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Apr 24 2012

Right. Let’s See How Good My Geology-Fu Is

I’ve struck up a bit of an online friendship on G+ with someone who made an off-hand comment about one of the geology posts I shared. It was the one about 10 Reasons Geologists Are Weird. When a person reshares your reshare, and their profile photo shows them standing by some particularly yummy geology, wonderful things can happen. Like, being shown some delights and asked to identify them.

Case in point:

Grooves in rocks leading from the beach out to the sea. Flinders, Victoria. Photo Credit: S. Travers.

Here’s the accompanying description:

Grooves in rocks leading from the beach out to the sea. Flinders Victoria. The romantic in me wants them to have been made by glacial activity, but I don’t seriously think that’s the cause. They don’t appear to be manmade and they’re not regular – at the widest about maybe 30 – 40 cm, but only about 20cm or so through the rocks in shadow toward the top of the photo.

There are fantastic rock pools right through this area, lots of seaweed washed up on shore and great surf for those hardy souls not worried about sharks (there’s a seal colony not far away) or being swept over the rocks.

Right. So I’m supposed to be writing a review of Victor Stenger’s new book, researching for the metamorphic post on Rosetta Stones, beginning endless reading for a huge series on Mount St. Helens, planning for my upcoming trip to New Hampshire to see The Doctor (Evelyn, that is), and about ten billion other things, including research for another set of photos that S. Travers dangled in front of me (and are delicious). However. Comma. This can’t be resisted. This is a chance for me to see if all this studying geology really taught me anything at all.

So, here’s my thought process: Two strikes against glacial striations. They don’t seem quite right. They’re just not giving me that striation vibe. Too curvy, for one. And they’re in Australia, for another. My Australian geological knowledge is teh pathetic, but I don’t recall Australia being heavily glaciated recently.

It’s hard to tell from this one photo, but I’m getting a faulty feeling. I’m seeing possible faults. And the rock looks sort of a bit volcanic. I’ll crop the photo a bit, and changed the exposure, to get a better look:

Crop of Flinders photo. Photo Credit: S. Travers.

So what could we have? Cracks and fractures in a volcanic rock, seems to me. Now, I could be spectacularly wrong. Certainly, I could be, and that’s why I’m posting this: so that my geologically-savvy readers can confirm or correct.

But let’s see if I can find some evidence to bolster my position.

First, we need to know where Flinders, Victoria is.

View Larger Map

Right. That helps. See, I found this site while searching for Flinders, Victoria geology, and it talks about stuff near Barwon Heads. That turns out to be within the same area. So, what can we find here? Ah-ha! “The collision of the Australian Plate and the Pacific Plate during the Miocene created the SE-NW compressional stress field that Victoria currently experiences.” Now, that looks promising! Could be faults, indeed. There’s also mention of basalt. Excellent!

Now, let’s have a look for faults. I love finding faults. And I found plenty round Victoria. Dunno if that map’s public domain, so I’ll just have you give it a click and return.

Right. So. We’ve got faults, we’ve got basalt, but none of them specifically saying, “Flinders, Victoria has got basalt and faults all over the place.” So what else can we do to make the case? Look at the geologic map. Here, I’ll even clip the bit and point toward Flinders. It’s a government-produced map, hopefully they won’t mind.

Geological map of Flinders, Victoria and surrounding area. Image credit: Geological Society of Australia.

So that bright yellow is noted as Older Volcanic Group. Yup. Volcanics abound there. I think we’ve got some pretty decent evidence, although without boots on the ground, I can’t be certain what I’m seeing. And, again, I could be spectacularly wrong. But even if so, I’ve just learned something about Flinders, Victoria, Australia that I didn’t know before. And got to see a photo of some very lovely rocks.

Also, you guys get something that is not so Pacific Northwest-centric. Along with a chance to kick my arse if I’m wrong. Which is what geology is all about, really, aside from the beer and the hammering and the stories rocks tell and the geologists (who rock) and – we could be here all day if I keep saying what geology’s all about, so I shall stop now. Over to my geos, who can tell us more.

