M.Chapman

Geological feature ID

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Any geologists out there?

 

I've uploaded some pictures of what I think is an exposed vertical volcanic rock dyke on Skye. This shows a general view.

 

fine-straight-vertical-volcanic-dyke-on- 

 

I also took some closeups of the unusual geometric fractured surface of the side of this rock band.

 

closeup-of-geometric-fractured-rock-surf

 

My is question is - Is there a technical term for this kind of fractured surface, or any other suggestions for my caption and keywords?

 

Thanks in advance.

 

Mark

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We have at least one professional geologist on the forum so I will hold back a little on my amateurish suggestions for now. 

 

That certainly looks like a dyke - I'd definitely use words such as igneous, intrusion, intrusive, also eroded/erosion/differential erosion/rates of erosion, wave cut platform. I'd also include sill as a keyword just in case because the rocks could be tilted/folded.

 

I don't know the technical term but joints/jointed/weathered/weathering. If I had to give an answer in a quiz I'd probably guess something along the lines of pressure release jointing.

 

But as I said...hopefully a professional will be along soon. Us human geographers are more comfortable with waffly concepts than hard science:)

Edited by geogphotos
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I'm a former professional geologist (not sure if it was me that geogphotos was referring to as there is at least one more who occasionally frequents the forum). I've never been to Skye but it is famous geologically for its igneous rocks. That is definitely a dyke, almost certainly a basaltic dyke. The rock it is intruding looks to be a lot more silicic, possibly (probably) a very fine grained or glassy rhyolite (looking at the second image which I presume is a side view of the same rock). Glassy rocks like rhyolite break with a conchoidal fracture which is what this rock is exhibiting but I am not sure offhand how you would describe the entire surface. Check out the BGS (British Geological Survey) website as they were (and presumably still are) publishing online geological maps  so you can check the geology if you know exactly where you were. These rocks are relatively young (around 60 million years or so) for Scotland and have not undergone any deformation such as folding. All the igneous rocks of this age in Scotland and Northern Ireland (Giant's Causeway) are related to the opening (formation) of the North Atlantic Ocean.

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Thank you, MDM, for that interesting post. I didn't know that Skye's igneous rocks are from the formation of the Atlantic.

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Many thanks guys.

 

MDM - The second photo is a closeup of the vertical exposed face of the dyke and the fractures are straight not curved (so not conchoidal?). Excellent tip about the BGS website. This intrusion is marked on there. I wonder if it's sensible to include the National Grid ref in the caption as I know exactly where this feature is.

 

Mark

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2 hours ago, M.Chapman said:

Many thanks guys.

 

MDM - The second photo is a closeup of the vertical exposed face of the dyke and the fractures are straight not curved (so not conchoidal?). Excellent tip about the BGS website. This intrusion is marked on there. I wonder if it's sensible to include the National Grid ref in the caption as I know exactly where this feature is.

 

Mark

 

 

Sorry I didn't explain what I meant about the conchoidal fractures very well. I wasn't talking about the linear and sub-linear joints or cracks - these may be cooling joints and the horizontal ones may have formed if overlying rocks were eroded which often results in sub-horizontal joint formation depending on the rock. What I really mean was the appearance of the fresh rock surfaces (pink and yellow) which look extremely smooth and fine-grained (probably originally glassy) and which might fracture conchoidally if hit with a hammer (which might be illegal here). Some of these fresher exposed surfaces look like they have that type of fracture but no I wouldn't use that in the description. It is best to avoid interpretation anyway without detailed study. From experience I would think it is rhyolitic which essentially means a fine-grained or glassy igneous rock with high silica content which could have extruded as a lava flow or lava dome or it could be a high-level intrusive rock which cooled very rapidly. But it could be a weathered basalt as you infer  if it is the same dyke as the dyke is so dark that I reckon it must be basaltic or thereabouts. Without seeing it in the field I would hesitate to say anything further about the rocks really but I was thinking that the second picture was of the rock that the dyke intrudes. Without understanding the field relations and detailed study it would be impossible to say which is where the BGS description and the scientific literature come in.

 

However, I  would think such pics are unlikely to sell as they are too generic and vague. The dyke is different - that appears to be a good example of a generic basic or mafic dyke. But I think the market for geological images is probably very small. 

