Post by: Geo4dood on 23/08/2018 11:02:46
In a recent hike, I've came across with this rock structure.
TR.jpg (106.08 kB . 494x338 - viewed 5434 times)I appreciate you to tell me what's it's name, geologically speaking, and what kind of fractures are those.
Thanks in advance.
Post by: Bill S on 24/08/2018 22:43:36
The left side of the arch looks as though it has parallel surfaces, which could be bedding planes. This would suggest it is a sedimentary rock that has been folded. Having said that, I've seen weathered granite domes that have shown quite similar features. Some more details would be valuable. Where is it?
The fractures at right-angles to these "planes" are likely to be stress fractures.
The structure is an anticline.
I'm only an amateur, and it's a long time since I dabbled in any geology, so hopefully the experts will chip in.
Post by: chris on 24/08/2018 22:49:24
Post by: Bill S on 24/08/2018 23:29:50
Post by: chris on 26/08/2018 00:24:46
True, but I'd still like to know where it is.Yes, indeed - @Geo4dood - where was this picture taken?
Post by: Geo4dood on 26/08/2018 18:30:44
The picture was taken in a recent hike I have made in the small azorean island of Corvo, (more or less 39° 41.087'N 31° 6.903'W).
I haven't approached to much, but I wonder that it's made of basaltic rock, like the most part of the island.
I agree on Bill S' remarks, but I ask you if a basaltic structure could it have the "bedding planes" and /or "stress fractures" that you've mentioned.
Thanks again!
Post by: Ophiolite on 27/08/2018 21:58:04
I wouldn't agree with the suggestion that it is an anticline. The top left to bottom right lineation in the rocks appears to be continued on both sides of the arch. that being the case there is no folding and so it cannot be an anticline.
"Bedding planes" can exist between individual lava flows, but they would not be as thin as is the case here. Thicker bodies of intruded magma (large sills, lacoliths, lopoliths, etc.) often have layering caused by the sequential crystallisation and settling of different minerals. Injections of fresh magma can cause this layering to be repeated many times. That's a possibility here.
The lack of a close up and the exposure of a fresh surface make it difficult to determine just what the rock type is. If much of the native rock colur is showing through lichen and weathered material then this is not basalt - far too light. The jointing is consistent with any body of cooling magma. i.e stress fractures are consistent with any igneous body, but especially a lava, rather than an intrusive mass.
Another possibility that occurs to me is volcanic ash, or pumice. This would explain the apparent layering, since multiple ash falls would be expected during an eruption. This would also be lighter in colour - another match with what we observe.
Summary: insufficient data to reach a conclusion, but a neat photograph and an intriguing feature.
Post by: Bass on 16/09/2018 01:01:43
Post by: Geo4dood on 18/09/2018 11:19:48
I never though that this structure could be the remains of a lava tube. Spectacular!
If that's the case, the type of rock has to be basalt, correct?
Post by: Bass on 05/10/2018 23:59:34
Post by: Bill S on 07/10/2018 15:13:35
Quote from: Bill
The structure is an anticline.
Write 100 times; I must not rush in without due care and attention. :(