That was fun. Very much so. So much so, in fact, that I’d like to extend an invitation for more photos. I can’t promise I’ll ever get to them, but if you want to send me photos of some delicious geology, along with the location they were taken and any observations you may have about them, it’s quite possible you, too, can see me attempt some geology-fu-by-photo on your very own mystery geology. Email them to dhunterauthor at yahoo dot com, if the fancy takes you.

10 comments

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  1. 1
    Matt Herod

    Hi Dana,

    Thanks for the reposting! I am glad that it led to such a cool connection and experiment. The geoblogosphere has proven to be such great place to make global connections. I am loving being a part of it.

  2. 2
    Gregory in Seattle

    It has been a long time since I’ve studied geology, but let me see if I understand. The basalt is fractured, probably quite deeply; the sediment and tidal currents have weathered the upper parts of the fractures, wearing them into wider channels; the cracks are filled with non-basaltic sediment, giving them the appearance of being smooth. The result is what appear to be (but are not) shallow grooves in the rock surface.

  3. 3
    cope

    Well, here is my dos centavo’s worth.

    The rocks in question appear (in the full-sized picture) to have some kind of layering that dips very slightly to the left-hand side of the picture. The zone with the “grooves” seems to have a slightly different (lighter) color and extends right the way down to the water’s edge.

    I’m gonna go with an explanation that involves gently dipping beds (whatever the rock type) with a slightly softer/lighter layer that was differentially eroded. The grooves are simply an expression of the internal layering of that more easily eroded layer…kinda like taking a book and tilting it slightly to the left and then cutting the top off horizontally to expose the pages/grooves.

    It really makes me want to get down on my hands and knees and take a closer look at it.

    Anyway, it’s a thought.

  4. 4
    frankb

    I am going with the simplest explanation. It looks like a dirt bike path.

  5. 5
    Lockwood

    It looks to me like a shear zone preferentially weathering & eroding down in basalt, as you suggest. To clarify, a “shear zone,” as opposed to a “fault,” could be a broad, interwoven (check out the word “anastamosing,” which might most simply be defined as “splitting and rejoining”) set of smaller faults which together accommodate a single offset- in other words, a bunch of little faults instead of one big one. The term is also used to describe a zone of plastic deformation that accommodates an offset, typically in metamorphic rocks. Cataclastite and augen gneiss are good examples of rock types found in the latter usage sense of shear zone. The shear zone appears to be tightening up into the distance, probably becoming more like a single fault.

    Good call, Dana.

    Looking back over this, I should point out that thinking of cataclastite as an example of plastic deformation may not be quite right- it’s more like a combination of brittle, plastic and granular deformation- and the only person likely to approve of my coining the term “granular deformation” there is Michael Welland.

    Also, the standard caveat is in order: photos are best used to support direct field observations. Using them as a proxy for direct observations is problematic, and any conclusions we draw from doing so are necessarily tentative.

  6. 6
    Callan Bentley

    The ones in the puddles look mineralized (white). So joints/veins is my guess. Have your photo contributor get in close to one of those and snap a couple of macro shots!

  7. 7
    Cole Kingsbury

    I will have to agree with Callan on his interpretation. The joints/veins also appear anastomosing. An alternative explanation is that this could also be a fault zone. Would love to check it out in person.

  8. 8
    Lockwood

    Not sure why mineralization (which I agree appears to be present) would be more indicative of joints than faults. And, in retrospect, to conclude faulting, one would want to see offset, crushing, or slickensides- none of which are apparent in thee pictures. On the other hand, why would a joint set- a result of tensile stress- be confined to such a narrow zone? Once the first joint formed, wouldn’t that relieve the local tension, and cause subsequent joints to form farther away?

    An additional observation: in the near foreground, there appear to be amygdules, which would support the idea of a basaltic/volcanic bedrock.

  9. 9
    F [i'm not here, i'm gone]

    Lockwood said,

    …the only person likely to approve of my coining the term “granular deformation” there is Michael Welland.

    Outstanding!

  10. 10
    StevoR : Free West Papua, free Tibet, let the Chagossians return!

    Reminds me of the cart wheel tracks from the gypsum mine off the beach at Inneston, Yorke peninsula, South Australia, visible at low tide where they used to take a short cut with laden wagons.

    I could , of course be horribly mistaken about that but wonder if that’s the answer – 19th century or so cart track from low tide traversing?

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