Monday, February 13, 2017

Never stick your hand into a viscous material

-Alison

“If there is one thing I’ve learned, it’s never to stick your hand into a viscous material” I came across that quote again in the signature of a colleague's email. It is from the 2004 Van Helsing movie. The movie is a bit cheesey, but that advice is very sound.


So what does viscous mean? The term viscosity is not a word that most people use every day, but a really useful one if you want to know anything about a fluid or anything that flows. It gets used by your mechanic when discussing different types of oils to put in a car’s engine. Or occasionally in movies involving evil scientists, monsters and gooey things (see above). Even TSA has to have a basic understanding of viscous things as they limit all things that pour, spreads or smears. This covers a range of things that, while they behave like fluids (which means they deform under a force), you might not immediately think of them. Unfortunately, TSA is as just as likely to take away your hair gel or peanut butter as your water bottle. Here at In the Company of Volcanoes, our favorite viscous material is of course, lava.

Sampling this viscous fluid is a unique experience. I used a rock hammer while wearing thick leather gloves while I was a volunteer at the Hawaii Volcano Observatory and it was still uncomfortably hot! Kilauea 2009. Even the duck was wearing personal protective gear. 

Volcanologists care about viscosity because it is a useful means of discussing how magma and lava move. A practical example is using viscosity of a lava flow to help estimate how far it will travel. Viscosity is, in its simplest form, a way to describe how a material, usually a fluid, flows. Viscosity is technically defined as the resistance to flow. The more viscous it is, the more sticky and slow it will be. So oil is more viscous than water, and toothpaste is more viscous than oil. If you ever take a geology course that has a good volcano section you may get to race various fluids down a slope, including things like syrup, oil, or ketchup.
 
There are lots of web resources to design these experiments for students of all ages. Everyone likes making a mess. Image from Science Sparks a website full of science activities for kids.

Another fascinating thing about viscosity is that it changes with temperature. You can think about honey. When it is a warm day honey will pour nicely from the container (squeezey bear for me) and runs quickly. When it is a cold day, or you keep your poor honey bear in the fridge, the honey is much thicker and doesn’t flow as easily. This temperature relationship with viscosity is pretty common. I’m sure you can think of lots of edible examples from your kitchen (chocolate and peanut butter are my personal favorites). 

Honey bear, backlit, from Wikimedia Commons.
Viscosity is also dependent on things like bubbles, or crystals inside the fluid. The presence of these things change the way the flow will respond to deformation. Crystals can get locked up and prevent the flow from moving, for example. Our honey analogy works here too. When honey sits around too long it can start to crystallize. These crystals resist flow and makes it much harder to get any honey out of that bear and into your tea. At some point, the honey becomes nothing but crystals and then it isn’t a fluid anymore. The trick is to reheat it and melt those crystals. For lava flows we have to consider the temperature, the crystal content, and the gas content, which are all constantly changing while at the surface because it is trying to cool down.

This lava from Tolbachick has lots of holes in it that are preserved bubbles, or vesicles. Some of them are stretched meaning they were formed while the lava was still flowing, these vesicles can influence the lava's viscosity. Image by J. Krippner.
Ok, so what units do we even measure this in???  The one we use in volcanology is the Pascal second (Pa s).  A pascal measures pressure, Europeans who own cars should already know this from when they check their tire pressure. In the US, and a few other holdouts, we use pounds per square inch, or PSI.. Blaise Pascal was a French Mathematician. I remember this because one of the top schools for Volcanology in the world is in France and named after him (though we more frequently refer to the city where it was built, Clermont Ferrand). When we add the seconds it becomes about the pressure required for deformation over time. This also can be called Poise (this is useful for really small viscosities as 10 Poise= 1 Pa s). We can also measure this as a function of distance mPa s (dynamic viscosity) or by area m^2/s (kinematic viscosity). 

Most of us, though, don’t have a good calibrated Pa s in their brain to be able to compare how much more viscous motor oil is than lava. So what are a few examples to put this in context? Water, one fluid I’m sure all of us have encountered enough to have a good feel for how runny it is, has a viscosity of 8.9 x 10-4 Pa s. So that is 0.00089 Pa s. So not very much resistance to flow. In fact we call it a Newtonian Fluid, or one that responds to any deformation in a linear way. When you add a force it responds quickly and proportionately. 

So lets look at these for different orders of magnitude. That is, each step is 10 times less runny than the previous. All numbers are rounded and for room temperature (70 F or 21 C).
Water              0.001 Pa s
Milk               0.03 Pa s
Motor oils      0.1-1 Pa s
Karo Syrup     5 Pa s
Honey            10 Pa s
Peanut butter  250 Pa s
Silly Putty      8,000 Pa s
Glaciers (average)         1,000,000,000,000 Pa s

Ok, so now we have some items to compare against lava flows.

