Answer to a Geologic Riddle: Well-Rounded

I expected this one to be simplicity itself, which is why I salted the clues with ambiguity – and apparently ended up being too deceptive! But all of you who guessed “pillow basalt!” shouldn’t feel bad. For one thing, the clues can easily be construed that way. For another, that’s the first thing I shouted when I saw that outcrop, too. And I had my nose to it!

But there’s another thing that can cause that nicely rounded shape: spheroidal weathering. This is a fine example of spheroidal weathering in basalt. Ron Schott wins a – um, well, there aren’t any prizes yet, so he wins our adulation, I suppose.

Spheroidal weathering made this lovely little basalt glob.

All of you who guessed basalt also win adulation. ‘Tis basalt! Well, I thought it was basalt, anyway. I was pretty sure I’d remembered that right, but there’s a lot of gabbro on Marys Peak as well, and I couldn’t remember if this was that or not, and I could have taken the rock hammer to it to get a fresh surface, but it’s such a nice little round shape, a perfect exemplar of spheroidal weathering, and… so I decided to check with Lockwood. He confirmed basalt:

Siletz River Volcanics, so basalt. That sample is a bit coarser grained than is usual for basalt, but still nowhere near coarse enough for me to be tempted to call it diabase- intermediate between basalt and gabbro. The fact mafic magmas are less viscous, and have ionic mobilities higher than felsic magma, means crystals can grow more easily and  rapidly- which means you can easily get grain sizes readily visible to the naked eye, but not really large enough to ID readily. In turn, I think this is why we have the term diabase as an intermediate to basalt and gabbro, while, as far as I know, there is no equivalent term for intermediate or felsic igneous rocks.

I’ve seen plenty like that in lava flows in Arizona, where crystals gleam and sparkle in flows that obviously didn’t spend ages slowly cooling underground. This one seems quite like those.

The reason I threw in the clue about its weight is because I didn’t want anyone haring off after ideas that it might be a concretion in a sedimentary formation: this piece I collected is little, but quite a bit heavier than an equal bit of sandstone would be. Yet, at first glance, it does look like a very dark sandstone.

Here’s the outcrop from whence it hails:

Basalt dike through volcanic breccia, Marys Peak, OR

Lockwood explains:

It’s a dike through a coarse volcanic breccia. Some poor sorting and bedding is visible in the breccia in some spots. The thing that particularly impressed me on this visit was the degree of weathering. That cut was made shortly before I moved out here, so about 35 years ago. Hard to believe, when I first saw this spot, it was basically pristine and fresh.

Now it’s all crumbly and weathered to hell. But still fun. Look! Baby spheroidal weathering!

Incipient spheroidal weathering in basalt, Marys Peak, OR

That bulgy bit on the right, with the little white spot of lichen or something, is aspiring to be the next well-rounded bit of basalt you can hold in your hand. Adorable!

Loved all of your guesses. If you’ve no objections, I believe we’ll keep doing riddles. This is awesome good fun!

Answer to a Geologic Riddle: Well-Rounded

5 thoughts on “Answer to a Geologic Riddle: Well-Rounded

  1. 1

    “Spheroidaly weathered basalt”…that’s what I MEANT to say.

    Good one, Dana. Occam’s Razor is successfully wielded yet again. Good thing I didn’t go with my original interpretation of “hyper-mega-tektites created by the K-T impact”.

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