up discussing gravitational sling-shot effects on space probes with a student and his parents who were incredibly interested in spaceflight. I also got quizzed about why the snow forecasts had been so bad lately, when the next big earthquake would be, and a myriad of other questions. Before talking to any public group, it's also good to make sure you are relatively up-to-date on current events, general theory, and are ready to critically think about questions that sound deceptively simple!

The last point I want to bring up today is the idea of comparisons. These are numbers that one of my committee members likes to say he "carries around in his shirt pocket." These are numbers that let us, as scientists, relate to others that are non-specialists and give us some physical attachment to a measurement.  What do I mean? Let's say that I tell you that tectonic plates move anywhere from 2-15 cm/year. Great, first, since we are in the U.S.A., everyone will hold out their fingers to try to get an idea of what this means in imperial units.... not quite 1-6 in/year. That's better, but a year is a long time and I can't really visualize moving that slowly since nothing I'm used to seeing everyday is that slow... or is it? Turns out that fingernails, on average, grow 3.6 cm/year and hair grows about 15 cm/year. Close enough! In Earth science we have lots of approximate numbers, so these tiny differences are not really that bad. Now let's revise our statement to the kids to say: "The Earth is made of big blocks of rock called plates. These move around at about the speed your finger nails or hair grow!" Now it is something that anyone can relate to, and next time they clip their nails or get a hair cut, they just might remember something about plate tectonics! It's not about having exact figures in the minds of everyone, it's about providing a hand-hold that anybody can relate to! This deserves a post to itself though.

That's all for now, but I'd love to hear back from anyone who has elementary education experience or has their own "shirt pocket numbers."


Exploding Ice and Rock - Booms Heard a Result of "Cryoseisms"

Ice Hanging From Rock

UPDATE 1/13/14: Frost-quake creates 100ft long crack here.

Over the past few days (starting around Christmas eve), there have been reports of large booming sounds associated with minor ground shaking across the northern states, as well as in Canada.  The Toronto events have a nice string of tweets that are associated with them as well.  Are these really explosions? Earthquakes? Sonic booms? The truth, as it turns out, is a rare event that produces what are known as "cryoseisms".  Oddly enough, these "frostquakes", as they are commonly known, have been discussed in the literature since about 1818!  Having a background in both meteorology and geophysics, cryoseisms are just one example of how closely related to two fields are.

So, what happens to produce such loud and potentially startling events? It's all about ice.  Cryoseisms occur when there are seasonal frost conditions, no insulating blanket of snow, lots of rain/thaw to saturate the ground, and a sharp drop in temperature.

Surface water penetrates into sufficiently permeable soil/rocks, but then is rapidly frozen with a fast drop in surface temperature.  Normally temperature drops slowly enough that the ice gradually freezes, giving the surrounding soil/rock time to adjust.  When really fast temperature drops occur and freezing is rapid, the surrounding areas are stressed by the expanding force of the ice.

The freezing process is actually a very powerful mechanism, and is one of the geologist's favorite ways to explain physical weathering of large boulders.  Freeze/thaw cycling has even been used as a quarrying technique in granite!

Expansion during this rapid freezing of infiltrated ground water stores energy in the surrounding rock/soil, like a spring, until..... BAM! Failure occurs in much the same way faults fail.  Here the driving force isn't tectonic though.

Cryoseisms can do light damage to structures in the immediate vicinity, but their intensity falls off very quickly with distance.  For the seismology buffs out there, the zero focal depth produces lots of surface waves, but these events are generally not recorded on seismic networks.

Want to know more about cryoseisms? The literature isn't too robust, but check out Barosh (2000), Nikonov (2010), and Voss & Herrmann (1980) for some starting points!

*Cryoseism is also used to refer to earthquakes at the base of glaciers as well.  That's a whole other story for another day!


Fun With Office Supplies - Geometric Cohesion and Staples

After the small rash of tape theft resulting from my suggestion at a talk that the audience go home and unroll scotch tape to see the resulting electrical dischange (which deserves a blog post soon) it's time for another attempt to make Swingline Co. stock soar.

