A Geologic Riddle: Well-Rounded

Right. I’m not ready with the big Quartzville riddle, so here’s a small one from Marys Peak instead. Hopefully, you all will enjoy it as much as the last one.

I’m a well-rounded rock.

Riddle Rock I

I’m something of a chameleon. At a glance, I look like something I’m not. And I’m heavier than I look.

Riddle Rock II. Dollar coin for scale. This small rock is surprisingly hefty.

I sparkle now: once, I glowed.

Riddle Rock III

I give road cuts an almost pillowy appearance, but you wouldn’t want to rest your head on me.

Riddle Rock IV

You just can’t assume anything about me.

What am I, and how did I form?

A Geologic Riddle: Well-Rounded

15 thoughts on “A Geologic Riddle: Well-Rounded

  1. Ole

    From the hints, it must be a lump of basalt. The roadcut shows pillow lava, a peculiar shape that forms when hot lava meets seawater. And basalt “once glowed”.

    Last time I found something that was far heavier than it looked, it was a lump of redeposited ferricrete – limonite-cemented sand.

  2. rq

    Basalt, because pillow basalt was in the list of things from your trip. :)
    It’s an igneous rock, which means it forms from cooling lava. More than that, and I need to do a lot more research than Wikipedia.

    I’m pretty sure it’s also the national rock/mineral of my current country, but I’m not 100% since I’m not sure if I’m translating correctly.

  3. 3

    Definitely volcanic. I’d guess that it froze in mid-air. What do you call them, volcanic bombs? The photo of the road cut doesn’t look quite like the one and only example of pillow lava that I’ve seen.

  4. 5

    On first glance it looks like a corestone, probably of basaltic composition. In this interpretation the rounding is the result of spheroidal weathering. This explanation doesn’t specifically account for the higher density though, unless that density is more of a relative measure compared to the surrounding weathered material.

  5. 8

    Whne you said it was heavier than it looks, my first thought was meteorite, but the last pict looks like a bunch of them imbedded in a really old lava flow. This riddle brings up many more questions than answers for this particular non-geologist. For example, are the shiny flecks pyrite, quartz or some other mineral?

  6. 9

    My first thought was volcanic bomb, but if I remember right, most of them show some sort of striation-like patterns from the lava ‘flowing’ or something. Also, I’m not sure that basalt is much heavier then it looks.

    And as Ron Schott mentioned, it does look like spheroidal weathering with the coarse texture.

    Something iron-rich that was either caught up in the initial flow, or possible filled in vesicles/voids after cooling?

  7. 10

    I will argue that it’s gabbro or diabase rather than basalt, from the grain size and crystallinity. I give this heavy, chameleonic creature about which little can be assumed the name of xenolith.

  8. 12

    @#11: Your paper certainly seems to have them in the right area. However, Dana indicates that they’re pretty dense. I can’t say for sure, but I don’t think that works for the zeolites. Also the glowing part — my impression is that the zeoloites would have formed long after the lava cooled.

  9. 13

    Agreed – not so sure about the density and the glowing part. Like I said, I’ve never encountered anything quite like this. We have some pillow lavas here in MI (got a piece on my front porch), but I’m not aware of anything quite like this.

  10. 14

    I’m double-checking a detail with Lockwood before I post the answer. Good guesses, everyone! Hope this has been fun and not a sort of torture. I’ve enjoyed it, but I’m concerned I might be a sadist.

    If anyone else decides to start riddling, please let me know so I can link you, okay?

  11. 15

    I’m late to the ball on this one (spending time with the family on Sanibel Island) but I’ll also weigh in on the pillow basalt side of things. Just a couple of additional points I’d like to make. Pillow lavas need not form in the ocean, any body of water (lake, river) will do if it gets in the way of flowing lava. I’m guessing these are pillows all mixed up in what might have been sediment. Typical oceanic pillows pile up on top of one another and tend to deform before completely hardening. This tends to give the tops of the pillows a rounded form while the bottoms contort and deform to accommodate the shape of the surface on which they formed. This is a useful “which way was up” indicator for ancient lava flows which may have been deformed.

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