Srsly, Obsidian at North Creek?!

Places are like people: you never know them as well as you think you do. They can still surprise you. I’ve walked round the North Creek area of Bothell several times a year for, oh, going on four years now. I drive those same roads, nearly every day. And yet, every time I go out, there’s something new.

This time it was obsidian.

Obsidian with Daffodil

I only ever noticed it because of the daffodil. Since Ophelia mentioned daffodils, I’ve been paying attention to them. Most of the ones I encounter are typical yellow varieties, but this one had an interesting petal structure with a little bit of orange in it, and I leaned over close for a look, and ZOMG OBSIDIAN! Is it? Is that really obsidian?

Obsidian Peak

It is! That’s obsidian! That little landscaping rock, half-buried in moss and lichen and overshadowed by an intriguing daffodil, is obsidian!

Pardon the inordinate excitement. There are reasons. For one, there’s a distinct lack of geology round the immediate area, so the least little example of it becomes an occasion for squeeing. For another, obsidian and I go way back. It’s one of the first rocks I learned to recognize. Bits of it peeped shiny and deepest black from the tan earth in the forest where I lived in Flagstaff. Sometimes, we’d run across an obsidian arrowhead, and I learned that the Sinagua found its lovely conchoidal fracture patterns and extremely sharp edges quite conducive to forming such things.

In fact, we can have Misha demonstrate conchoidal fracture in obsidian.

Kitteh with Obsidian demonstrating conchoidal fracture. Awwww.

Glass fractures the same way. Obsidian is volcanic glass. Ergo, same fracture. And it really is easy to get a razor’s edge on it. I did just that, in fact, experimenting around with a hunk of it I’d found in the forest. Chips right off with a simple pocket knife, leaving nice, sharp edges. Of course, I wasn’t paying so much attention to my incipient arrowhead so much, because a nice long, razor-sharp sliver flew off and speared me in the thumb. It cut so fast and smooth I didn’t feel it, but the copious amounts of blood pouring from said pad o’ my thumb and the fact there was a great black splinter sticking out of the flood pointed toward something being not quite right. It does take a certain skill to chip arrowheads, which I have demonstrated I do not possess.

This is why I have no trouble believing doctors use obsidian scalpels (pdf) for surgery. There’s also this study showing obsidian scalpels cause less damage than steel. I can attest to the fact that obsidian is sharp as shit and leaves less of a scar.

Back to our chunk o’ obsidian, then. By now, you’re noticing something.

Macro shot of obsidian texture, penny for scale.

It’s… gray. And kind of bubbly.

Very bubbly.

Hit it with a hammer, and you’re going to get crumbles, not a nice conchoidal fracture. Make this into a surgical knife, and your scars will make you look like Frankenstein. None of us are rude enough to heave it out of the landscaping, which someone obviously took some pains with, and heave it into the street, but it seems that if you did, it would be a lot lighter than a chunk of solid black obsidian. It certainly looks lighter.

And really, could such a diverse community of lichens and mosses get started on silky-smooth non-porous obsidian?

Lichen some obsidian. Ahahaha. Sorry.

This rock is full of vesicles, and the vesicles, in many places, are full o’ life.

Lichen on obsidian macro.
Moar lichen on obsidian, because I can't help myself.

All right, so it’s pumaceous obsidian. It’s obsidian that’s frothy. It’s pretty much pumice, but it’s still obsidian.

And if you look closely, there are some magnificent vitreous portions.

Really vitreous bit

Vitreous basically means “glassy.” Which this is. Very much so.

So this delighted me. I mean, the last thing you expect walking up the little road along by the wastewater plant and the Shell station is obsidian. Lovely, lichen-covered, unobtrusive obsidian.

Streetside view of obsidian.

You can even see a bit of what looks like flow banding in there.

So, someone clever decided that instead of the usual granite or basalt boulders, they’d go with obsidian. And they didn’t just leave it at one chunk. There’s a few more scattered in there.

Moar obsidian!

And this one had a nice pattern to it.

Obsidian with pattern.

I’m not sure precisely what’s causing it. Perhaps one of the savvier geos in the audience can tell us.

Pattern closeup.

I suspect it’s got something to do with water or other gasses entrapped in the flow, but I could be completely wrong. And I wish I could tell you how obsidian forms, but the old ideas of lava cooling so quickly it doesn’t have time to crystalize are in doubt just now. The Big Obsidian Flow at Newberry Crater kind of calls that into question. How can something that thick cool rapidly enough to form glass? There’s probably a different mechanism. If someone’s figured that out and published a paper, I’d be very grateful for a PDF.

One thing I do know about this obsidian: it’s rhyolite. Bound to be. It’s nearly identical to the pumaceous obsidian we saw at the Big Obsidian Flow, which is undoubtedly rhyolitic. That means it’s around 72% silica, with aluminum making up most of the rest. A bit of iron is what makes it black, and there are traces of other oxides in there – sodium, potassium, magnesium, calcium, titanium, manganese. Microscopic bubbles of water probably makes this particular variety gray, or it could have something to do with how frothy it is.

I’m loving the fact a landscaper decided to put this here. For my next field trip, I may have to go down to the places that sell landscaping rocks to see if I can acquire a chunk o’ me own.

Oh, and the daffodil was nice, too.

Srsly, Obsidian at North Creek?!

5 thoughts on “Srsly, Obsidian at North Creek?!

  1. 2

    There may well be a more local source, but I know pumiceous (not pumaceous) obsidian is quarried for use in landscaping and facing stones from one of the coulees in the Mono-Inyo chain just north of Long Valley Caldera in California. I’ve been to that quarry and I’ve got some very large and frothy samples of it in my garage – it looks very similar to what you’ve got here.

  2. 3

    I read a paper back in the 80’s in which the author(s) proposed obsidian may be a particularly volatile-poor variety of rhyolite. Magma tends to have (if I remember correctly) 0.5-3.5 weight percent of their mass as water, and more fractionated melts- such as granite/rhyolite tend to be on the higher end. A rhyolitic melt with a very low weight percentage of water (<1-0.5%) would be too "sticky" too crystallize very quickly. That is, the silica tetrahedra would get tangled up with each other (water helps silica stay "unstuck" from itself), and as a result would only organize into crystals very slowly. Incidentally, pegmatites are the result at the opposite end of H2O activity: very high water content, very large crystals.

    I've never read anything following up on that article, either to confirm or challenge, and I don't have any idea what the title, author(s), or journal were. so I can't vouch for it. But it always made sense to me.

  3. 4

    Here is a link to an Oregon State paper on Obsidian:
    It takes a very SiO2 rich magma (70% plus) that got close enough to the surface to outgas all the water. This leads to a very viscous lava, that is so viscous that the atoms can not rearrange themselves to crystalize, and thus become a glass. The article notes the pumice in the middle of the flow at Newberry, indicating offgassing. It also notes that an ant could outrun an obsidian flow.

  4. 5

    I’ve never taken a geology class, and I generally find rocks to be…rocky, but your excitement has been wearing my rock apathy down, Dana. You got me glancing at roadside gravel and river bluffs, wondering what secrets are hiding in plain sight!

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