Monday, May 19, 2008

Plumose structure

Here's a photo one of my Audubon students (Albert) took this past Saturday on the Berma Road, in C&O Canal National Historical Park. The lighting was just right, so that when we passed by this outcrop of metagraywacke, we saw an illuminated example of plumose structure:

plumose

Plumose structure is something that forms when rock breaks. The fracture starts at one point, and then grows, propagating thorough the rock and leaving behind a telling signature of its growth. In this case, the fracture (also known as a joint) started at point A and propagated through the rock to point B (central 'shaft'), expanding laterally (feathery 'plumes') at the same time.

Sometimes, concentric 'ribs' form, perpendicular to all these feathery plumes, showing the actual leading edge of the growing fracture surface. An example most people are probably familiar with is the "clamshell" shape of a classic conchoidal fracture. Check out this image to see how the two relate to one another.

When we saw this lovely example, I pointed out to the students that if we had been there fifteen minutes earlier or later, this subtle topography would either have been obscured totally in shadow, or washed out in full light. It was only because the light was at juuuuuust the right angle relative to these mm-scale variations that we noticed it.

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5 Comments:

Blogger Kim said...

Great photo! (May I use a copy of it in my joints powerpoint?)

May 19, 2008 5:27 PM  
Blogger Callan Bentley said...

Kim,

Of course. You want a bigger version of it?

I can e-mail it from my NOVA account to your uni account...

C

May 19, 2008 5:48 PM  
Blogger Kim said...

Yes, please e-mail it - it would be good to have it at high resolution for projecting.

Thanks!

May 20, 2008 8:35 AM  
Blogger Callan Bentley said...

Kim,

Did you get it? (Sent it yesterday AM, but haven't heard back).

Also sent the hackle photo.

C

May 21, 2008 10:52 AM  
Anonymous Anonymous said...

I agree - very nice photo

January 20, 2010 10:35 AM  

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