Naked Science Forum

Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: VistaHunter on 14/08/2012 01:15:19

Title: What made this volcanic rock look like this?
Post by: VistaHunter on 14/08/2012 01:15:19

Hello,

I have an interesting rock that I would like some help in identifying. I found this rock on my friends property up by Mt. St. Helens in Washington State. For a long time I actually thought this rock was a meteorite because it appears oriented, is weakly magnetic, and has very thin black coatings in areas that I thought may have been remnant fusion crust.

I know now that it is not a meteorite because I sent a sample in to New England Meteoritical Services for petrographic analysis. They informed me that it is a volcaniclastic rock composed of chalcedony.

The rock fascinates me very much and I would like to learn more about it.

How did it form?

Clearly it came from the bowels of Mt. St. Helens right?

Was it once a giant molten geode?

Did brecciation occur on impact?

Any insight would be much appreciated!!

Title: Re: Volcaniclastic Brecciation
Post by: VistaHunter on 14/08/2012 01:16:31

Here is a slab photo.

Title: Re: Volcaniclastic Brecciation
Post by: VistaHunter on 14/08/2012 01:17:41

What are the black inclusions within the matrix that I can scrap out with my fingernail? Carbonates?

Title: Re: Volcaniclastic Brecciation
Post by: VistaHunter on 14/08/2012 01:18:59

What is the brown substance throughout the matrix that gives the agate a yellow golden hue? Sulfides?

Title: Re: Volcaniclastic Brecciation
Post by: VistaHunter on 14/08/2012 01:21:13

One Crystal appears to have formed wavy bands or perhaps was melted and warped. Why?

Title: Re: Volcaniclastic Brecciation
Post by: VistaHunter on 14/08/2012 01:21:59

Another crystal has formed or was recrystallized right in the middle of dense agate. Why?

Title: Re: Volcaniclastic Brecciation
Post by: VistaHunter on 14/08/2012 01:23:15

Closeup of breccia.

Title: Re: Volcaniclastic Brecciation
Post by: VistaHunter on 14/08/2012 01:23:38

More Slabs.

Title: Re: What made this volcanic rock look like this?
Post by: Bass on 17/08/2012 04:24:38

What you found is not uncommon in near volcanoes.  Your rock is probably a lava flow breccia that had its voids filled with chalcedonic silica.  The banded "wavey" agate is typical, as the hot fluids circulated through the breccia and precipitated silica and/or replaced precipitated calcite.

Title: Re: What made this volcanic rock look like this?
Post by: VistaHunter on 17/08/2012 06:15:46

Thanks Bass.

Can anyone tell me what the soft black inclusions within the matrix might be?

What might the brown substance be that leached into the agate and quartz?