 

Finally identifying rocks and their relationships is a lot different from identifylng biological identification. If I was studying that I would want to see it under a microscope and get a chemical analysis before I would say anything much about composition

 

Edited by MDM

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6 hours ago, MDM said:

 

 

Sorry I didn't explain what I meant about the conchoidal fractures very well. I wasn't talking about the linear and sub-linear joints or cracks - these may be cooling joints and the horizontal ones may have formed if overlying rocks were eroded which often results in sub-horizontal joint formation depending on the rock. What I really mean was the appearance of the fresh rock surfaces (pink and yellow) which look extremely smooth and fine-grained (probably originally glassy) and which might fracture conchoidally if hit with a hammer (which might be illegal here). Some of these fresher exposed surfaces look like they have that type of fracture but no I wouldn't use that in the description. It is best to avoid interpretation anyway without detailed study. From experience I would think it is rhyolitic which essentially means a fine-grained or glassy igneous rock with high silica content which could have extruded as a lava flow or lava dome or it could be a high-level intrusive rock which cooled very rapidly. But it could be a weathered basalt as you infer  if it is the same dyke as the dyke is so dark that I reckon it must be basaltic or thereabouts. Without seeing it in the field I would hesitate to say anything further about the rocks really but I was thinking that the second picture was of the rock that the dyke intrudes. Without understanding the field relations and detailed study it would be impossible to say which is where the BGS description and the scientific literature come in.

 

However, I  would think such pics are unlikely to sell as they are too generic and vague. The dyke is different - that appears to be a good example of a generic basic or mafic dyke. But I think the market for geological images is probably very small. 

 

Finally identifying rocks and their relationships is a lot different from identifylng biological identification. If I was studying that I would want to see it under a microscope and get a chemical analysis before I would say anything much about composition

 

 

Thanks for your help and clarifying that.

 

Yes it was strange that the dyke itself was very dark and appeared to be basalt. But the side face, when viewed from the right angle appeared quite different, as if it been coated with a reflective / coloured material.  You're right that such images don't sell very frequently, but when they do the use is typically in educational books and the fees have been well worth it. You're right about the geology of Skye, it's fabulous. I imagine it's one of the most geologically diverse places in the UK. I'll be keywording a couple of other unusual geological images shortly and will post on the forum for comment.

 

Mark

Edited by M.Chapman

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1 hour ago, M.Chapman said:

 

Thanks for your help and clarifying that.

 

Yes it was strange that the dyke itself was very dark and appeared to be basalt. But the side face, when viewed from the right angle appeared quite different, as if it been coated with a reflective / coloured material.  You're right that such images don't sell very frequently, but when they do the use is typically in educational books and the fees have been well worth it. You're right about the geology of Skye, it's fabulous. I imagine it's one of the most geologically diverse places in the UK. I'll be keywording a couple of other unusual geological images shortly and will post on the forum for comment.

 

Mark

 

If the vertical face is definitely part of the same dyke, then it is just chemical weathering. 

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23 hours ago, MDM said:

Check out the BGS (British Geological Survey) website as they were (and presumably still are) publishing online geological maps  so you can check the geology if you know exactly where you were.

1

 

Download the iGeology smartphone app - highly recommended (free).

 

http://www.bgs.ac.uk/igeology/

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21 minutes ago, Vincent Lowe said:

 

Download the iGeology smartphone app - highly recommended (free).

 

http://www.bgs.ac.uk/igeology/

 

Very useful. I've not been on the BGS site for some time. Will check that out.

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On 5/19/2018 at 09:05, Vincent Lowe said:

 

Download the iGeology smartphone app - highly recommended (free).

 

http://www.bgs.ac.uk/igeology/

 

Excellent, thanks. I've downloaded and installed it. It's a shame it seems to need internet access to display a map when on site (so not so good in remote locations) and the maps aren't as detailed (on an iPhone anyway) as those available by going to the BGS website.

 

Mark

Edited by M.Chapman

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On 19/05/2018 at 16:22, M.Chapman said:

You're right that such images don't sell very frequently

 

True but you never know, I managed to sell one of blue asbestos.

 

FMPG5E.jpg

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