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Volcanic Formation Texture - please help identify!

 
 
Reply Wed 25 Nov, 2015 05:22 pm
Hi guys

I am trying to name the correct texture for the image but I haven't got a clue. Picture was taken from a volcanic formation in Wales, i.e. rhyolite in/extrusions with sedimentary rocks and volcanic ash/tuffs. My initial thought was that it's a brecciated deposits laid on top of a sedimentary rock (note: base of the picture)? Not entirely sure though.

http://1drv.ms/1T5hNPP

Thanks very much!

Jafr.
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farmerman
 
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Reply Thu 26 Nov, 2015 05:30 am
@jafreese,
You mean like aphanitic or phaneritic? Most descriptions should be "unblessed" with a lot of the old fashioned bullshit so that you can explain clearly what youve found and what it means. Id use the least amount of jargon and get to what the actual "ROCK FABRIC" is telling you.
Unfortunately you have the picture taken as an over-grainy black and white(It actually looks like layers of cannel coal or lignite). Id shoot it in color and let the light come in from a steepr angle in the AM or late PM (either way). Then Id try to describe it.
The web is full of syllabi from various colleges' teachers who are running petrology or rock structure courses. Find one(try ' IGNEOUS ROCK TEXTURE)

Are you sure its igneous?
At the least Id use aphanitic and go from there.
1.Color, mineral habit and rock structure
2. overall structure(describe the entire outcrop)
3.is it really an unconformity?(is the ldge just underlying another igneous layer or a sedimentary contact? (If sedimentary theres a whole big story of how the rocks got next to each other that involves texture and habit and fracturing etc)

You could write a long boring page on this outcrop.
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