Monday, February 22, 2010

Andesite, shale and sandstone

I'm a bit late getting out my Monday's Show and Tell, but here goes. This weekend I went on a bit of an adventure to test out some new iphone apps and see about collecting a couple of rock samples. I used GeologyWA/OR to find the rock types I was interested in and pinpoint possible areas of investigation (which obviously worked quite well- gotta love those geologic maps!), and to take images I decided to put Theodolite to the test. Unfortunately I didn't check to see if I had the "crosshairs" turned off, so the images from Theodolite ended up with them included in the images, but it wasn't entirely disastrous. The images weren't half bad, however you can't take the images in "portrait" without getting an elongated distortion. It displays the gps coordinates etc in the upper left hand corner of your image, so it isn't too bad when you want to keep track of where you have taken a particular photograph. It also zooms up to 4x, but you lose quality with each zoom. I prefer to just leave it as it is and edit in photoshop later. The image to the top left is of a bridge I hate crossing (note the rickety looking timbers) but I like what is on the other side so it is a necessary evil.

The drop down is apparently pretty far (from the picture Husband took) but I wasn't about to look for myself. As embarrassing as it is to admit, I tend to get vertigo over gorges and the like. Since this has caused me to miss out on some of the things I would love to see, it's a fault I've been trying to overcome. Thankfully I'm not as bad as I used to be when crossing bridges,  as there once was a time I would have had my eyes shut with my hands over my hears humming to myself. Progress is a good thing :)

One of the coolest rocks I found on my little excursion was andesite. When I first saw the rock I had to take a double take because it reminded me of a stromatolite at first glance. Upon picking it up I could see it was andesite, although I hadn't seen it eroded in such a way so it was a very nice find!I like rocks such as this one, as I find beauty in their form. The majority of the rocks I have about my home are in the raw as that is how I prefer them. (Minerals are a bit different- I do like polished malachite etc.)  It is at this point that I should mention that I am not overly familiar with volcanics, but that is probably the only fun part (for me) about being a military family. Each time I move, I have to learn the local geology. Since my entire undergraduate program has been so varied, I have a bit more diversity than the normal undergraduate. However I have not been in any areas with volcanics to the extent that this one provides.

I found some coal-which is really awesome since I had yet to obtain a sample of coal. It looks to be like a high grade of bituminous coal. Then I won the lottery when I came across a piece of shale that had beautiful leaf  and grass imprints embedded into it. I think it is quite the loveliest piece of shale I have ever seen. I put it in a special spot in my livingroom. The other rock I was wanting to find was the arkosic sandstone, and not only did I find that- but also a gorgeous piece of fine grained sandstone.

To conclude today's "show and tell" (although this isn't everything I discovered on my little expedition- I'll save the rest for another day) is a piece I am uncertain about. It has structures about it that would indicate it to be lithified wood, but  I am unfamiliar with this type of structure, having never actually even SEEN a piece of fossilized wood in person. The other thought I had was maybe it is just an imprint of wood on flow lava. I honestly am perplexed. I'm still researching it, however if by looking at the images below you can offer any insight, please feel free to point me in the right direction. The literature online hasn't offered me much in the way of assistance, married with the aspect that search engines are spitting back useless drivel.  Pictures below:


Lockwood said...

I can't tell from the pictures whether that last is petrified wood or not. What you want to look for are the growth rings. The striations on the left side of the upper right photo are suggestive, but again, not enough to really convince me. I've seen flow banded lavas that take a very careful examination with a hand lens to see that they're not wood, and vice versa.

Still, wood tends to silicify in the Cascades, it is pretty abundant, and quite durable. If you keep looking, you'll find it before too long (look for small chips in river-rounded road gravel). If you have access to a decent binoc-dissecting scope, examining the above sample might clarify what you have.

Cannibal Panda said...

Looked through some old books of mine and permineralization is definitely not what occurred with this rock. I'm thinking more along the lines of an impression possibly caught up in a flow, but I've yet to find that journal article I need to either confirm or deny that possibility. No, I wasn't looking for it (pet. wood) as it doesn't really interest me as far as collecting. I just picked this rock up because it looked interesting and I wanted to bring it back to inspect in more detail. Unfortunately I don't have a scope, just a mini gadget that does 100X and my hand lens. And, a thin sample would make this sooo easy to know what it is, for sure! I'm much better with those than hand samples :)

Interesting aspect to this that makes me think of a flow (as you suggested) is there are black XLs that have the appearance of obsidian in some areas - you can see it on the top left image. Have you seen anything like that before?

Lockwood said...

If those little sparkly bits are glassy- I can't tell from the picture, look for conchoidal fracture- it might be a hyaloclastite, lava that shattered on contact with water. It's pretty common in both Cascades and older basalts of the Coast Range, though I don't know if I've ever seen such a narrow zone like that.

Cannibal Panda said...

They look like obsidian but aren't- as the associations don't seem right. I don't know what they are.

(okay, I went and got a knife and was able to get up underneath one of them- they are in fact, a high grade of coal. I also was able to flake away a few pieces and noticed even more imprints that looked distinctly plant-like. I'm thinking maybe this just may be a shale, but the shape is really throwing me off. It shouldn't- but my cookie cutter mind is fighting what my gut wants to conclude. I'll take some better pictures tomorrow and see if those little subtle clues I see makes sense to you.

Oh, and when I flaked some of the black pieces off it became very apparent that the entire structure was quite fissile.