In a recent Physics Today article "Geometric Cohesion in Granular Materials", Scott Franklin of the Rochester Institute of Technology showed some very interesting data regarding how the shape of a material effects how likely it is to stay together as a coherent mass.  Before we delve into the article though, let's talk about cohesion in general.  Then we can come up with a couple of fun experiments that you can do at home.

Cohesion is just the tendency of a material to stick together.  This is different than adhesion though.  Consider a water drop on a dish in your dish drainer.  The water drop is sticking to another material (the ceramic) which is adhesion.  The water drop is also a coherent mass; water molecules are sticking together to form a raised droplet on the surface of the plate.  In essence the water is 'sticking to itself', which is due to electrostatic forces of water being a polar molecule.  Electrostatic forces give water a property of surface tension that causes lots of wonderful things that can make up a whole other post at some point.

Other things can cause a material to stick to itself though.  Remember playing the old pickup sticks game as a kid? The object was to extract a stick from a pile of sticks on the table without disturbing other sticks.  This turns out to be pretty challenging.  Your pile of sticks appeared to be stuck together or interconnected, which is cohesion in a macroscopic or broad sense.  In the case of the sticks electrostatic forces certainly aren't the cause.  Electrostatics forces are relatively weak, and the sticks don't have enough mass for their gravitational attraction to cause this cohesiveness of the pile.  What is the mysterious factor then? It's just their shape. When objects act cohesive because of their geometry or shape it's called geometric cohesion.

First grab a couple of tablespoons of sugar, place it in a small container (an old medicine bottle works great) and turn the container upside down on the table.  Now remove the container... the sugar spreads out in a small pile.  The roughly spherical grains of the sugar don't stick together that well and the pile isn't very tall.  The angle of the pile from horizontal is called the angle of repose, and is around 30-35 degrees for lots of things.  My pile of sugar sat at about 30 degrees.  The angle of repose really tells us how hard it is for the grains to slide past one another.  If it's easy, the pile is a very low angle, and if the pile is made of large, angular hunks of rock, it becomes more steep.

The angle of repose can even be thought of as a proxy for the coefficient of static friction, or how hard it is for the grains to move past each other from a dead stop.  A mathematical relationship can be derived from some geometry, but it turns out the tangent of the angle is about the coefficient of friction. So the tangent of 30 degrees is about 0.6, which is the general number for static friction of lots of materials.  This result means all is well in the world of static newtonian physics and we can think about something more interesting than approximate spheres of sugar.

What about rods? Going back to our game of pickup sticks, the shape of the long, narrow rods seems to be the main factor holding things together.  It's easy for lots of objects to interact and become 'locked together'.  Who cares about rods locking together? Manufacturers often have automated assembly machinery that may have large hoppers of screws, and when things get locked together it costs money.  The long narrow shape of screws can jam hoppers in seconds and hold up the entire line.  While not many people have a pile of tiny screws at home, I bet you have staples.

Staples are a funny shape really.  Standard office staples are about 7mm along the long upper shank and  around 5mm at the two barbs.  These measurements give the staple a barb to shank ratio of about 0.7, which as it turns out, is a governing number describing how well staples can stick together.  Franklin's group did a whole series of experiments with staples of different barb ratios and found that staples with a ratio near 0.4 were the most cohesive.  Even though our staples aren't the ideal ratio let's repeat the sugar experiment.

Using a stapler, eject a bunch of staples into the same container (or one with a slightly larger mouth if you have it).  Now shake the container up for a bit, turn it upside down, and remove the container.  This time the mass didn't spread like the sugar, but retained the relatively sharp edges of the container shape.  Retention of shape tells us that the staples are cohesive, and their high angle of repose means that the coefficient of static friction is very very high.  Franklin's group is currently seeing how strong these piles are by pulling on the ends, but they are actually pretty robust.

The big question is why does any of this matter other than being interesting? Well, cohesion is a big deal when we study soils.  Cohesion can determine if the ground can support a building, if a landslide is due, or if the machine powder coating your morning doughnuts gives you a plain doughnut.  While some of these are more life threatening that others, it's import to study cohesion to keep tabs of impending disasters, especially avalanches and landslides. I know that there aren't that many staples in soils generally, but there are clays which are shaped like plates.  Different minerals/materials in the soil with different shapes can greatly change the strength of the soil and how likely it is to slide as a mass.