Basalt, which is what most people think of when they think of lava, erupts at high temperatures around 1200 C, which is 2,500 F.  Hot runny basalt can have viscosities of as low as 10-100 Pa s. That means it can be as runny as honey in your kitchen (we don't see this often). A recent video of lava entering the ocean from Kilauea volcano in Hawaii shows a good example of low viscosity basaltic lava. The color and the velocity of this lava tells us it has a low viscosity (for lava) and thus very hot. This footage is so impressive that its getting a lot of attention, even in Hawaii. The video below is from Big Island Video News.

 

At lower temperatures basalt becomes more sticky and the viscosity increases. This makes the lava ooze more than gush. The viscosity of the lava in the video below is probably closer to 100-1,000 Pa s. That makes it closer to that of peanut butter. It is still smearable, but it takes more force to make it move. This video below shows the oozing lava collected by Hawaii Outdoor Guides.



Even though flows like this take their time moving, as long as there is more lava pushing behind it the flow can keep going for miles. You can out run most lava flows, but they can still cause damage by covering roads, damaging houses and farms. Sometimes the viscosity of the flow is high enough that it doesn't even look like a liquid anymore, more like a pile of moving rubble. A'a lava flows have viscosities that range from 1,000-10,000 Pa s. That is closer to the viscosity of silly putty at room temperature. Inside that rubble is molten lava that is pushing the whole pile forward. The footage below was taken at Kilauea volcano on 1 June, 2010 by volcanochaser.



As I mentioned before viscosity is controlled by temperature (seen in videos above), but also composition. The above examples are basalt lava. They contain between 45-52% silica by weight. Silica in rock hangs out with four oxygens to make a tetrahedron. If we add more silica to a lava the tetrahedra start sharing oxygen, linking together and making chains. As they link up, the flow gets harder to deform, more viscous. As silica increases further the chains link together, then the tetrahedra form a framework.

One silicon (grey) hangs out with four oxygens (red) to make a tetrahedron. This is the building block of most minerals that occur in lava. As the amount of silica increases the tetrahedra link together to make chains, and then double chains, and then a framework. As these structures get more interconnected the viscosity of the lava increases. Image using silica tetrahedra from wikimedia commons. 

Lavas with more silica have progressively higher viscosities. Andesite lava, which has silica contents closer to 60 % silica by weight have viscosities that are a few orders of magnitude higher than that of basalt (1,000,000 Pa s). However, as the viscosity increases it becomes more dangerous to film these lavas moving. They tend to form steep volcano and trap more gas. Trapped gas can build up and cause explosive eruptions. I haven't yet found a good video of an andesite lava flow, as so many andesite flows occur while explosive activity happens at the same time.

The next step in viscosity is dacite, or lavas with 60-70% silica by weight. With this much silica the viscosities get really high and the lava doesn't look like a fluid anymore. Most often when we see dacite lava moving it is in lava domes. These look like piles of rubble, or even fins of rock that grown and change with time. The inside of this pile is still hot viscous lava that pushes the cooler dome material above it out of the way. To see them move we really need time lapse photography. This video of the Mount St Helens 2004-2008 lava dome from the USGS is really quite impressive.

At the far end of the silica spectrum we get rhyolite, which is up to 77% by weight silica. So half again as much as the basalt. The effect on the viscosity is significant, up to 1,000,000,000,000 Pa s, that is the same order of magnitude of a glacier, which moves very slowly. So not quite doubling the silica content of runny basalt makes the viscosity go up 10 orders of magnitude. All that silica makes a difference. The difference we saw between similar composition lavas but different temperatures spanned from 100-10,000 Pa s. The effect of adding more silica is much greater. Rhyolite is the stickiest of lavas. It is so sticky, that it rarely flows at all. We don't get to see it very often because the viscosity traps gases which means the volcano is more likely to explode than ooze a lava flow. There have been a few cases, including the video below from Lancaster University showing an eruption of Cordon Caulle, Chile in 2011. Hugh Tuffen and colleagues were lucky enough to see the high silica lava flow in motion. They call it obsidian because it is very glassy (the preview image of the video is not of rhyolite, you have to click the video to see the obsidian lava).



You may not have thought about viscosity before reading this blog (and you may never again), but if you want to, you will be able to compare some numbers if you are trying to buy fancy olive oils, translate a physics text, digest a hazard model, look up the safety features on your shampoo, or pick an oil for your car. More importantly, you now can talk about how viscous lava is, a party subject I'm sure you've been longing to bring up. There are lots of good exercises online for studying viscosity including this and this that have great teaching tips to give another explanation of what was covered in this blog. Don't forget our long list of other volcano teaching resource from our blog last year.

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