Otherwise known as a geolantern, subspecies of the common jack o’ lantern.
And yes, I painted the damned thing. Trust me, you don’t want to let me near a pumpkin with a knife. The results could put you in mind of Jack the Ripper crime scene photos.
This all came about through an interesting confluence of events. As I said, I am teh suck at pumpkin carving, so I was going to give Michael Klaas’s Accretionary Wedge topic a miss. Didn’t have time, tools, or a pumpkin, right? Busy doing NaNo, even so. But then, on Sunday, on the way out to the car to retrieve soda, I saw this beauty of a pristine pumpkin sitting forlornly by the dumpster. So I fetched it up the stairs. I have a soft spot in my heart for orphaned members of the squash family.
Hmm, I thought. I could turn it into a migmatite.
And there it was. Inspiration.
It’s too bad I suck almost as much at painting as I do carving, but it didn’t turn out too badly for all that.
That photo fascinated me and started a conversation on Twitter. It’s one of those things that probably got sold as granite, but there’s so much more going on here. You’ve got a pale host rock, and then these darker inclusions. It looks like something felsic gobbled up something mafic. That happens sometimes, where a mass of intruding magma grabs up bits of something else and incorporates them. The incorporated bits sometimes melt, but not always.
And then Lockwood mentioned reaction rinds. I think that’s what really grabbed my attention, because I hadn’t heard of those before.
See those halos of darker rock around the blueish-gray interior? That’s what he was talking about. Translating from Twitterese to English, here’s what he said: “When a xenolith/inclusion isn’t stable in a magmatic environment, it reacts with the magma to form new minerals. These can protect the inclusion, and/or slow the reaction to the point where the melt cools before it all reacts. This leaves a mostly pristine inclusion surrounded by a ‘reaction rind.'”
The inclusion in question is probably anorthosite, a variety of gabbro, according to Ron. The lovely sort-of blue in the dark inclusions probably results from the mineral labradorite. If you haven’t yet had the opportunity to enjoy some, get thee to your local rock shop and pick some up. Turn it round. Watch it shimmer and shine in a magnificent sort of iridescence. It’s gorgeous.
Ron’s not sure what’s causing the deep, rich brown color in the reaction rinds (or coronas, if you’d like a less gourd-like synonym for the phenomenon, though rind fits rather well with the spirit of this Accretionary Wedge theme, don’t you think?). Could be hornblende. Could be biotite. It’s not always possible to determine such things through a photograph of a countertop, no matter how awesome. We could grab a hand sample and find out for sure, but the countertop owner may not be entirely willing to allow that, even if it’s for science.
So, that’s the photo that inspired an Accretionary Wedge post and launched a whole new topic. Watch for countertop geology in the future! And watch Volcanoclast’s blog. Great stuff there, plus an intriguing mystery countertop to come!
In the meantime, I’ll leave you with a photo of chat meurtrier avec la geolanterne.
With gratitude to Volcanoclast, Lockwood DeWitt, and Ron Schott for their inspiration and assistance.