Exploding Rocks for Dummies

Permeability and porosity

Staffers at the Antisocial Network long ago lost count of the number of freelancers tripped up by the arbitrary minimum word count (MWC) forced on contributors to eHow.com and its sister sites. Way too many times we've found someone who was cable to come up with a simple (and sometimes even correct) answer ten or twenty words long, and then sink the post with padding full of misinformation and errors. That's precisely what happened with today's nominee, one Delia Rollow, whose "Bachelor of Science in environmental studies" apparently didn't cover the information necessary to write a list of "Rocks That Explode Around Fire Pits" at (for reasons unknown) Hunker.com.

Rollow's answer was basically that,

"Air- and water-permeable rocks are much more likely to explode than dense non-permeable rocks,"

which is about half true. The problem being, of course, that to make that answer... less useless, she had to tell people what rocks fall in that classification. Here: we'll do it for her:

Scoria, aka "lava rock," and pumice.

In order to "explode," a rock exposed to high temperatures (e.g., a fire pit) would need to be not only porous, but the pores would have to be water-filled. That's possible for some sedimentary rocks, but just about anything you find lying around the surface is unlikely to waterlogged. OK? OK.

Now that we have that out of the way, let's take a look at some of the dumbassery Delia employed to pad her answer to the required 300-500 words:

• "Igneous rock is the result of volcanic activity throwing out hot lava..." – Only some igneous rocks are the result of cooling lava. Delia prattles a lot about granite which is not one of them.
• "Sedimentary rock is the product of many layers of ocean or lake sediment settling on the ocean or lake floor. " – Ummm, there's a lot more to it than that, Delia.
• "Metamorphic is rock that was once either igneous or sedimentary and has been heated and compressed over time into something that is much more dense than the original rock." – Not necessarily more dense, just... rearranged.
• "Granite is an igneous rock that was never exposed to air or water as it cooled down, allowing it to solidify in a very dense form." – If you're that hung up on density, Delia, compare the density of granite to basalt... oh, wait: you weren't using the scientific meaning of "dense," were you...
• "Softer rocks, like sandstone, limestone and pumice, are not as dense as granite or marble; there is more space in between the molecules that make up the stone." – And this is the sentence that got our staffer running to the morning meeting waving the URL. "Molecules"? Is this moron unaware that rocks are composed of mineral grains? 

• "...sandstone and limestone are sedimentary rocks that were formed without a great deal of compression, thus allowing for their high levels of permeability. " – A lack of "compression" notwithstanding, permeability is not the same thing as porosity. 
• "Air- and water-permeable rocks are much more likely to explode than dense non-permeable rocks." – In fact, that's backwards: a porous but low-permeability rock is more likely to explode than a porous and permeable rock, which is why pumice and scoria are more likely candidates for rock popcorn.

Not only did Rollow get half (or more) of the petrology wrong, she also confused porosity and permeability. The combination of the two species of dumbassery was all we needed to name Delia our Dumbass of the Day.

SI - PETROLOGY