Thursday, June 7, 2012

Rocks in Campfires

Sometimes we get extremely practical questions - so mundane that no one has ever spent time scientifically researching or studying them. Another way to put this: a million people have conducted a million unreported independent experiments. However, equally many people have opinions!

Case in point: putting wet rocks from a river in a campfire:

Hi there!
     First let me say I was so happy to find a result when I googled "ask a geologist".  The internet continues to impress.
     A friend and I recently were talking about rocks in campfires, and the safety of it.  She was convinced that solid rocks can explode with shrapnel-like effect if overheated.  I ceded that I believe rocks may able to explode, but rocks that were solid and found in a dry area would probably be safe, and that it were more likely to simply crack than to actually provide enough force to send a fragment out at high velocity.
     So obviously when we returned I did a bit of research, and found that a lot of people talk about this, but no-one seems to have any concrete evidence.  It's all either anecdotal or stated as theory.  Most of these involve "river rocks", rocks which have been exposed to water over long periods, "soft rocks" such as sandstone or pumice, the combination of the two, or simply "rocks which have air or liquid in them".
     I don't doubt for a minute that there are circumstances where gas or liquid inside of a rock can expand and cause the rock to break.  What I question is whether or not the explosion can produce a shrapnel-like effect.  Whether or not the force can be great enough to send a piece
of the rock out at great velocity, or if it would be more likely for it to simply crack with little effect.  I did a tiny bit of research only to realize that the mechanics regarding density of rock and vapor
pressure were pretty deep. The engineer in me realizes that it relies on many variables, the distance of the pressurized water to the surface, the shape of the rock, and of course it's density and the amount of vaporizable water, and that water's coordination within the rock.
     So whaddya think?  Can rocks explode like grenades?
- Andy B.


That's a classic question with thousands of anecdotal answers. I have personally seen a river rock, deposited in the middle of a roaring campfire, explode. There was a distinct bang sound (several, actually), but I don't remember any pieces flying off. Others around the campfire told me that yes, they had seen fragments fly out of other wet-rocks-in-a-fire experiments, and considered anyone putting a water-soaked rock in a hot campfire as being unusually foolish. I've seen worse: in a field camp in the deep Venezuelan jungle, I watched obreros throw half-used cans of insecticide spray into a campfire with predictable consequences.

The engineer in you has homed in on the best answer (I hesitate to use "correct" here, meaning that it has been experimentally verified, or verified from personal experience, take your pick). The problem is there are too many variables.These include how "tight" the rock is, how fractured it is, how much porosity, how much transmissivity (how interconnected the pore spaces are), how fast it heats up, and how large a volume. There are probably others.

It comes down to this: to constrain the variables, one must do thousands of experiments to get anything statistically meaningful. I suspect this experiment has been "conducted" millions of times by millions of kids around campfires, but no one ever collected and compiled the results. I can't imagine anyone other than Myth Busters having the time and resources to do an appropriate set of experiments. I would be surprised, however, if they have NOT done experiments like this.


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